Part VI
=113. Comparison of the ionization produced by the α and β rays=. With unscreened active material the ionization produced between two parallel plates, placed as in Fig. 17, is mainly due to the α rays. On account of the slight penetrating power of the α rays, the current due to them practically reaches a maximum with a small thickness of radio-active material. The following saturation currents were observed[181] for different thicknesses of uranium oxide between parallel plates sufficiently far apart for all the α rays to be absorbed in the gas between them.
Surface of uranium oxide 38 sq. cms.
Weight of uranium Saturation current oxide in grammes per in amperes per sq. sq. cm. of surface cm. of surface
·0036 1·7 × 10⁻¹³
·0096 3·2 × 10⁻¹³
·0189 4·0 × 10⁻¹³
·0350 4·4 × 10⁻¹³
·0955 4·7 × 10⁻¹³
The current reached about half its maximum value for a weight of oxide ·0055 gr. per sq. cm. If the α rays are cut off by a metallic screen, the ionization is then mainly due to the β rays, since the ionization produced by the γ rays is small in comparison. For the β rays from uranium oxide it has been shown (section 86) that the current reaches half its maximum value for a thickness of 0·11 gr. per sq. cm.
Meyer and Schweidler[182] have found that the radiation from a water solution of uranium nitrate is very nearly proportional to the amount of uranium present in the solution.
On account of the difference in the penetrating power of the α and β rays, the ratio of the ionization currents produced by them depends on the thickness of the radio-active layer under examination. The following comparative values of the current due to the α and β rays were obtained for very thin layers of active matter[183]. A weight of ⅒ gramme of fine powder, consisting of uranium oxide, thorium oxide, or radium chloride of activity 2000, was spread as uniformly as possible over an area of 80 sq. cms. The saturation current was observed between parallel plates 5·7 cms. apart. This distance was sufficient to absorb most of the α rays from the active substances. A layer of aluminium ·009 cm. thick absorbed all the α rays.
Current Current Ratio of due to α due to β currents rays rays β/α
Uranium 1 1 ·0074
Thorium 1 ·27 ·0020
Radium 2000 1350 ·0033
In the above table the saturation current due to the α and β rays of uranium is, in each case, taken as unity. The third column gives the ratio of the currents observed for equal weights of substance. The results are only approximate in character, for the ionization due to a given weight of substance depends on its fineness of division. In all cases, the current due to the β rays is small compared with that due to the α rays, being greatest for uranium and least for thorium. As the thickness of layer increases, the ratio of currents β/α steadily increases to a constant value.
=114. Comparison of the energy radiated by the α and β rays=. It has not yet been found possible to measure directly the energy of the α and β rays. A comparison of the energy radiated in the two forms of rays can, however, be made indirectly by two distinct methods.
If it be assumed that the same amount of energy is required to produce an ion by either the α or the β ray, and that the same proportion of the total energy is used up in producing ions, an approximate estimate can be made of the ratio of the energy radiated by the α and β rays by measuring the ratio of the total number of ions produced by them. If λ is the coefficient of absorption of the β rays in air, the rate of production of ions per unit volume at a distance x from the source is
$$ q₀ e^{–λ x} $$
where q₀ is the rate of ionization at the source.
The total number of ions produced by complete absorption of the rays is
$$ \int₀^{\infty} q₀ e^{–λ x} dx = \frac {q₀} {λ} $$,
Now λ is difficult to measure experimentally for air, but an approximate estimate can be made of its value from the known fact that the absorption of β rays is approximately proportional to the density of any given substance.
For β rays from uranium the value of λ for aluminium is about 14, and λ divided by the density is 5·4. Taking the density of air as ·0012, we find that for air
λ = ·0065.
The total number of ions produced in air is thus 154_q₀_ when the rays are completely absorbed.
Now from the above table the ionization due to the β rays is ·0074 of that produced by α rays, when the β rays passed through a distance of 5·7 cms. of air.
Thus we have approximately
Total number of ions produced by β rays ·0074 ---------------------------------- = ----- × 154 = 0·20. Total number of ions produced by α rays 5·7
Therefore about ⅙ of the total energy radiated into air by a thin layer of uranium is carried by the β rays or electrons. The ratio for thorium is about ¹⁄₂₂ and for radium about ¹⁄₁₄, assuming the rays to have about the same average value of λ.
This calculation takes into account only the energy which is radiated out into the surrounding gas; but on account of the ease with which the α rays are absorbed, even with a thin layer, the greater proportion of the radiation is absorbed by the radio-active substance itself. This is seen to be the case when it is recalled that the α radiation of thorium or radium is reduced to half value after passing through a thickness of about 0·0005 cm. of aluminium. Taking into consideration the great density of the radio-active substances, it is probable that most of the radiation which escapes into the air is due to a thin skin of the powder not much more than ·0001 cm. in thickness.
* * * * *
An estimate, however, of the relative rate of emission of energy by the α and β rays from a thick layer of material can be made in the following way:—For simplicity suppose a thick layer of radio-active substance spread uniformly over a large plane area. There seems to be no doubt that the radiations are emitted uniformly from each portion of the mass; consequently, the radiation, which produces the ionizing action in the gas above the radio-active layer, is the sum total of all the radiation which reaches the surface of the layer.
* * * * *
Let λ₁ be the average coefficient of absorption of the α rays in the radio-active substance itself and σ the specific gravity of the substance. Let E₁ be the total energy radiated per sec. per unit mass of the substance when the absorption of the rays in the substance itself is disregarded. The energy per sec. radiated to the upper surface by a thickness dx of a layer of unit area at a distance x from the surface is given by
$$ \frac {1} {2} E_1 \sigma e^{–λ_1 x} dx $$ .
The total energy W₁ per unit area radiated to the surface per sec. by a thickness d is given by
$$ W_1 = \frac {1}{2} \int₀^d E_1 \sigma e^{–λ_1 x} dx $$ $$ = \frac {E_1 \sigma} {2 λ_1} (1 − e^{–λ_1 d}) $$ $$ = \frac {E_1 \sigma} {2 λ_1} $$
if λ₁_d_ is large.
* * * * *
In a similar way it may be shown that the energy W₂ of the β rays reaching the surface is given by
$$ W_2 = \frac {E_2 \sigma} {2 λ_2} $$
where E₂ and λ₂ are the values for the β rays corresponding to E₁ and λ₁ for the α rays. Thus it follows that
E₁ λ₁_W₁_ ---- = ------ E₂ λ₂_W₂_
λ₁ and λ₂ are difficult to determine directly for the radio-active substance itself, but it is probable that the ratio λ₁/λ₂ is not very different from the ratio for the absorption coefficients for another substance like aluminium. This follows from the general result that the absorption of both α and β rays is proportional to the density of the substance; for it has already been shown in the case of the β rays from uranium that the absorption of the rays in the radio-active material is about the same as for non-radio-active matter of the same density.
With a thick layer of uranium oxide spread over an area of 22 sq. cms., it was found that the saturation current between parallel plates 6·1 cms. apart, due to the α rays, was 12·7 times as great as the current due to the β rays. Since the α rays were entirely absorbed between the plates and the total ionization produced by the β rays is 154 times the value at the surface of the plates,
W₁ total number of ions due to α rays ----- = ---------------------------------- W₂ total number of ions due to β rays
12·7 × 6·1 = ------ = 0·5 approximately. 154
Now the value of λ₁ for aluminium is 2740 and of λ₂ for the same metal 14, thus
E₁ λ₁_W₁_ ----- = ---------- = 100 approximately E₂ λ₂_W₂_
This shows that the energy radiated from a thick layer of material by the β rays is only about 1 per cent. of the energy radiated in the form of α rays.
This estimate is confirmed by calculations based on independent data. Let m₁, m₂ be the masses of the α and β particles respectively and v₁, v₂ their velocities.
$$ \frac {Energy of one \alpha particle} {Energy of one \beta particle} = \frac {m_1 v_1^2} {m_2 v_2^2} $$ $$ = \frac {\frac {m_1} {e} v_1^2} {\frac {m^2} {e} v_2^2} $$ .
Now it has been shown that for the α rays of radium
v₁ = 2·5 × 10⁹,
e ---- = 6 × 10³. m₁
The velocity of the β rays of radium varies between wide limits. Taking for an average value
v₂ = 1·5 × 10¹⁰,
e ---- = 1·8 × 10⁷, m₂
it follows that the energy of the α particle from radium is almost 83 times the energy of the β particle. If equal numbers of α and β particles are projected per second, the total energy radiated in the form of α rays is about 83 times the amount in the form of β rays.
Evidence will be given later (section 253) to show that the number of α particles projected is probably four times the number of β particles; so that a still greater proportion of the energy is emitted in the form of α rays. These results thus lead to the conclusion that, from the point of view of the energy emitted, the α rays are far more important than the β rays. This conclusion is supported by other evidence which is discussed in chapters XII and XIII, where it will be shown that the α rays play by far the most important part in the changes occurring in radio-active bodies, and that the β rays only appear in the latter stages of the radio-active processes. From data based on the relative absorption and ionization of the β and γ rays in air, it can be shown that the γ rays carry off about the same amount of energy as the β rays. These conclusions are confirmed by direct measurement of the heating effect of radium, which is discussed in detail in chapter XII.
Footnote 111:
In an examination of uranium the writer (Phil. Mag. p. 116, Jan. 1899) found that the rays from uranium consist of two kinds, differing greatly in penetrating power, which were called the α and β rays. Later, it was found that similar types of rays were emitted by thorium and radium. On the discovery that very penetrating rays were given out by uranium and thorium as well as by radium, the term γ was applied to them by the writer. The word “ray” has been retained in this work, although it is now settled that the α and β rays consist of particles projected with great velocity. The term is thus used in the same sense as by Newton, who applied it in the Principia to the stream of corpuscles which he believed to be responsible for the phenomenon of light. In some recent papers, the α and β rays have been called the α and β “emanations.” This nomenclature cannot fail to lead to confusion, since the term “radio-active emanation” has already been generally adopted in radio-activity as applying to the material substance which gradually diffuses from thorium and radium compounds, and itself emits rays.
Footnote 112:
This method of illustration is due to Mme Curie, Thèse présentée à la Faculté des Sciences de Paris, 1903.
Footnote 113:
Giesel, Annal. d. Phys. 69, p. 834, 1899.
Footnote 114:
Meyer and Schweidler, Phys. Zeit. 1, pp. 90, 113, 1899.
Footnote 115:
Becquerel, C. R. 129, pp. 997, 1205, 1899.
Footnote 116:
Curie, C. R. 130, p. 73, 1900.
Footnote 117:
Rutherford, Phil. Mag. January, 1899.
Footnote 118:
Rutherford and Grier, Phil. Mag. September, 1902.
Footnote 119:
Becquerel, C. R. 130, pp. 206, 372, 810, 979. 1900.
Footnote 120:
M. and Mme Curie, C. R. 130, p. 647, 1900.
Footnote 121:
The activity of the radium preparation was not stated in the paper.
Footnote 122:
Dorn, Phys. Zeit. 4, No. 18, p. 507, 1903.
Footnote 123:
Strutt, Phil. Mag. Nov. 1903.
Footnote 124:
Wien, Phys. Zeit. 4, No. 23, p. 624, 1903.
Footnote 125:
Dorn, C. R. 130, p. 1129, 1900.
Footnote 126:
Becquerel, C. R. 130, p. 809, 1900.
Footnote 127:
Kaufmann, Phys. Zeit. 4, No. 1 b, p. 54, 1902.
Footnote 128:
Abraham, Phys. Zeit. 4, No. 1 b, p. 57, 1902.
Footnote 129:
Kaufmann, Nachrichten d. Ges. d. Wiss. zu Gött., Nov. 8, 1901.
Footnote 130:
Simon, Annal. d. Phys. p. 589, 1899.
Footnote 131:
Kaufmann, Phys. Zeit. 4, No. 1 b, p. 54, 1902.
Footnote 132:
Paschen, Annal. d. Phys. 14, p. 389, 1904.
Footnote 133:
Meyer and Schweidler, Phys. Zeit. pp. 90, 113, 209, 1900.
Footnote 134:
Lenard, Annal. d. Phys. 56, p. 275, 1895.
Footnote 135:
Strutt, Nature, p. 539, 1900.
Footnote 136:
Seitz, Phys. Zeit. 5, No. 14, p. 395, 1904.
Footnote 137:
It is presumed that the results were corrected, if necessary, for the discharging action due to the ionized gas, although no direct mention of this is made in the paper by Seitz.
Footnote 138:
Strutt, Phil. Trans. A, p. 507, 1901.
Footnote 139:
Crookes, Proc. Roy. Soc. 1902. Chem. News, 85, p. 109, 1902.
Footnote 140:
Mme Curie, C. R. 130, p. 76, 1900.
Footnote 141:
Rutherford, Phil. Mag. Feb. 1903. Phys. Zeit. 4, p. 235, 1902.
Footnote 142:
Becquerel, C. R. 136, p. 199, 1903.
Footnote 143:
Becquerel, C. R. 136, p. 431, 1903.
Footnote 144:
Des Coudres, Phys. Zeit. 4, No. 17, p. 483, 1903.
Footnote 145:
Becquerel, C. R. 136, p. 1517, 1903.
Footnote 146:
Bragg, Phil. Mag. Dec. 1904; Bragg and Kleeman, Phil. Mag. Dec. 1904.
Footnote 147:
Further experimental results bearing on this important question are given in an Appendix to this book.
Footnote 148:
Bakerian Lecture, Phil. Trans. A, p. 169, 1904.
Footnote 149:
Strutt, Phil. Mag. Aug. 1904.
Footnote 150:
J. J. Thomson, Proc. Camb. Phil. Soc. 13, Pt. I. p. 39, 1905. Nature, Dec. 15, 1904.
Footnote 151:
Rutherford, Nature, March 2, 1905. J. J. Thomson, Nature, March 9, 1905.
Footnote 152:
Crookes, Proc. Roy. Soc. 81, p. 405, 1903.
Footnote 153:
Elster and Geitel, Phys. Zeit. No. 15, p. 437, 1903.
Footnote 154:
Glew, Arch. Röntgen Ray, June 1904.
Footnote 155:
Becquerel, C. R. 137, Oct. 27, 1903.
Footnote 156:
Tommasina, C. R. 137, Nov. 9, 1903.
Footnote 157:
An interesting side-light is thrown on this question by the experiments described in Appendix A of this book.
Footnote 158:
Rutherford and Miss Brooks, Phil. Mag. July 1902.
Footnote 159:
In order to obtain a thin layer, the compound to be tested is ground to a fine powder and then sifted through a fine gauge uniformly over the area, so that the plate is only partially covered.
Footnote 160:
Rutherford, Phil. Mag. Jan. 1899.
Footnote 161:
Owens, Phil. Mag. Oct. 1899.
Footnote 162:
Rutherford and Miss Brooks, Phil. Mag. July, 1900.
Footnote 163:
Since the ionization at any point above the plate is the resultant effect of the α particles coming from all points of the large radio-active layer, λ is not the same as the coefficient of absorption of the rays from a point source. It will however be proportional to it. For this reason λ is called the “absorption constant.”
Footnote 164:
Townsend, Phil. Mag. Feb. 1901.
Footnote 165:
Durack, Phil. Mag. July 1902, May 1903.
Footnote 166:
Bragg and Bragg and Kleeman, Phil. Mag. Dec. 1904.
Footnote 167:
Villard, C. R. 130, pp. 1010, 1178, 1900.
Footnote 168:
Becquerel, C. R. 130, p. 1154, 1900.
Footnote 169:
Rutherford, Phys. Zeit. 3, p. 517, 1902.
Footnote 170:
McClelland, Phil. Mag. July 1904.
Footnote 171:
Paschen, Phys. Zeit. 5, No. 18, p. 563, 1904.
Footnote 172:
A. S. Eve, Phil. Mag. Nov. 1904.
Footnote 173:
Paschen, Annal. d. Physik, 14, p. 114, 1904; 14, 2, p. 389, 1904. Phys. Zeit. 5, No. 18, p. 563, 1904.
Footnote 174:
Paschen, Phys. Zeit. 5, No. 18, p. 563, 1904.
Footnote 175:
Rutherford and Barnes, Phil. Mag. May 1905. Nature, p. 151, Dec. 15, 1904.
Footnote 176:
Barkla, Nature, March 17, 1904.
Footnote 177:
Becquerel, C.R. 132, pp. 371, 734, 1286. 1901.
Footnote 178:
Mme Curie, Thèse présentée à la Faculté des Sciences, Paris 1903, p. 85.
Footnote 179:
A. S. Eve, Phil. Mag. Dec. 1904.
Footnote 180:
In a recent paper (Phil. Mag. Feb. 1905), McClelland has, in the main, confirmed the experimental results obtained by Eve. An electrometer was used instead of an electroscope. He finds, in addition, that the amount of secondary radiation depends on the angle of incidence of the primary rays, and is greatest for an angle of 45°. In a letter to Nature (Feb. 23, p. 390, 1905), he states that more recent experiments have shown that the amount of secondary radiation from different substances is a function of their atomic weights rather than of their densities. In every case examined, the amount of secondary radiation increases with the atomic weight, but is not proportional to it.
Footnote 181:
Rutherford and McClung, Phil. Trans. A. p. 25, 1901.
Footnote 182:
Meyer and Schweidler, Wien Ber. 113, July, 1904.
Footnote 183:
Rutherford and Grier, Phil. Mag. Sept. 1902.
V. PROPERTIES OF THE RADIATIONS
=115.= Besides their power of acting on a photographic plate, and of ionizing gases, the radiations from active bodies are able to produce marked chemical and physical actions in various substances. Most of these effects are due either to the α or β rays. The γ rays produce little effect in comparison. Since the β rays are similar in all respects to high velocity cathode rays, it is to be expected that they will produce effects similar in character to those produced by the cathode rays in a vacuum tube.
Phosphorescent action.
Becquerel[184] has studied the action of radium rays in producing phosphorescence in various bodies. The substance to be tested was placed above the radium in the form of powder on a very thin mica plate. Examination was made of the sulphides of calcium and strontium, ruby, diamond, varieties of spar, phosphorus and hexagonal blende. Substances like the ruby and spar, which phosphoresce under luminous rays, did not phosphoresce under the radium rays. On the other hand, those which were made luminous by ultra-violet light were also luminous under the action of radium rays. The radium rays show distinct differences from X rays. For example, a diamond which was very luminous with radium rays was unaffected by X rays. The double sulphate of uranium and potassium is more luminous than hexagonal blende under X rays, but the reverse is true for radium rays; under the influence of these rays, sulphide of calcium gave a blue luminosity but was hardly affected by X rays.
The following table shows the relative phosphorescence excited in various bodies.
Substance Without screen. Across screen Intensity of black paper
Hexagonal blende 13·36 ·04
Platino-cyanide of 1·99 ·05 barium
Diamond 1·14 ·01
Double sulphate of 1·00 ·31 Uranium and Potassium
Calcium Fluoride ·30 ·02
In the last column the intensity without the screen is in each case taken as unity. The great diminution of intensity after the rays have passed through black paper shows that most of the phosphorescence developed without the screen is, in the majority of cases, due to the α rays.
Bary[185] has made a very complete examination of the class of substances which become luminous under radium rays. He found that the great majority of substances belong to the alkali metals and alkaline earths. All these substances were also phosphorescent under the action of X rays.
Crystalline zinc sulphide (Sidot’s blende) phosphoresces very brightly under the influence of the rays from radium and other very active substances. This was observed by Curie and Debierne in their study of the radium emanation and the excited activity produced by it. It has also been largely used by Giesel as an optical means of detecting the presence of emanations from very active substances. It is an especially sensitive means of detecting the presence of α rays, when it exhibits the “scintillating” property already discussed in section 96. In order to show the luminosity due to the α rays, the screen should be held close to the active substance, as the rays are absorbed in their passage through a few centimetres of air. Zinc sulphide is also luminous under the action of the β rays, but the phosphorescence is far more persistent than when produced by the α rays.
Very beautiful luminous effects are produced by large crystals of the platinocyanides exposed to the radium rays. Those containing lithium give a brilliant pink colour. The calcium and barium salts fluoresce with a deep green light, and the sodium compound with a lemon yellow. The mineral willemite (zinc silicate) was recently found by Kunz to be an even more sensitive means of detecting the presence of the radiations than platinocyanide of barium. It fluoresces showing a beautiful greenish colour, and a piece of mineral exposed to the action of the rays appears quite translucent. The crystals of the platinocyanides of barium and lithium are especially suited for showing the action of the γ rays, and, in this respect, are superior to willemite.
A very striking effect is shown by the mineral kunzite—a new variety of spodumene discovered by Kunz[186]. This is a transparent gem like crystal, often of very large size, which glows with a beautiful reddish colour under the action of the β or γ rays, but does not appear to be sensitive to the α rays. The luminosity extends throughout the crystal, but is not so marked as in the platinocyanides or willemite. The mineral sparteite[187], a form of calcite containing a few per cent. of manganese, has been found by Ambrecht to fluoresce with a very deep orange light under the β and γ rays. The colour appears to depend on the intensity of the rays, and is deeper close to the radium than at some distance away.
If kunzite and sparteite are exposed to the action of the cathode rays in a vacuum tube, the colour is different from that produced by the radium rays. The former appears a deep yellow, instead of the deep red observed with the radium rays.
The different actions of the radium rays on these fluorescent substances can be illustrated very simply and beautifully by the following experiment. A small U tube is filled with fragments of the fluorescent substance arranged in layers. The U tube is immersed in liquid air and the emanation from about 30 mgrs. of radium bromide is condensed in the tube. On closing the tube and removing it from the liquid air, the emanation distributes itself uniformly in the tube. The shades of colour produced in the different substances are clearly seen.
It is observed that all the crystals increase in luminosity for several hours, on account of the excited activity produced by the emanation. This effect is especially observed in kunzite, which at first hardly responds to the rays, since the β and γ rays, which causes it to fluoresce, are not given out by the emanation itself but by one of its later products. The intensity of the β and γ rays is, in consequence, small at first but rises to a maximum after several hours; the luminosity observed varies in a corresponding manner.
Sir William Crookes[188] has made an examination of the effect of continued exposure of a diamond to the radium rays. An “off-colour” diamond, of a pale yellow colour, was placed inside a tube with radium bromide. After 78 days’ exposure, the diamond had darkened and become bluish green in tint; when heated at 50° in a mixture of potassium chlorate for ten days, the diamond lost its dull surface colour and was bright and transparent, and its tint had changed to a pale bluish green. The rays have thus a double action on the diamond; the less penetrating β rays produce a superficial darkening due to the change of the surface into graphite, while the more penetrating β rays and the γ rays produce a change of colour throughout its mass. The diamond phosphoresced brightly during the whole course of its exposure to the rays. Crookes also observed that the diamond still retained enough activity to affect a photographic plate 35 days after removal, although, during the period of 10 days, it was heated in a mixture sufficiently powerful to remove the outer skin of graphite. This residual activity may possibly be due to a slow transformation product of the emanation which is deposited on the surface of bodies (see chapter XI).
Marckwald observed that the α rays from radio-tellurium produced marked phosphorescence on some kinds of diamonds. An account of the various luminous effects produced on different gems by exposure to the radium and actinium rays has been given by Kunz and Baskerville[189].
Both zinc sulphide and platinocyanide of barium diminish in luminosity after exposure for some time to the action of the rays. To regenerate a screen of the latter, exposure to solar light is necessary. A similar phenomenon has been observed by Villard for a screen exposed to Röntgen rays. Giesel made a screen of platinocyanide of radio-active barium. The screen, very luminous at first, gradually turned brown in colour, and at the same time the crystals became dichroic. In this condition the luminosity was much less, although the active substance had increased in activity after preparation. Many of the substances which are luminous under the rays from active substances lose this property to a large extent at low temperatures[190].
=116. Luminosity of radium compounds.= All radium compounds are spontaneously luminous. This luminosity is especially brilliant in the dry haloid salts, and persists for long intervals of time. In damp air the salts lose a large amount of their luminosity, but they recover it on drying. With very active radium chloride, the Curies have observed that the light changes in colour and intensity with time. The original luminosity is recovered if the salt is dissolved and dried. Many inactive preparations of radiferous barium are strongly luminous. The writer has seen a preparation of impure radium bromide which gave out a light sufficient to read by in a dark room. The luminosity of radium persists over a wide range of temperature and is as bright at the temperature of liquid air as at ordinary temperatures. A slight luminosity is observed in a solution of radium, and if crystals are being formed in the solution, they can be clearly distinguished in the liquid by their greater luminosity.
=117. Spectrum of the phosphorescent light of radium and actinium.= Compounds of radium, with a large admixture of barium, are usually strongly self-luminous. This luminosity decreases with increasing purity, and pure radium bromide is only very feebly self-luminous. A spectroscopic examination of the slight phosphorescent light of pure radium bromide has been made by Sir William and Lady Huggins[191]. On viewing the light with a direct vision spectroscope, there were faint indications of a variation of luminosity at different points along the spectrum. In order to get a photograph of the spectrum within a reasonable time, they made use of a quartz spectroscope of special design which had been previously employed in a spectroscopic examination of faint celestial objects. After three days’ exposure with a slit of ¹⁄₄₅₀ of an inch in width, a negative was obtained which showed a number of bright lines. The magnified spectrum is shown in Fig. 46 A. The lines of this spectrum were found to agree not only in position but also in relative intensity with the band spectrum of nitrogen. The band spectrum of nitrogen and also the spark spectrum[192] of radium are shown in the same figure.
Some time afterwards Sir William Crookes and Prof. Dewar showed that this spectrum of nitrogen was not obtained if the radium was contained in a highly exhausted tube. Thus it appears that the spectrum is due to the action of the radium rays either on occluded nitrogen or the nitrogen in the atmosphere surrounding the radium.
It is very remarkable that a phosphorescent light, like that of radium bromide, should show a bright line spectrum of nitrogen. It shows that radium at ordinary temperatures is able to set up radiations which are produced only by the electric discharge under special conditions.
Sir William and Lady Huggins were led to examine the spectrum of the natural phosphorescent light of radium with the hope that some indications might be obtained thereby of the processes occurring in the radium atom. Since the main radiation from radium consists of positively charged atoms projected with great velocity, radiations must be set up both in the expelled body and in the system from which it escapes.
Giesel[193] observed that the spectrum of the phosphorescent light of actinium consists of three bright lines. Measurements of the wave length were made by Hartmann[194]. The luminosity was very slight and a long exposure was required. The lines observed were in the red, blue and green. The wave length λ and velocity are shown below.
Line Intensity λ
1 10 4885·4 ± 0·1 Ångström units
2 6 5300 ± 6 „
3 1 5909 ± 10 „
The line 4885 was very broad; the other two lines were so feeble that it was difficult to determine their wave length with accuracy. Hartmann suggests that these lines may be found in the spectrum of the new stars. The lines observed have no connection with radium or its emanation[195].
=118. Thermo-luminescence.= E. Wiedemann and Schmidt[196] have shown that certain bodies after exposure to the cathode rays or the electric spark become luminous when they are heated to a temperature much below that required to cause incandescence. This property of thermo-luminescence is most strikingly exhibited in certain cases where two salts, one of which is much in excess of the other, are precipitated together. It is to be expected that such bodies would also acquire the property when exposed to the β or cathodic rays of radium. This has been found to be the case by Wiedemann[197]. Becquerel showed that fluor-spar, exposed to the radium rays, was luminous when heated. The glass tubes in which radium is kept are rapidly blackened. On heating the tube, a strong luminosity is observed, and the coloration to a large extent disappears. The peculiarity of many of these bodies lies in the fact that the property of becoming luminous when heated is retained for a long interval of time after the body is removed from the influence of the exciting cause. It appears probable that the rays cause chemical changes in these bodies, which are permanent until heat is applied. A portion of the chemical energy is then released in the form of visible light.
Physical actions.
=119. Some electric effects.= Radium rays have the same effect as ultra-violet light and Röntgen rays in increasing the facility with which a spark passes between electrodes. Elster and Geitel[198] showed that if two electrodes were separated by a distance such that the spark just refused to pass, on bringing near a specimen of radium the spark at once passes. This effect is best shown with short sparks from a small induction coil. The Curies have observed that radium completely enveloped by a lead screen 1 cm. thick produces a similar action. The effect in that case is due to the γ rays alone. This action of the rays can be very simply illustrated by connecting two spark-gaps with the induction coil in parallel. The spark-gap of one circuit is adjusted so that the discharge just refuses to pass across it, but passes by the other. When some radium is brought near the silent spark-gap, the spark at once passes and ceases in the other[199].
Hemptinne[200] found that the electrodeless discharge in a vacuum tube began at a higher pressure when a strong preparation of radium was brought near the tube. In one experiment the discharge without the rays began at 51 mms. but with the radium rays at 68 mms. The colour of the discharge was also altered.
Himstedt[201] found that the resistance of selenium was diminished by the action of radium rays in the same way as by ordinary light.
F. Henning[202] examined the electrical resistance of a barium chloride solution containing radium of activity 1000, but could observe no appreciable difference between it and a similar pure solution of barium chloride. This experiment shows that the action of the rays from the radium does not produce any appreciable change in the conductivity of the barium solution.
Kohlrausch and Henning[203] have recently made a detailed examination of the conductivity of pure radium bromide solutions, and have obtained results very similar to those for the corresponding barium solutions. Kohlrausch[204] found that the conductivity of water exposed to the radiations from radium increased more rapidly than water which had not been exposed. This increase of conductivity may have been due to an increase of the conductivity of the water itself, or to an increased rate of solution of the glass of the containing vessel.
Specimens of strongly active material have been employed to obtain the potential at any point of the atmosphere. The ionization due to the active substance is so intense that the body to which it is attached rapidly takes up the potential of the air surrounding the active substance. In this respect it is more convenient and rapid in its action than the ordinary taper or water dropper, but on account of the disturbance of the electric field by the strong ionization produced, it is probably not so accurate a method as that of the water dropper.
=120. Effect on liquid and solid dielectrics.= P. Curie[205] made the very important observation that liquid dielectrics became partial conductors under the influence of radium rays. In these experiments the radium, contained in a glass tube, was placed in an inner thin cylinder of copper. This was surrounded by a concentric copper cylinder, and the liquid to be examined filled the space between. A strong electric field was applied, and the current through the liquid measured by means of an electrometer.
The following numbers illustrate the results obtained:
Substance Conductivity in megohms per 1 cm.³
Carbon bisulphide 20 × 10⁻¹⁴
Petroleum ether 15 „
Amyline 14 „
Carbon chloride 8 „
Benzene 4 „
Liquid air 1·3 „
Vaseline oil 1·6 „
Liquid air, vaseline oil, petroleum ether, amyline, are normally nearly perfect insulators. The conductivity of amyline and petroleum ether due to the rays at −17° C. was only ⅒ of its value at 0° C. There is thus a marked action of temperature on the conductivity. For very active material the current was proportional to the voltage. With material of only ¹⁄₅₀₀ of the activity, it was found that Ohm’s law was not obeyed.
The following numbers were obtained:
Volts Current 50 109 100 185 200 255 400 335
For an increase of voltage of 8 times, the current only increases about 3 times. The current in the liquid thus tends to become “saturated” as does the ordinary ionization current through a gas. These results have an important bearing on the ionization theory, and show that the radiation probably produces ions in the liquid as well as in the gas. It was also found that X rays increased the conductivity to about the same extent as the radium rays.
Becquerel[206] has recently shown that solid paraffin exposed to the β and γ rays of radium acquires the property of conducting electricity to a slight extent. After removal of the radium the conductivity diminishes with time according to the same law as for an ionized gas. These results show that a solid as well as a liquid and gaseous dielectric is ionized under the influence of radium rays.
=121. Effect of temperature on the radiations.= Becquerel[207], by the electric method, determined the activity of uranium at the temperature of liquid air, and found that it did not differ more than 1 per cent. from the activity at ordinary temperatures. In his experiments, the α rays from the uranium were absorbed before reaching the testing vessel, and the electric current measured was due to the β rays alone. P. Curie[208] found that the luminosity of radium and its power of exciting fluorescence in bodies were retained at the temperature of liquid air. Observations by the electric method showed that the activity of radium was unaltered at the temperature of liquid air. If a radium compound is heated in an open vessel, it is found that the activity, measured by the α rays, falls to about 25 per cent. of its original value. This is however not due to a change in the radio-activity, but to the release of the radio-active emanation, which is stored in the radium. No alteration is observed if the radium is heated in a closed vessel from which none of the radio-active products are able to escape.
=122. Motion of radium in an electric field.= Joly[209] found that a disc, one side of which is coated with a few milligrams of radium bromide, exhibits, when an electrified body is brought near it, motions very different to those observed in the case of an inactive substance. The electrified body, whether positive or negative, repels the suspended body if brought up to it on the side coated with radium, but attracts it if presented to the naked side.
This effect is very simply shown by constructing a small apparatus like a radiometer. Two covered glasses are attached to the end of a glass fibre about 6 cms. long, the surfaces lying in the same plane. The apparatus is free to rotate on a pivot. The two vanes are coated on alternate faces with radium bromide, and the whole apparatus contained within a glass receiver. If an electrified rod of ebonite or sealing wax is brought up close to the receiver, a rotation is communicated to the vane which increases as the pressure of the air is lowered to 5 or 6 cms. of mercury. By placing the apparatus between parallel plates connected with the terminals of a Wimshurst machine, a steady rotation is communicated to the vanes. The rotation is always in such a direction that the radium coated surface is repelled from the electrified body.
This action was examined still further by attaching the vanes to the glass beam of a Coulomb’s balance. A metal sphere, which could be charged from without, was fixed facing the side coated with radium. A repulsion was always observed except when the charge was very strong and the vane near the sphere. If, however, the two vanes were connected by a light wire and a similar sphere placed exactly opposite the other, an attraction was observed if one sphere was charged, but a repulsion if both were charged. These effects were observed whether the vanes were of aluminium or glass.
Joly found that the effect could not be explained by any direct action due to the movement of the ions in an electric field. The recoil, due to the expulsion of α particles from one side of the vane, is far too small to account for the movement observed.
This effect can, I think, be simply accounted for by taking into consideration the difference in conductivity of the gas on the two sides of the radium coated vane. If a small vane, coated uniformly with radium on both sides, and mounted on an insulating support, be brought near a charged body kept at a constant potential, it acts like a water dropper and rapidly acquires very nearly the average potential which existed at that point before the vane was brought up. The mechanical force acting on the vane will, in consequence, be small. If, however, the vane is only coated with radium on the side near the charged body, the ionization and consequently the conductivity of the gas is much greater between the vane and the charged body than on the opposite side. Suppose, for simplicity, the body is charged to a positive potential. On account of the greater conductivity of the gas on the side facing the charged body, it will rapidly acquire a positive charge, and the potential of the vane will reach a higher value than existed at that place before the vane was introduced. This will result in a repulsion of the vane. This also accounts for the attraction observed in the experiment with the Coulomb’s balance already referred to. Suppose that one sphere is positively charged and the other earthed, and the two vanes metallically connected together. The vane next to the charged body will become charged positively, but this charge will be dissipated rapidly on account of the ionization of the gas close to the opposite vane, and, in most conditions, this loss of charge will be so rapid that the potential of the vane is unable to reach the value which would exist at that place in the field, if the vane were removed. There will, in consequence, be an attracting force acting on the vane towards the sphere.
The repulsion observed by Joly is thus only an indirect result of the ionization in the gas produced by the radium, and should be shown under conditions where similar unequal distribution of ionization is produced by any other sources.
Since radium gives out heat at a fairly rapid rate, a radiometer in which the vanes were coated on one side with radium instead of lampblack, should rotate at low pressure of the gas, even if no source of light is brought near it. This should evidently be the case, since the face coated with radium should reach a slightly higher temperature than the other. This experiment has been tried, but the effect seems too small to produce rotation of the vanes.
Chemical actions.
=123.= Rays from active radium preparations change oxygen into ozone[210]. Its presence can be detected by the smell or by the action on iodide of potassium paper. This effect is due to the α and β rays from the radium, and not to the luminous rays from it. Since energy is required to produce ozone from oxygen, this must be derived from the energy of the radiations.
The Curies found that radium compounds rapidly produced coloration in glass. For moderately active material the colour is violet, for more active material it is yellow. Long continued action blackens the glass, although the glass may have no lead in its composition. This coloration gradually extends through the glass, and is dependent to some extent on the kind of glass used.
Giesel[211] found that he could obtain as much coloration in rock-salt and fluor-spar by radium rays, as by exposure to the action of cathode rays in a vacuum tube. The coloration, however, extended much deeper than that produced by the cathode rays. This is to be expected, since the radium rays have a higher velocity, and consequently greater penetrating power, than the cathode rays produced in an ordinary vacuum tube. Goldstein observed that the coloration is far more intense and rapid when the salts are melted or heated to a red heat. Melted potassium sulphate, under the action of a very active preparation of radium, was rapidly coloured a strong greenish blue which gradually changed into a dark green. Salomonsen and Dreyer[212] found that plates of quartz were coloured by exposure to radium rays. When examined minutely, plates cut perpendicular to the optic axis showed the presence of lines and striae, parallel to the binary axes. Adjacent portions of the striated system differed considerably in intensity of coloration and clearly revealed the heterogeneity of structures of the crystal.
The cause of these colorations by cathode and radium rays has been the subject of much discussion. Elster and Geitel[213] observed that a specimen of potassium sulphate, coloured green by radium rays, showed a strong photo-electric action, i.e. it rapidly lost a negative charge of electricity when exposed to the action of ultra-violet light. All substances coloured by cathode rays show a strong photo-electric action, and, since the metals sodium and potassium themselves show photo-electric action to a very remarkable degree, Elster and Geitel have suggested that the colorations are caused by a solid solution of the metal in the salt.
Although the coloration due to radium rays extends deeper than that due to the cathode rays, when exposed to light the colour fades away at about the same rate in the two cases.
Becquerel[214] found that white phosphorus is changed into the red variety by the action of radium rays. This action was shown to be due mainly to the β rays. The secondary radiation set up by the primary rays also produced a marked effect. Radium rays, like ordinary light rays, also caused a precipitate of calomel in the presence of oxalic acid.
Hardy and Miss Willcock[215] found that a solution of iodoform in chloroform turned purple after exposure for 5 minutes to the rays from 5 milligrams of radium bromide. This action is due to the liberation of iodine. By testing the effect of screens of different thicknesses, over the radium, this action was found to be mainly due to the β rays from the radium. Röntgen rays produce a similar coloration.
Hardy[216] also observed an action of the radium rays on the coagulation of globulin. Two solutions of globulin from ox serum were used, one made electro-positive by adding acetic acid, and the other electro-negative by adding ammonia. When the globulin was exposed close to the radium in naked drops, the opalescence of the electro-positive solution rapidly diminished, showing that the solution became more complete. The electro-negative solution was rapidly turned to a jelly and became opaque. These actions were found to be due to the α rays of radium alone.
This is further evidence in favour of the view that the α rays consist of projected positively charged bodies of atomic dimensions, for a similar coagulation effect is produced by the metallic ions of liquid electrolytes, and has been shown by W. C. D. Whetham[217] to be due to the electric charges carried by the ions.
=124. Gases evolved from radium.= Curie and Debierne[218] observed that radium preparations placed in a vacuum tube continually lowered the vacuum. The gas evolved was always accompanied by the emanation, but no new lines were observed in its spectrum. Giesel[219] has observed a similar evolution of gas from solutions of radium bromide. Giesel forwarded some active material to Runge and Bödlander, in order that they might test the gas spectroscopically. From 1 gram of a 5 per cent. radium preparation they obtained 3·5 c.c. of gas in 16 days. This gas was found, however, to be mainly hydrogen, with 12 per cent. of oxygen. In later experiments Ramsay and Soddy[220] found that 50 milligrams of radium bromide evolved gases at the rate of about 0·5 c.c. per day. This is a rate of evolution about twice that observed by Runge and Bödlander. On analysing the gases about 28·9 per cent. consisted of oxygen, and the rest hydrogen. The slight excess of hydrogen over that attained in the decomposition of water, they consider to be due to the action of oxygen on the grease of the stop-cocks. The radio-active emanation from radium has a strong oxidizing action and rapidly produces carbon dioxide, if carbonaceous matter is present. The production of gas is probably due to the action of the radiations in decomposing water. The amount of energy required to produce the rate of decomposition observed by Ramsay and Soddy—about 10 c.c. per day for 1 gram of radium bromide—corresponds to about 30 gram-calories per day. This amount of energy is about two per cent. of the total energy emitted in the form of heat.
Ramsay and Soddy (loc. cit.) have also observed the presence of helium in the gases evolved by solution of radium bromide. This important result is considered in detail in section 267.
Physiological actions.
=125.= Walkhoff first observed that radium rays produce burns of much the same character as those caused by Röntgen rays. Experiments in this direction have been made by Giesel, Curie and Becquerel, and others, with very similar results. There is at first a painful irritation, then inflammation sets in, which lasts from 10 to 20 days. This effect is produced by all preparations of radium, and appears to be due mainly to the α and β rays.
Care has to be taken in handling radium on account of the painful inflammation set up by the rays. If a finger is held for some minutes at the base of a capsule containing a radium preparation, the skin becomes inflamed for about 15 days and then peels off. The painful feeling does not disappear for two months.
Danysz[221] found that this action is mainly confined to the skin, and does not extend to the underlying tissue. Caterpillars subjected to the action of the rays lost their power of motion in several days and finally died.
Radium rays have been found beneficial in certain cases of cancer. The effect is apparently similar to that produced by Röntgen rays, but the use of radium possesses the great advantage that the radiating source can be enclosed in a fine tube and introduced at the particular point at which the action of the rays is required. The rays have also been found to hinder or stop the development of microbes[222].
It would be out of place here to give an account of the numerous experiments that have been made by physicists and physiologists on the action of the rays of radium and of other radio-active substances on different organisms, such as caterpillars, mice and guinea-pigs. In some cases, the experiments have been carried out by placing the organisms in an atmosphere impregnated with the radium emanation. The effect of an exposure under such conditions for several days or weeks has been found generally harmful and in many cases fatal. The literature in this new department of study is already large and is increasing rapidly.
Another interesting action of the radium rays has been observed by Giesel. On bringing up a radium preparation to the closed eye, in a dark room, a sensation of diffuse light is observed. This effect has been examined by Himstedt and Nagel[223] who have shown that it is due to a fluorescence produced by the rays in the eye itself. The blind are able to perceive this luminosity if the retina is intact, but not if the retina is diseased. Hardy and Anderson[224] have examined this effect in some detail. The sensation of light is produced both by the β and γ rays. The eyelid practically absorbs all the β rays, so that the luminosity observed with a closed eye is due to the γ rays alone. The lens and retina of the eye are strongly phosphorescent under the action of the β and γ rays. Hardy and Anderson consider that the luminosity observed in a dark room with the open eye (the phosphorescent light of the radium itself being stopped by black paper) is to a large extent due to the phosphorescence set up in the eyeball. The γ rays, for the most part, produce the sensation of light when they strike the retina.
Tommasina stated that the air exhaled by man contained a larger proportion of ions than ordinary air, and, in consequence, caused an increased rate of discharge of an electroscope. The experiment was repeated by Elster and Geitel but with negative results. On the other hand, they found that the breath of Dr Giesel, of Braunschweig, who had been engaged continuously in the chemical separation of the radio-active bodies, caused a rapid loss of charge of an electroscope. This increased rate of discharge was probably mainly due to the radium emanation, with which his system had become impregnated by inhaling the emanation-laden air of the laboratory.
Footnote 184:
Becquerel, C. R. 129, p. 912, 1899.
Footnote 185:
Bary, C. R. 130, p. 776, 1900.
Footnote 186:
Kunz and Baskerville, Amer. Journ. Science XVI. p. 335, 1903.
Footnote 187:
See Nature, p. 523, March 31, 1904.
Footnote 188:
Crookes, Proc. Roy. Soc. 74, p. 47, 1904.
Footnote 189:
Kunz and Baskerville, Science XVIII, p. 769, Dec. 18, 1903.
Footnote 190:
Beilby in a recent communication to the Royal Society (Feb. 9 and 23, 1905) has examined in some detail the production of phosphorescence by the β and γ rays of radium and has put forward a theory to account for the different actions observed.
Footnote 191:
Huggins, Proc. Roy. Soc. 72, pp. 196 and 409, 1903.
Footnote 192:
The spark spectrum of the radium bromide showed the H and K lines of calcium and also faintly some of the strong lines of barium. The characteristic lines of radium of wave-lengths 3814·59, 3649·7, 4340·6 and 2708·6, as shown by Demarçay and others are clearly shown in the figure. The strong line of wave-length about 2814 is due to radium.
Footnote 193:
Giesel, Ber. d. D. Chem. Ges. 37, p. 1696, 1904.
Footnote 194:
Hartmann, Phys. Zeit. 5, No. 18, p. 570, 1904.
Footnote 195:
In a recent paper, Giesel (Ber. d. D. Chem. Ges. No. 3, p. 775, 1905) has shown that the bright lines are due to didymium, which is present as an impurity. Exposure of didymium to the radium rays also causes the appearance of the lines.
Footnote 196:
Wiedemann and Schmidt, Wied. Annal. 59, p. 604, 1895.
Footnote 197:
Wiedemann, Phys. Zeit. 2, p. 269, 1901.
Footnote 198:
Elster and Geitel, Annal. d. Phys. 69, p. 673, 1899.
Footnote 199:
Willows and Peck (Phil. Mag. March, 1905) found that under some conditions, especially for long sparks, the rays of radium hindered the passage of the spark.
Footnote 200:
Hemptinne, C. R. 133, p. 934, 1901.
Footnote 201:
Himstedt, Phys. Zeit. p. 476, 1900.
Footnote 202:
Henning, Annal. d. Phys. p. 562, 1902.
Footnote 203:
Kohlrausch and Henning, Verh. Deutsch. Phys. Ges. 6, p. 144, 1904.
Footnote 204:
Kohlrausch, Verh. Deutsch. Phys. Ges. 5, p. 261, 1904.
Footnote 205:
P. Curie, C. R. 134, p. 420, 1902.
Footnote 206:
Becquerel, C. R. 136, p. 1173, 1903.
Footnote 207:
Becquerel, C. R. 133, p. 199, 1901.
Footnote 208:
P. Curie, Société de Physique, March 2, 1900.
Footnote 209:
Joly, Phil. Mag. March, 1904.
Footnote 210:
S. and P. Curie, C. R. 129, p. 823, 1899.
Footnote 211:
Giesel, Verhandlg. d. D. Phys. Ges. Jan. 5, 1900.
Footnote 212:
Salomonsen and Dreyer, C. R. 139, p. 533, 1904.
Footnote 213:
Elster and Geitel, Phys. Zeit. p. 113, No. 3, 1902.
Footnote 214:
Becquerel, C. R. 133, p. 709, 1901.
Footnote 215:
Hardy and Miss Willcock, Proc. Roy. Soc. 72, p. 200, 1903.
Footnote 216:
Hardy, Proc. Physiolog. Soc. May 16, 1903.
Footnote 217:
Whetham, Phil. Mag. Nov. 1899; Theory of Solution, Camb. 1902, p. 396.
Footnote 218:
Curie and Debierne, C. R. 132, p. 768, 1901.
Footnote 219:
Giesel, Ber. D. d. Chem. Ges. 35, p. 3605, 1902.
Footnote 220:
Ramsay and Soddy, Proc. Roy. Soc. 72, p. 204, 1903.
Footnote 221:
Danysz, C. R. 136, p. 461, 1903.
Footnote 222:
Aschkinass and Caspari, Arch. d. Ges. Physiologie, 86, p. 603, 1901.
Footnote 223:
Himstedt and Nagel, Drude’s Annal. 4, p. 537, 1901.
Footnote 224:
Hardy and Anderson, Proc. Roy. Soc. 72, p. 393, 1903.
VI. CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER
=126.= An account will now be given of some experiments which have thrown much light, not only on the nature of the processes which serve to maintain the radio-activity of the radio-active bodies, but also on the source of the energy continuously emitted by those bodies. In this chapter, for simplicity, the radio-activity of uranium and thorium will alone be considered, for it will be seen later that the changes taking place in these two substances are typical of those which occur in all radio-active substances.
We have seen (section 23) that there is some doubt whether the radio-activity of thorium is due to that element itself, or to an unknown radio-active constituent associated with it. This uncertainty, however, will present no serious difficulty when we are discussing the radio-activity of thorium, for the general conclusions are, for the most part, independent of whether thorium is the primary radio-active constituent or not. For simplicity, however, it will be assumed for the present that the radio-activity is due to thorium itself. If future research should definitely show that the radio-activity, ordinarily observed in thorium, is due to a new radio-active element mixed with it, the radio-active processes considered will refer to this new element.
=127. Uranium X.= The experiments of Mme Curie show that the radio-activity of uranium and radium is an atomic phenomenon. The activity of any uranium compound depends only on the amount of that element present, and is unaffected by its chemical combination with other substances, and is not appreciably affected by wide variations of temperature. It would thus seem probable, since the activity of uranium is a specific property of the element, that the activity could not be separated from it by chemical agencies.
In 1900, however, Sir William Crookes[225] showed that, by a single chemical operation, uranium could be obtained photographically inactive while the whole of the activity could be concentrated in a small residue free from uranium. This residue, to which he gave the name of Ur X, was many hundred times more active photographically, weight for weight, than the uranium from which it had been separated. The method employed for this separation was to precipitate a solution of the uranium with ammonium carbonate. On dissolving the precipitate in an excess of the reagent, a light precipitate remained behind. This was filtered, and constituted the Ur X. The active substance Ur X was probably present in very small quantity, mixed with impurities derived from the uranium. No new lines were observed in its spectrum. A partial separation of the activity of uranium was also effected by another method. Crystallized uranium nitrate was dissolved in ether, when it was found that the uranium divided itself between the ether and water present in two unequal fractions. The small part dissolved in the water layer was found to contain practically all the activity when examined by the photographic method, while the other fraction was almost inactive. These results, taken by themselves, pointed very strongly to the conclusion that the activity of uranium was not due to the element itself, but to some other substance, associated with it, which had distinct chemical properties.
Results of a similar character were observed by Becquerel[226]. It was found that barium could be made photographically very active by adding barium chloride to the uranium solution and precipitating the barium as sulphate. By a succession of precipitations the uranium was rendered photographically almost inactive, while the barium was strongly active.
The inactive uranium and the active barium were laid aside; but, on examining them a year later, it was found that the uranium had completely regained its activity, while that of the barium had completely disappeared. The loss of activity of uranium was thus only temporary in character.
In the above experiments, the activity of uranium was examined by the photographic method. The photographic action produced by uranium is due almost entirely to the β rays. The α rays, in comparison, have little if any effect. Now the radiation from Ur X consists entirely of β rays, and is consequently photographically very active. If the activity of uranium had been measured electrically without any screen over it, the current observed would have been due very largely to the α rays, and little change would have been observed after the removal of Ur X, since only the constituent responsible for the β rays was removed. This important point is discussed in more detail in section 205.
=128. Thorium X.= Rutherford and Soddy[227], working with thorium compounds, found that an intensely active constituent could be separated from thorium by a single chemical operation. If ammonia is added to a thorium solution, the thorium is precipitated, but a large amount of the activity is left behind in the filtrate, which is chemically free from thorium. This filtrate was evaporated to dryness, and the ammonium salts driven off by ignition. A small residue was obtained which, weight for weight, was in some cases several thousand times more active than the thorium from which it was obtained, while the activity of the precipitated thorium was reduced to less than one half of its original value. This active constituent was named Th X from analogy to Crookes’ Ur X.
The active residue was found to consist mainly of impurities from the thorium; the Th X could not be examined chemically, and probably was present only in minute quantity. It was also found that an active constituent could be partly separated from thorium oxide by shaking it with water for some time. On filtering the water, and evaporating down, a very active residue was obtained which was analogous in all respects to Th X.
On examining the products a month later, it was found that the Th X was no longer active, while the thorium had completely regained its activity. A long series of measurements was then undertaken to examine the time-rate of these processes of decay and recovery of activity.
The results are shown graphically in Fig. 47, where the final activity of the thorium and the initial activity of the Th X are in each case taken as 100. The ordinates represent the activities determined by means of the ionization current, and the abscissae represent the time in days. It will be observed that both curves are irregular for the first two days. The activity of the Th X increased at first, while the activity of the thorium diminished. Disregarding these initial irregularities of the curves, which will be explained in detail in section 208, it will be seen that, after the first two days, the time taken for the thorium to recover half its lost activity is about equal to the time taken by the Th X to lose half its activity. This time in each case is about four days. The percentage proportion of the activity regained by the thorium, over any given interval, is approximately equal to the percentage proportion of the activity lost by the Th X during the same interval.
If the recovery curve is produced backwards to meet the vertical axis, it does so at a minimum of 25 per cent., and the above conclusions hold more accurately, if the recovery is assumed to start from this minimum. This is clearly shown by Fig. 48, where the percentages of activity recovered, reckoned from the 25 per cent. minimum, are plotted as ordinates. In the same figure the decay curve, after the second day, is shown on the same scale. The activity of the Th X decays with the time according to an exponential law, falling to half value in about four days. If I₀ is the initial activity and I{t} is the activity after a time t, then
$$ \frac {I₀} {I_t} = e^{–λt} $$, where λ is a constant and e the natural base of logarithms. The experimental curve of the rise of activity from a minimum to a maximum value is therefore expressed by the equation
$$ \frac {I_t} {I₀} = 1 − e^{–λt} $$,
where I₀ is the amount of activity recovered when the state of constant activity is reached, I{t}_ the activity recovered after a time t, and λ is the same constant as before.
=129. Uranium X.= Similar results were obtained when uranium was examined. The Ur X was separated by Becquerel’s method of successive precipitations with barium. The decay of the separated activity and the recovery of the lost activity are shown graphically in Fig. 49. A more detailed discussion of this experiment is given in section 205.
The curves of decay and recovery exhibit the same peculiarities and can be expressed by the same equations as in the case of thorium. The time-rate of decay and recovery is, however, much slower than for thorium, the activity of the Ur X falling to half its value in about 22 days.
A large number of results of a similar character have been obtained from other radio-active products, separated from the radio-elements, but the cases of thorium and uranium will suffice for the present to form a basis for the discussion of the processes that are taking place in radio-active bodies.
=130. Theory of the phenomena.= These processes of decay and recovery go on at exactly the same rate if the substances are removed from the neighbourhood of one another, or enclosed in lead, or placed in a vacuum tube. It is at first sight a remarkable phenomenon that the processes of decay and recovery should be so intimately connected, although there is no possibility of mutual interaction between them. These results, however, receive a complete explanation on the following hypotheses:
(1) That there is a constant rate of production of fresh radio-active matter by the radio-active body;
(2) That the activity of the matter so formed decreases according to an exponential law with the time from the moment of its formation.
Suppose that q₀ particles of new matter are produced per second from a given mass of matter. The rate of emission of energy due to the particles produced in the time dt, is, at the moment of their formation, equal to _Kq₀__dt_, where K is a constant.
It is required to find the activity due to the whole matter produced after the process has continued for a time T.
The activity dI, due to the matter produced during the time dt at the time t, decays according to an exponential law during the time T − t that elapses before its activity is estimated, and in consequence is given by
$$ dI = Kq₀e^{–λ (T-t)} dt $$,
where λ is the constant of decay of activity of the active matter. The activity I{T}_ due to the whole matter produced in the time T is thus given by
$$ I_t = \int₀^T Kq₀e^{–λ (T-t)} dt $$
$$ = \frac {Kq₀} {λ} (1 − e^{–λ T}) $$ .
The activity reaches a maximum value I₀ when T is very great, and is then given by
Kq₀ I₀ = ----- λ
thus
$$ \frac {I_T} {I₀} = 1 − e^{–λ T} $$ .
This equation agrees with the experimental results for the recovery of lost activity. Another method for obtaining this equation is given later in section 133.
A state of equilibrium is reached when the rate of loss of activity of the matter already produced is balanced by the activity supplied by the production of new active matter. According to this view, the radio-active bodies are undergoing change, but the activity remains constant owing to the action of two opposing processes. Now, if this active matter can at any time be separated from the substance in which it is produced, the decay of its activity, as a whole, should follow an exponential law with the time, since each portion of the matter decreases in activity according to an exponential law with the time, whatever its age may be. If I₀ is the initial activity of the separated product, the activity I{t}_ after an interval t is given by
$$ \frac {I_T} {I₀} = e^{–λt} $$ .
Thus, the two assumptions—of uniform production of active matter and of the decay of its activity in an exponential law from the moment of its formation—satisfactorily explain the relation between the curves of decay and recovery of activity.
=131. Experimental evidence.= It now remains to consider further experimental evidence in support of these hypotheses. The primary conception is that the radio-active bodies are able to produce from themselves matter of chemical properties different from those of the parent substance, and that this process goes on at a constant rate. This new matter initially possesses the property of activity, and loses it according to a definite law. The fact that a proportion of the activity of radium and thorium can be concentrated in small amounts of active matter like Th X or Ur X does not, of itself, prove directly that a material constituent responsible for the activity has been chemically separated. For example, in the case of the separation of Th X from thorium, it might be supposed that the non-thorium part of the solution is rendered temporarily active by its association with thorium, and that this property is retained through the processes of precipitation, evaporation, and ignition, and finally manifests itself in the residue remaining. According to this view it is to be expected that any precipitate capable of removing the thorium completely from its solution should yield active residues similar to those obtained from ammonia. No such case has, however, been observed. For example, when thorium nitrate is precipitated by sodium or ammonium carbonate, the residue from the filtrate after evaporation and ignition is free from activity and the thorium carbonate obtained has the normal amount of activity. In fact, ammonia is the only reagent yet found capable of completely separating Th X from thorium. A partial separation of the Th X can be made by shaking thorium oxide with water owing to the greater solubility of Th X in water.
Thorium and uranium behave quite differently with regard to the action of ammonia and ammonium carbonate. Ur X is completely precipitated with the uranium in an ammonia solution and the filtrate is inactive. Ur X is separated by ammonium carbonate, while Th X under the same conditions is completely precipitated with the thorium. The Ur X and the Th X thus behave like distinct types of matter with well-marked chemical properties quite distinct from those of the substances in which they are produced. The removal of Ur X by the precipitation of barium is probably not directly connected with the chemical properties of Ur X. The separation is probably due to the dragging down of the Ur X with the dense barium precipitate. Sir William Crookes found that the Ur X was dragged down by precipitates when no question of insolubility was involved, and such a result is to be expected if the Ur X exists in extremely minute quantity. It must be borne in mind that the actual amount of the active constituents Th X and Ur X, separated from thorium and uranium, is probably infinitesimal, and that the greater proportion of the residues is due to impurities present in the salt and the reagents, a very small amount of active matter being mixed with them.
=132. Rate of production of Th X.= If the recovery of the activity of uranium or thorium is due to the continuous production of new active matter, it should be possible to obtain experimental evidence of the process. As the case of thorium has been most fully investigated, a brief account will be given of some experiments made by Rutherford and Soddy[228] to show that Th X is produced continuously at a constant rate. Preliminary experiments showed that three successive precipitations were sufficient to remove the Th X almost completely from the thorium. The general method employed was to precipitate a solution of 5 grams of thorium-nitrate with ammonia. The precipitate was then redissolved in nitric acid and the thorium again precipitated as before, as rapidly as possible, so that the Th X produced in the time between successive precipitations should not appreciably affect the results. The removal of the Th X was followed by measurements of the activity of the residues obtained from successive filtrates. In three successive precipitations the activities of the residues were proportional to 100, 8, 1·6 respectively. Thus two precipitations are nearly sufficient to free the thorium from Th X.
The thorium freed from Th X was then allowed to stand for a definite time, and the amount of Th X formed during that time found by precipitating it, and measuring its radio-activity. According to the theory, the activity I{t}_ of the thorium formed in the time t is given by
$$ \frac {I_t} {I₀} = 1 − e^{–λt} $$,
where I₀ is the total activity of Th X, when there is radio-active equilibrium.
If λ_t_ is small,
I{t}_ ------ = λ_t_. I₀
Since the activity of Th X falls to half value in 4 days, the value of λ expressed in hours = ·0072. After standing a period of 1 hour about ¹⁄₁₄₀, after 1 day ⅙, after 4 days ½ of the maximum should be obtained. The experimental results obtained showed an agreement, as good as could be expected, with the equation expressing the result that the Th X was being produced at a constant rate.
The thorium-nitrate which had been freed from Th X was allowed to stand for one month, and then it was again subjected to the same process. The activity of the Th X was found to be the same as that obtained from an equal amount of the original thorium-nitrate. In one month, therefore, the Th X had been regenerated, and had reached a maximum value. By leaving the thorium time to recover fully its activity, this process can be repeated indefinitely, and equal amounts of Th X are obtained at each precipitation. Ordinary commercial thorium-nitrate and the purest nitrate obtainable showed exactly the same action, and equal amounts of Th X could be obtained from equal weights. These processes thus appear to be independent of the chemical purity of the substance[229].
The process of the production of Th X is continuous, and no alteration has been observed in the amount produced in the given time after repeated separations. After 23 precipitations extending over 9 days, the amount produced in a given interval was about the same as at the beginning of the process.
These results are all in agreement with the view that the Th X is being continuously produced from the thorium compound at a constant rate. The amount of active matter produced from 1 gram of thorium is probably extremely minute, but the electrical effects due to its activity are so large that the process of production can be followed after extremely short intervals. With a sensitive electrometer the amount of Th X produced per minute in 10 grams of thorium-nitrate gives a rapid movement to the electrometer needle. For larger intervals it is necessary to add additional capacity to the system to bring the effects within range of the instrument.
=133. Rate of decay of activity.= It has been shown that the activity of Ur X and Th X decays according to an exponential law with the time. This, we shall see later, is the general law of decay of activity in any type of active matter, obtained by itself, and freed from any secondary active products which it may, itself, produce. In any case, when this law is not fulfilled, it can be shown that the activity is due to the superposition of two or more effects, each of which decays in an exponential law with the time. The physical interpretation of this law still remains to be discussed.
It has been shown that in uranium and thorium compounds there is a continuous production of active matter which keeps the compound in radio-active equilibrium. The changes by which the active matter is produced must be chemical in nature, since the products of the action are different in chemical properties from the matter in which the changes take place. The activity of the products has afforded the means of following the changes occurring in them. It now remains to consider the connection between the activity at any time, and the amount of chemical change taking place at that time.
In the first place, it is found experimentally that the saturation ionization current i{t}_, after the active product has been allowed to decay for a time t, is given by
$$ \frac {i_t} {i₀} = e^{–λt} $$,
where i₀ is the initial saturation current and λ the constant of decay.
Now the saturation current is a measure of the total number of ions produced per second in the testing vessel. It has already been shown that the α rays, which produce the greater proportion of ionization in the gas, consist of positively charged particles projected with great velocity. Suppose for simplicity that each atom of active matter, in the course of its change, gives rise to one projected α particle. Each α particle will produce a certain average number of ions in its path before it strikes the boundaries or is absorbed in the gas. Since the number of projected particles per second is equal to the number of atoms changing per second, the number of atoms n{t}_ which change per second at the time t is given by
$$ \frac {n_t} {n₀} = e^{–λt} $$,
where n₀ is the initial number which change per second. On this view, then, the law of decay expresses the result that the number of atoms changing in unit time, diminishes according to an exponential law with the time. The number of atoms N{t}_ which remain unchanged after an interval t is given by
$$ N_t = \int_t^{\infty} n_t dt $$
$$ = \frac {n₀} {λ} e^{–λt} $$ .
If N₀ is the number of atoms at the beginning,
$$ N₀ = \frac {n₀} {λ} $$,
Thus
$$ \frac {N_t} {N_₀} = e^{–λt} $$ (1).
or the law of decay expresses the fact that the activity of a product at any time is proportional to the number of atoms which remain unchanged at that time.
This is the same as the law of monomolecular change in chemistry, and expresses the fact that there is only one changing system. If the change depended on the mutual action of two systems, the law of decay would be different, since the rate of decay in that case would depend on the relative concentration of the two reacting substances. This is not so, for not a single case has yet been observed in which the law of decay was affected by the amount of active matter present.
From the above equation (1)
dN{t}_ ------- = –λ_N_{t}_, dt
or the number of systems changing in unit time is proportional to the number unchanged at that time.
In the case of recovery of activity, after an active product has been removed, the number of systems changing in unit time, when radio-active equilibrium is produced, is equal to λ_N₀_. This must be equal to the number q₀ of new systems applied in unit time, or
q₀ = λ_N₀_,
q₀ and λ = ------; N₀
λ has thus a distinct physical meaning, and may be defined as the proportion of the total number of systems present which change per second. It has different values for different types of active matter, but is invariable for any particular type of matter. For this reason, λ will be termed the “radio-active constant” of the product.
We are now in a position to discuss with more physical definiteness the gradual growth of Th X in thorium, after the Th X has been completely removed from it. Let q₀ particles of Th X be produced per second by the thorium, and let N be the number of particles of Th X present at any time t after the original Th X was removed. The number of particles of Th X which change every second is λ_N_, where λ is the radio-active constant of Th X. Now, at any time during the process of recovery, the rate of increase of the number of particles of Th X = the rate of production − the rate of change; that is
dN ------ = q₀ − λ_N_. dt
The solution of this equation is of the form
$$ N = ae^{–λt} + b $$,
where a and b are constants.
Now when t is very great, the number of particles of Th X present reach a maximum value N₀.
Thus, since N = N₀ when t = infinity, b = N₀;
since N = 0 when t = 0,
a + b = 0; hence b = -a = N₀,
and the equation becomes
$$ \frac {N} {N₀} = 1 − e^{–λt} $$ .
This is equivalent to the equation already obtained in section 130, since the intensity of the radiation is always proportional to the number of particles present.
=134. Influence of conditions on the rate of decay.= Since the activity of any product, at any time, may be taken as a measure of the rate at which chemical change takes place, it may be used as a means of determining the effect of conditions on the changes occurring in radio-active matter. If the rate of change should be accelerated or retarded, it is to be expected that the value of the radio-active constant λ will be increased or decreased, i.e. that the decay curve will be different under different conditions.
No such effect, however, has yet been observed in any case of radio-active change, where none of the active products produced are allowed to escape from the system. The rate of decay is unaltered by any chemical or physical agency, and in this respect the changes in radio-active matter are sharply distinguished from ordinary chemical changes. For example, the rate of decay of activity from any product takes place at the same rate when the substance is exposed to light as when it is kept in the dark, and at the same rate in a vacuum as in air or any other gas at atmospheric pressure. Its rate of decay is unaltered by surrounding the active matter by a thick layer of lead under conditions where no ordinary radiation from outside can affect it. The activity of the matter is unaffected by ignition or chemical treatment. The material giving rise to the activity can be dissolved in acid and re-obtained by evaporation of the solution without altering the activity. The rate of decay is the same whether the active matter is retained in the solid state or kept in solution. When a product has lost its activity, resolution or heat does not regenerate it, and as we shall see later, the rate of decay of the active products, so far examined, is the same at a red heat as at the temperature of liquid air. In fact, no variation of physical or chemical conditions has led to any observable difference in the decay of activity of any of the numerous types of active matter which have been examined.
=135. Effect of conditions on the rate of recovery of activity.= The recovery of the activity of a radio-element with time, when an active product is separated from it, is governed by the rate of production of fresh active matter and by the decay of activity of that already produced. Since the rate of decay of the activity of the separated product is independent of conditions, the rate of recovery of activity can be modified only by a change of the rate of production of fresh active matter. As far as experiments have gone, the rate of production, like the rate of decay, is independent of chemical or physical conditions. There are indeed certain cases which are apparent exceptions to this rule. For example, the escape of the radio-active emanations from thorium and radium is readily affected by heat, moisture and solution. A more thorough investigation, however, shows that the exception is only apparent and not real. These cases will be discussed more in detail in chapter VII, but it may be stated here that the differences observed are due to differences in the rate of escape of the emanations into the surrounding gas, and not to differences in the rate of production. For this reason it is difficult to test the question at issue in the case of the thorium compounds, which in most cases readily allow the emanation produced by them to escape into the air.
In order to show that the rate of production is independent of molecular state, temperature, etc., it is necessary in such a case to undertake a long series of measurements extending over the whole time of recovery. It is impossible to make accurate relative comparisons to see if the activity is altered by the conversion of one compound into another. The relative activity in such a case, when measured by spreading a definite weight of material uniformly on a metal plate, varies greatly with the physical conditions of the precipitate, although the total activity of two compounds may be the same.
The following method[230] offers an accurate and simple means of studying whether the rate of production of active matter is influenced by molecular state. The substance is chemically converted into any compound required, care being taken that active products are recovered during the process. The new compound is then spread on a metal plate and compared with a standard sample of uranium for several days or weeks as required. If the rate of production of active matter is altered by the conversion, there should be an increase or decrease of activity to a new steady value, where the production of active matter is again balanced by the rate of decay. This method has the great advantage of being independent of the physical condition of the precipitate. It can be applied satisfactorily to a compound of thorium like the nitrate and the oxide which has been heated to a white heat, after which treatment only a slight amount of emanation escapes. The nitrate was converted into the oxide in a platinum crucible by treatment with sulphuric acid and ignition to a white heat. The oxide so obtained was spread on a plate, but no change of its activity was observed with time, showing that in this case the rate of production was independent of molecular state. This method, which is limited in the case of thorium, may be applied generally to the uranium compounds where the results are not complicated by the presence of an emanation.
No differences have yet been observed in the recovery curves of different thorium compounds after the removal of Th X. For example, the rate of recovery is the same whether the precipitated hydroxide is converted into the oxide or into the sulphate.
=136. Disintegration hypothesis.= In the discussion of the changes in radio-active bodies, only the active products Ur X and Th X have been considered. It will, however, be shown later that these two products are only examples of many other types of active matter which are produced by the radio-elements, and that each of these types of active matter has definite chemical as well as radio-active properties, which distinguish it, not only from the other active products, but also from the substance from which it is produced.
The full investigation of these changes will be shown to verify in every particular the hypothesis that radio-activity is the accompaniment of chemical changes of a special kind occurring in matter, and that the constant activity of the radio-elements is due to an equilibrium process, in which the rate of production of fresh active matter balances the rate of change of that already formed.
The nature of the process taking place in the radio-elements, in order to give rise to the production at a constant rate of new kinds of active matter, will now be considered. Since in thorium or uranium compounds there is a continuous production of radio-active matter, which differs in chemical properties from the parent substance, some kind of change must be taking place in the radio-element. This change, by which new matter is produced, is very different in character from the molecular changes dealt with in chemistry, for no chemical change is known which proceeds at the same rate at the temperatures corresponding to a red heat and to liquid air, and is independent of all physical and chemical actions. If, however, the production of active matter is supposed to be the result of changes, not in the molecule, but in the atom itself, it is not to be expected that the temperature would exert much influence. The general experience of chemistry in failing to transform the elements by the action of temperature is itself strong evidence that wide ranges of temperature have not much effect in altering the stability of the chemical atom.
The view that the atoms of the radio-elements are undergoing spontaneous disintegration was put forward by Rutherford and Soddy as a result of evidence of this character. The discovery of the material nature of the α rays added strong confirmation to the hypothesis; for it has been pointed out (section 95) that the expulsion of α particles must be the result of a disintegration of the atoms of the radio-element. Taking the case of thorium as an example, the processes occurring in the atom may be pictured in the following way. It must be supposed that the thorium atoms are not permanently stable systems, but, on an average, a constant small proportion of them—about one atom in every 10¹⁶ will suffice—break up per second. The disintegration consists in the expulsion from the atom of one or more α particles with great velocity. For simplicity, it will be supposed that each atom expels one α particle. It has been shown that the α particle of radium has a mass about twice that of the hydrogen atom. From the similarity of the α rays from thorium and radium, it is probable that the α particle of thorium does not differ much in mass from that of radium, and may be equal to it. The α particles expelled from the thorium atoms as they break up constitute what is known as the “non-separable activity” of thorium. This activity, measured by the α rays, is about 25 per cent. of the maximum. After the escape of an α particle, the part of the atom left behind, which has a mass slightly less than that of the thorium atom, tends to rearrange its components to form a temporarily stable system. It is to be expected that it will differ in chemical properties from the thorium atom from which it was derived. The atom of the substance Th X is, on this view, the thorium atom minus one α particle. The atoms of Th X are far more unstable than the atoms of thorium, and one after the other they break up, each atom expelling one α particle as before. These projected α particles give rise to the radiation from the Th X. Since the activity of Th X falls to half its original value in about four days, on an average half of the atoms of Th X break up in four days, the number breaking up per second being always proportional to the number present. After an atom of Th X has expelled an α particle, the mass of the system is again reduced, and its chemical properties are changed. It will be shown (section 154) that the Th X produces the thorium emanation, which exists as a radio-active gas, and that this in turn is transformed into matter which is deposited on solid bodies and gives rise to the phenomena of excited activity. The first few successive changes occurring in thorium are shown diagrammatically below (Fig. 50).
Thus as a result of the disintegration of the thorium atom, a series of chemical substances is produced, each of which has distinctive chemical properties. Each of these products is radio-active, and loses its activity according to a definite law. Since thorium has an atomic weight of 237, and the weight of the α particle is about 2, it is evident that, if only one α particle is expelled at each change, the process of disintegration could pass through a number of successive stages and yet leave behind, at the end of the process, a mass comparable with that of the parent atom.
It will be shown later that a process of disintegration, very similar to that already described for thorium, must be supposed to take place also in uranium, actinium and radium. The full discussion of this subject cannot be given with advantage until two of the most important products of the three substances thorium, radium and actinium, viz. the radio-active emanations and the matter which causes excited activity, have been considered in detail.
=137. Magnitude of the changes.= It can be calculated by several independent methods (see section 246) that, in order to account for the radio-activity observed in thorium, about 3 × 10⁴ atoms in each gram of thorium suffer disintegration per second. It is well known (section 39) that 1 cubic centimetre of hydrogen at atmospheric pressure and temperature contains about 3·6 × 10¹⁹ molecules. From this it follows that one gram of thorium contains 3·6 × 10²¹ atoms. The fraction which breaks up per second is thus about 10¹⁷. This is an extremely small ratio, and it is evident that the process could continue for long intervals of time, before the amount of matter changed would be capable of detection by the spectroscope or by the balance. With the electroscope it is possible to detect the radiation from 10⁻⁵ gram of thorium, i.e. the electroscope is capable of detecting the ionization which accompanies the disintegration of a single thorium atom per second. The electroscope is thus an extraordinarily delicate means for detection of minute changes in matter, which are accompanied, as in the case of the radio-elements, by the expulsion of charged particles with great velocity. It is possible to detect by its radiation the amount of Th X produced in a second from 1 gram of thorium, although the process would probably need to continue thousands of years before it could be detected by the balance or the spectroscope. It is thus evident that the changes occurring in thorium are of an order of magnitude quite different from that of ordinary chemical changes, and it is not surprising that they have never been observed by direct chemical methods.
Footnote 225:
Crookes, Proc. Roy. Soc. 66, p. 409, 1900.
Footnote 226:
Becquerel, C. R. 131, p. 137, 1900; 133, p. 977, 1901.
Footnote 227:
Rutherford and Soddy, Phil. Mag. Sept. and Nov. 1902. Trans. Chem. Soc. 81, pp. 321 and 837, 1902.
Footnote 228:
Rutherford and Soddy, Phil. Mag. Sept. 1902.
Footnote 229:
The general method of regarding the subject would be unchanged, even if it were proved that the radio-activity of thorium is not due to thorium at all but to a small constant amount of a radio-active impurity mixed with it.
Footnote 230:
Rutherford and Soddy, Phil. Mag. Sept. 1902.
VII. RADIO-ACTIVE EMANATIONS
=138. Introduction.= A most important and striking property possessed by radium, thorium, and actinium, but not by uranium or polonium, is the power of continuously emitting into the surrounding space a material emanation, which has all the properties of a radio-active gas. This emanation is able to diffuse rapidly through gases and through porous substances, and may be separated from the gas with which it is mixed by condensation by the action of extreme cold. This emanation forms a connecting link between the activity of the radio-elements themselves and their power of exciting activity on surrounding objects, and has been studied more closely than the other active products on account of its existence in the gaseous state. The emanations from the three active bodies all possess similar radio-active properties, but the effects are more marked in the case of the emanation from radium, on account of the very great activity of that element.
Thorium Emanation.
=139. Discovery of the emanation.= In the course of examination of the radiations of thorium, several observers had noted that some of the thorium compounds, and especially the oxide, were very inconstant sources of radiation, when examined in open vessels by the electrical method. Owens[231] found that this inconstancy was due to the presence of air currents. When a closed vessel was used, the current, immediately after the introduction of the active matter, increased with the time, and finally reached a constant value. By drawing a steady stream of air through the vessel the value of the current was much reduced. It was also observed that the radiations could apparently pass through large thicknesses of paper, which completely absorbed the ordinary α radiation.
In an investigation of these peculiar properties of thorium compounds, the writer[232] found that the effects were due to an emission of radio-active particles of some kind from the thorium compounds. This “emanation,” as it was termed for convenience, possesses the properties of ionizing the gas and acting on a photographic plate, and is able to diffuse rapidly through porous substances like paper and thin metal foil.
The emanation, like a gas, is completely prevented from escaping by covering the active matter with a thin plate of mica. The emanation can be carried away by a current of air; it passes through a plug of cotton-wool and can be bubbled through solutions without any loss of activity. In these respects, it behaves very differently from the ions produced in the gas by the rays from active substances, for these give up their charges completely under the same conditions.
Since the emanation passes readily through large thicknesses of cardboard, and through filters of tightly packed cotton-wool, it does not seem likely that the emanation consists of particles of dust given off by the active matter. This point was tested still further by the method used by Aitken and Wilson, for detecting the presence of dust particles in the air. The oxide, enclosed in a paper cylinder, was placed in a glass vessel, and the dust was removed by repeated small expansions of the air over a water surface. The dust particles act as nuclei for the formation of small drops and are then removed from the air by the action of gravity. After repeated expansions, no cloud was formed, and the dust was considered to be removed. After waiting for some time to allow the thorium emanation to collect, further expansions were made but no cloud resulted, showing that for the small expansions used, the particles were too small to become centres of condensation. The emanation then could not be regarded as dust emitted from thorium.
Since the power of diffusing rapidly through porous substances, and acting on a photographic plate, is also possessed by a chemical substance like hydrogen peroxide, some experiments were made to see if the emanation could be an agent of that character. It was found, however, that hydrogen peroxide is not radio-active, and that its action on the plate is a purely chemical one, while it is the radiation from the emanation and not the emanation itself that produces ionizing and photographic effects.
=140. Experimental arrangements.= The emanation from thorium is given off in minute quantity. No appreciable lowering of the vacuum is observed when an emanating compound is placed in a vacuum tube and no new spectrum lines are observed.
For an examination of the emanation, an apparatus similar in principle to that shown in Fig. 51 is convenient.
The thorium compound, either bare or enclosed in a paper envelope, was placed in a glass tube C. A current of air from a gasometer, after passing through a tube containing cotton-wool to remove dust particles, bubbled through sulphuric acid in the vessel A. It then passed through a bulb containing tightly packed cotton-wool to prevent any spray being carried over. The emanation, mixed with air, was carried from the vessel C through a plug of cotton-wool D, which removed completely all the ions carried with the emanation. The latter then passed into a long brass cylinder, 75 cm. in length and 6 cm. in diameter. The insulated cylinder was connected with a battery in the usual way. Three insulated electrodes, E, F, H, of equal lengths, were placed along the axis of the cylinder, supported by brass rods passing through ebonite corks in the side of the cylinder. The current through the gas, due to the presence of the emanation, was measured by means of an electrometer. An insulating key was arranged so that any one of the electrodes E, F, H could be rapidly connected with one pair of quadrants of the electrometer, the other two being always connected with earth. The current observed in the testing cylinder vessel was due entirely to the ions produced by the emanation carried into the vessel by the current of air. On substituting a uranium compound for the thorium, not the slightest current was observed. After a constant flow has passed for about 10 minutes, the current due to the emanation reaches a constant value.
The variation of the ionization current with the voltage is similar to that observed for the gas ionized by the radiations from the active bodies. The current at first increases with the voltage, but finally reaches a saturation value.
=141. Duration of the activity of the emanation.= The emanation rapidly loses its activity with time. This is very readily shown with the apparatus of Fig. 51. The current is found to diminish progressively along the cylinder, and the variation from electrode to electrode depends on the velocity of the flow of air.
If the velocity of the air current is known, the decay of activity of the emanation with time can be deduced. If the flow of air is stopped, and the openings of the cylinder closed, the current steadily diminishes with time. The following numbers illustrate the variation with time of the saturation current, due to the emanation in a closed vessel. The observations were taken successively, and as rapidly as possible after the current of air was stopped.
Time in seconds Current 0 100 28 69 62 51 118 25 155 14 210 6·7 272 4·1 360 1·8
Curve A, Fig. 52, shows the relation existing between the current through the gas and the time. The current just before the flow of air was stopped is taken as unity. The current through the gas, which is a measure of the activity of the emanation, diminishes according to an exponential law with the time like the activity of the products Ur X and Th X. The rate of decay is, however, much more rapid, the activity of the emanation decreasing to half value in about one minute. According to the view developed in section 136, this implies that half of the emanation particles have undergone change in one minute. After an interval of 10 minutes the current due to the emanation is very small, showing that practically all the emanation particles present have undergone change.
The rate of decay has been more accurately determined by Rossignol and Gimingham[233] who found that the activity fell to half value in about 51 seconds. Bronson[234], using the steady deflection method described in section 69, found the corresponding time 54 seconds.
The decrease of the current with the time is an actual measure of the decrease of the activity of the emanation, and is not in any way influenced by the time that the ions produced take to reach the electrodes. If the ions had been produced from a uranium compound the duration of the conductivity for a saturation voltage would only have been a fraction of a second.
The rate of decay of the activity of the emanation is independent of the electromotive force acting on the gas. This shows that the radio-active particles are not destroyed by the electric field. The current through the gas at any particular instant, after stoppage of the flow of air, was found to be the same whether the electromotive force had been acting the whole time or had been just applied for the time of the test.
The emanation itself is unaffected by a strong electric field and so cannot be charged. By testing its activity after passing it through long concentric cylinders, charged to a high potential, it was found that the emanation certainly did not move with a velocity greater than ·00001 cm. per second, for a gradient of 1 volt per cm., and there was no evidence to show that it moved at all. This conclusion has been confirmed by the experiments of McClelland[235].
The rate at which the emanation is produced is independent of the gas surrounding the active matter. If in the apparatus of Fig. 51 air is replaced by hydrogen, oxygen, or carbonic acid, similar results are obtained, though the current observed in the testing vessel varies for the different gases on account of the unequal absorption by them of the radiation from the emanation.
If a thorium compound, enclosed in paper to absorb the α radiation, is placed in a closed vessel, the saturation current due to the emanation is found to vary directly as the pressure. Since the rate of ionization is proportional to the pressure for a constant source of radiation, this experiment shows that the rate of emission of the emanation is independent of the pressure of the gas. The effect of pressure on the rate of production of the emanation is discussed in more detail later in section 157.
=142. Effect of thickness of layer.= The amount of emanation emitted by a given area of thorium compound depends on the thickness of the layer. With a very thin layer, the current between two parallel plates, placed in a closed vessel as in Fig. 17, is due very largely to the α rays. Since the α radiation is very readily absorbed, the current due to it practically reaches a maximum when the surface of the plate is completely covered by a thin layer of the active material. On the other hand the current produced by the emanation increases until the layer is several millimetres in thickness, and then is not much altered by adding fresh active matter. This falling off of the current after a certain thickness has been reached is to be expected, since the emanation, which takes several minutes to diffuse through the layer above it, has already lost a large proportion of its activity.
With a thick layer of thorium oxide in a closed vessel, the current between the plates is largely due to the radiation from the emanation lying between the plates. The following tables illustrate the way in which the current varies with the thickness of paper for both a thin and a thick layer.
TABLE I. Thin Layer.
Thickness of sheets of paper ·0027.
No. of Current layers of paper
0 1
1 ·37
2 ·16
3 ·08
TABLE II. Thick Layer.
Thickness of paper ·008 cm.
No. of Current layers of paper
0 1
1 ·74
2 ·74
5 ·72
10 ·67
20 ·55
The initial current with the unscreened compound is taken as unity. In Table I, for a thin layer of thorium oxide, the current diminished rapidly with additional layers of thin paper. In this case the current is due almost entirely to the α rays. In Table II the current falls to ·74 for the first layer. In this case about 26% of the current is due to the α rays, which are practically absorbed by the layer ·008 cm. in thickness. The slow decrease with additional layers shows that the emanation diffuses so rapidly through a few layers of paper that there is little loss of activity during the passage. The time taken to diffuse through 20 layers is however appreciable, and the current consequently has decreased. After passing through a layer of cardboard 1·6 mms. in thickness the current is reduced to about one-fifth of its original value. In closed vessels the proportion of the total current, due to the emanation, varies with the distance between the plates as well as with the thickness of the layer of active material. It also varies greatly with the compound examined. In the nitrate, which gives off only a small amount of emanation, the proportion is very much smaller than in the hydroxide, which gives off a large amount of emanation.
=143. Increase of current with time.= The current due to the emanation does not reach its final value for some time after the active matter has been introduced into the closed vessel. The variation with time is shown in the following table. The saturation current due to thorium oxide, covered with paper, was observed between concentric cylinders of 5·5 cms. and ·8 cm. diameter.
Immediately before observations on the current were made, a rapid stream of air was blown through the apparatus. This removed most of the emanation. However, the current due to the ionization of the gas by the emanation, as it was carried along by the current of air, was still appreciable. The current consequently does not start from zero.
Time in Current seconds
0 9
23 25
53 49
96 67
125 76
194 88
244 98
304 99
484 100
The results are shown graphically in Fig. 52, curve B. The decay of the activity of the emanation with time, and the rate of increase of the activity due to the emanation in a closed space, are connected in the same way as the decay and recovery curves of Th X and Ur X.
With the previous notation, the decay curve is given by
$$ \frac {I_T} {I₀} = e^{–λt} $$ .
and the recovery curve by
$$ \frac {I_t} {I₀} = 1 − e^{–λt} $$,
where λ is the radio-active constant of the emanation.
This relation is to be expected, since the decay and recovery curves of the emanation are determined by exactly the same conditions as the decay and recovery curves of Ur X and Th X. In both cases there is:
(1) A supply of fresh radio-active particles produced at a constant rate.
(2) A loss of activity of the particles following an exponential law with the time.
In the case of Ur X and Th X, the active matter produced manifests its activity in the position in which it is formed; in this new phenomenon, a proportion of the active matter in the form of the emanation escapes into the surrounding gas. The activity of the emanation, due to a thorium compound kept in a closed vessel, thus reaches a maximum when the rate of supply of fresh emanation particles from the compound is balanced by the rate of change of those already present. The time for recovery of half the final activity is about 1 minute, the same as the time taken for the emanation, when left to itself, to lose half its activity.
If q₀ is the number of emanation particles escaping into the gas per second, and N₀ the final number when radio-active equilibrium is reached, then (section 133),
q₀ = λ_N₀_.
Since the activity of the emanation falls to half value in 1 minute
λ = ¹⁄₈₇,
and N₀ = 87_q₀_, or the number of emanation particles present when a steady state is reached is 87 times the number produced per second.
Radium Emanation.
=144. Discovery of the emanation.= Shortly after the discovery of the thorium emanation, Dorn[236] repeated the results, and, in addition, showed that radium compounds also gave off radio-active emanations, and that the amount given off was much increased by heating the compound. The radium emanation differs from the thorium emanation in the rate at which it loses its activity. It decays far more slowly, but in other respects the emanations of thorium and radium have much the same properties. Both emanations ionize the gas with which they are mixed, and affect a photographic plate. Both diffuse readily through porous substances but are unable to pass through a thin plate of mica; both behave like a temporarily radio-active gas, mixed in minute quantity with the air or other gas in which they are conveyed.
=145. Decay of activity of the emanation.= Very little emanation escapes from radium chloride in the solid state, but the amount is largely increased by heating, or by dissolving the compound in water. By bubbling air through a radium chloride solution, or passing air over a heated radium compound, a large amount of emanation may be obtained which can be collected, mixed with air, in a suitable vessel.
Experiments to determine accurately the rate of decay of activity of the emanation have been made by P. Curie[237], and Rutherford and Soddy[238]. In the experiments of the latter, the emanation mixed with air was stored over mercury in an ordinary gas-holder. From time to time, equal quantities of air mixed with the emanation were measured off by a gas pipette and delivered into a testing vessel. The latter consisted of an air-tight brass cylinder carrying a central insulated electrode. A saturation voltage was applied to the cylinder, and the inner electrode was connected to the electrometer with a suitable capacity in parallel. The saturation current was observed immediately after the introduction of the active gas into the testing vessel, and was taken as a measure of the activity of the emanation present. The current increased rapidly with the time owing to the production of excited activity on the walls of the containing vessel. This effect is described in detail in chapter VIII.
The measurements were made at suitable intervals over a period of 33 days. The following table expresses the results, the initial activity being taken as 100.
Time in Relative hours Activity
0 100
20·8 85·7
187·6 24·0
354·9 6·9
521·9 1·5
786·9 0·19
The activity falls off according to an exponential law with the time, and decays to half value in 3·71 days. With the usual notation
$$ \frac {I_T} {I₀} = e^{–λt} $$ .
the mean value of λ deduced from the results is given by
λ = 2·16 × 10⁻⁶ = ¹⁄₄₆₃₀₀₀.
P. Curie determined the rate of decay of activity of the emanation by another method. The active matter was placed at one end of a sealed tube. After sufficient time had elapsed the portion of the tube containing the radium compound was removed. The loss of activity of the emanation, stored in the other part, was tested at regular intervals by observing the ionization current due to the rays which passed through the walls of the glass vessel. The testing apparatus and the connections are shown clearly in Fig. 53. The ionization current is observed between the vessels BB and CC. The glass tube A contains the emanation.
Now it will be shown later that the emanation itself gives off only α rays, and these rays are completely absorbed by the glass envelope, unless it is made extremely thin. The rays producing ionization in the testing vessel were thus not due to the α rays from the emanation at all, but to the β and γ rays due to the excited activity produced on the walls of the glass tube by the emanation inside it. What was actually measured was thus the decay of the excited activity derived from the emanation, and not the decay of activity of the emanation itself. Since, however, when a steady state is reached, the amount of excited activity is nearly proportional at any time to the activity of the emanation, the rate of decay of the excited activity on the walls of the vessel indirectly furnishes a measure of the rate of decay of the emanation itself. This is only true if the emanation is placed for four or five hours in the tube before observations begin, in order to allow the excited activity time to reach a maximum value.
Using this method P. Curie obtained results similar to those obtained by Rutherford and Soddy by the direct method. The activity decayed according to an exponential law with the time, falling to half value in 3·99 days.
The experiments were performed under the most varied conditions but the rate of decay was found to remain unaltered. The rate of decay did not depend on the material of the vessel containing the emanation or on the nature or pressure of the gas with which the emanation was mixed. It was unaffected by the amount of emanation present, or by the time of exposure to the radium, provided sufficient time had elapsed to allow the excited activity to reach a maximum value before the observations were begun. P. Curie[239] found that the rate of decay of activity was not altered by exposing the vessel containing the emanation to different temperatures, ranging from +450° to −180° C.
In this respect the emanations of thorium and radium are quite analogous. The rate of decay seems to be unaffected by any physical or chemical agency, and the emanations behave in exactly the same way as the radio-active products Th X and Ur X, already referred to. The radio-active constant λ is thus a fixed and unalterable quantity for both emanations, although in one case its value is about 5000 times greater than in the other.
Emanations from Actinium.
=146.= Debierne[240] found that actinium gives out an emanation similar to the emanation of thorium and radium. The loss of activity of the emanation is even more rapid than for the thorium emanation, for its activity falls to half value in 3·9 seconds. In consequence of the rapid decay of activity, the emanation is able to diffuse through the air only a short distance from the active matter before it loses the greater proportion of its activity. Giesel early observed that the radio-active substance separated by him, which we have seen (section 18) is identical in radio-active properties with actinium, gave off a large amount of emanation. It was in consequence of this property, that he gave it the name of the “emanating substance” and later “emanium.” The impure preparations of this substance emit the emanation very freely and in this respect differ from most of the thorium compounds. The emanation from actinium like those from thorium and radium possesses the property of exciting activity on inactive bodies, but it has not yet been studied so completely as the better known emanations of thorium and radium.
Experiments with large amounts of Radium Emanation.
=147.= With very active specimens of radium a large amount of emanation can be obtained, and the electrical, photographic, and fluorescent effects are correspondingly intense. On account of the small activity of thorium and the rapid decay of its emanation the effects due to it are weak, and can be studied only for a few minutes after its production. The emanation from radium, on the other hand, in consequence of the slow decay of its activity, may be stored mixed with air in an ordinary gas-holder, and its photographic and electrical actions may be examined several days or even weeks after, quite apart from those of the radium from which it was obtained.
It is, in general, difficult to study the radiation due to the emanation alone, on account of the fact that the emanation is continually producing a secondary type of activity on the surface of the vessel in which the emanation is enclosed. This excited activity reaches a maximum value several hours after the introduction of the emanation, and, as long as it is kept in the vessel, this excited activity on the walls decays at the same rate as the emanation itself, i.e. it falls to half its initial value in about 4 days. If, however, the emanation is blown out, the excited activity remains behind on the surface, but rapidly loses its activity in the course of a few hours. After several hours the intensity of the residual radiation is very small.
These effects and their connection with the emanation are discussed more fully in chapter VIII. Giesel[241] has recorded some interesting observations of the effect of the radium emanation on a screen of phosphorescent zinc sulphide. When a few centigrams of moist radium bromide were placed on a screen any slight motion of the air caused the luminosity to move to and fro on the screen. The direction of phosphorescence could be altered at will by a slow current of air. The effect was still further increased by placing the active material in a tube and blowing the air through it towards the screen. A screen of barium platinocyanide or of Balmain’s paint failed to give any visible light under the same conditions. The luminosity was not altered by a magnetic field, but it was affected by an electric field. If the screen were charged the luminosity was more marked when it was negative than when it was positive.
Giesel states that the luminosity was not equally distributed, but was concentrated in a peculiar ring-shaped manner over the surface of the screen. The concentration of luminosity on the negative, rather than on the positive, electrode is probably due to the excited activity, caused by the emanation, and not to the emanation itself, for this excited activity is concentrated chiefly on the negative electrode in an electric field (see chapter VIII).
An experiment to illustrate the phosphorescence produced in some substances by the rays from a large amount of emanation is described in section 165.
=148.= Curie and Debierne[242] have investigated the emanation from radium, and the excited activity produced by it. Some experiments were made on the amount of emanation given off from radium under very low pressures. The tube containing the emanation was exhausted to a good vacuum by a mercury pump. It was observed that a gas was given off from the radium which produced excited activity on the glass walls. This gas was extremely active, and rapidly affected a photographic plate through the glass. It caused fluorescence on the surface of the glass and rapidly blackened it, and was still active after standing ten days. When spectroscopically examined, this gas did not show any new lines, but generally those of the spectra of carbonic acid, hydrogen, and mercury. In the light of the results described in section 124 the gas, given off by the radium, was probably the non-active gases hydrogen and oxygen, in which the active emanation was mixed in minute quantity. It will be shown later (section 242) that the energy radiated from the emanation is enormous compared with the amount of matter involved, and that the effects observed, in most cases, are produced by an almost infinitesimal amount of the emanation.
In further experiments, Curie and Debierne[243] found that many substances were phosphorescent under the action of the emanation and the excited activity produced by it. In their experiments, two glass bulbs A and B (Fig. 54) were connected with a glass tube. The active material was placed in the bulb A and the substance to be examined in the other.
They found that, in general, substances that were phosphorescent in ordinary light became luminous. The sulphide of zinc was especially brilliant and became as luminous as if exposed to a strong light. After sufficient time had elapsed the luminosity reached a constant value. The phosphorescence is partly due to the excited activity produced by the emanation on its surface, and partly to the direct radiation from the emanation.
Phosphorescence was also produced in glass. Thuringian glass showed the most marked effects. The luminosity of the glass was found to be about the same in the two bulbs, but was more marked in the connecting tube. The effect in the two bulbs was the same even if connected by a very narrow tube.
Some experiments were also made with a series of phosphorescent plates placed in the vessel at varying distances apart. With the plates 1 mm. apart the effect was very feeble, but increased directly as the distance and was large for a distance of 3 cms.
These effects receive a general explanation on the views already put forward. When the radium is placed in the closed vessel, the emanation is given off at a constant rate and gradually diffuses throughout the enclosure. Since the time taken for diffusion of the emanation through tubes of ordinary size is small compared with the time required for the activity to be appreciably reduced, the emanation, and also the excited activity due to it, will be nearly equally distributed throughout the vessel.
The luminosity due to it should thus be equal at each end of the tube. Even with a capillary tube connecting the two bulbs, the gas continuously given off by the radium will always carry the emanation with it and cause a practically uniform distribution.
The gradual increase of the amount of emanation throughout the tube will be given by the equation
$$ \frac {N_t} {N₀} = 1 − e^{–λt} $$ .
where N{t}_ is the number of emanation particles present at the time t, N₀ the number present when radio-active equilibrium is reached, and λ is the radio-active constant of the emanation. The phosphorescent action, which is due partly to the radiations from the emanation and partly to the excited activity on the walls, should thus reach half the maximum value in four days and should practically reach its limit after three weeks’ interval.
The variation of luminosity with different distances between the screens is to be expected. The amount of excited activity deposited on the boundaries is proportional to the amount of emanation present. Since the emanation is equally distributed, the amount of excited activity deposited on the screens, due to the emanation between them, varies directly as the distance, provided the distance between the screens is small compared with their dimensions. Such a result would also follow if the phosphorescence were due to the radiation from the emanation itself, provided that the pressure of the gas was low enough to prevent absorption of the radiation from the emanation in the gas itself between the screens.
Measurements of Emanating Power.
=149. Emanating power.= The compounds of thorium in the solid state vary very widely in the amount of emanation they emit under ordinary conditions. It is convenient to use the term emanating power to express the amount of emanation given off per second by one gram of the compound. Since, however, we have no means of determining absolutely the amount of emanation present, all measurements of emanating power are of necessity comparative. In most cases, it is convenient to take a given weight of a thorium compound, kept under conditions as nearly as possible constant, and to compare the amount of emanation of the compound to be examined with this standard.
In this way comparisons of the emanating power of thorium compounds have been made by Rutherford and Soddy[244], using an apparatus similar to that shown in Fig. 51 on page 240.
A known weight of the substance to be tested was spread on a shallow dish, placed in the glass tube C. A stream of dry dust-free air, kept constant during all the experiments, was passed over the compound and carried the emanation into the testing vessel. After ten minutes interval, the current due to the emanation in the testing vessel reached a constant value. The compound was then removed, and the standard comparison sample of equal weight substituted; the saturation current was observed when a steady state was again reached. The ratio of these two currents gives the ratio of the emanating power of the two samples.
It was found experimentally that, for the velocities of air current employed, the saturation current in the testing vessel was directly proportional to the weight of thorium, for weights up to 20 grams. This is explained by the supposition that the emanation is removed by the current of air from the mass of the compound, as fast as it is formed.
Let i₁ = saturation current due to a weight ω₁ of the standard, i₂ = „ „ „ „ ω₂ of the sample to be tested.
(emanating power of specimen) i₂ ω₁ Then ---------------------------- = ---- --- (emanating power of standard) i₁ ω₂
By means of this relation the emanating power of compounds which are not of equal weight can be compared.
It was found that thorium compounds varied enormously in emanating power, although the percentage proportion of thorium present in the compound was not very different. For example, the emanating power of thorium hydroxide was generally 3 to 4 times greater than that of ordinary thoria, obtained from the manufacturer. Thorium nitrate, in the solid state, had only ¹⁄₂₀₀ of the emanating power of ordinary thoria, while preparations of the carbonate were found to vary widely among themselves in emanating power, which depended upon slight variations in the method of preparation.
=150. Effect of conditions on emanating power.= The emanating power of different compounds of thorium and radium is much affected by the alteration of chemical and physical conditions. In this respect the emanating power, which is a measure of the rate of escape of the emanation into the surrounding gas, must not be confused with the rate of decay of the activity of the emanations themselves, which has already been shown to be unaffected by external conditions.
Dorn (loc. cit.) first observed that the emanating power of thorium and radium compounds was much affected by moisture. In a fuller investigation of this point by Rutherford and Soddy, it was found that the emanating power of thoria is from two to three times greater in a moist than in a dry gas. Continued desiccation of the thoria in a glass tube, containing phosphorus pentoxide, did not reduce the emanating power much below that observed in ordinary dry air. In the same way radium chloride in the solid state gives off very little emanation when in a dry gas, but the amount is much increased in a moist gas.
The rate of escape of emanation is much increased by solution of the compound. For example, thorium nitrate, which has an emanating power of only ¹⁄₂₀₀ that of thoria in the solid state, has in solution an emanating power of 3 to 4 times that of thoria. P. Curie and Debierne observed that the emanating power of radium was also much increased by solution.
Temperature has a very marked effect on the emanating power. The writer[245] showed that the emanating power of ordinary thoria was increased three to four times by heating the substance to a dull red heat in a platinum tube. If the temperature was kept constant the emanation continued to escape at the increased rate, but returned to its original value on cooling. If, however, the compound was heated to a white heat, the emanating power was greatly reduced, and it returned on cooling to about 10% of the original value. Such a compound is said to be de-emanated. The emanating power of radium compounds varies in a still more striking manner with rise of temperature. The rate of escape of the emanation is momentarily increased even 10,000 times by heating to a dull red heat. This effect does not continue, for the large escape of the emanation by heating is in reality due to the release of the emanation stored up in the radium compound. Like thoria, when the compound has once been heated to a very high temperature, it loses its emanating power and does not regain it. It regains its power of emanating, however, after solution and re-separation.
A further examination of the effect of temperature was made by Rutherford and Soddy[246]. The emanating power of thoria decreases very rapidly with lowering of temperature, and at the temperature of solid carbonic acid it is only about 10% of its ordinary value. It rapidly returns to its original value when the cooling agent is removed.
Increase of temperature from 80° C. to a dull red heat of platinum thus increases the emanating power about 40 times, and the effects can be repeated again and again, with the same compound, provided the temperature is not raised to the temperature at which de-emanation begins. De-emanation sets in above a red heat, and the emanating power is then permanently diminished, but even long-continued heating at a white heat never entirely destroys the emanating power.
=151. Regeneration of emanating power.= An interesting question arises whether the de-emanation of thorium and radium is due to a removal or alteration of the substance which produces the emanation, or whether intense ignition merely changes the rate of escape of the emanation from the solid into the surrounding atmosphere.
It is evident that the physical properties of the thoria are much altered by intense ignition. The compound changes in colour from white to pink; it becomes denser and also far less readily soluble in acids. In order to test if the emanating power could be regenerated by a cyclic chemical process, the de-emanated thoria was dissolved, precipitated as hydroxide and again converted into oxide. At the same time a specimen of the ordinary oxide was subjected to an exactly parallel process. The emanating power of both these compounds was the same, and was from two to three times greater than that of ordinary thoria.
Thus de-emanation does not permanently destroy the power of thorium of giving out an emanation, but merely produces an alteration of the amount of the emanation which escapes from the compound.
=152. Rate of production of the emanation.= The emanating power of thorium compounds, then, is a very variable quantity, much affected by moisture, heat, and solution. Speaking generally, increased temperatures and solution greatly increase the emanating power of both thorium and radium.
The wide differences between the emanating powers of these substances in the solid state and in solution pointed to the conclusion that the differences were probably due to the rate of escape of the emanation into the surrounding gas, and not to a variation of the rate of reaction which gave rise to the emanation. It is obvious that a very slight retardation in the rate of escape of the thorium emanation from the compound into the gas, will, on account of the rapid decay of activity of the emanation, produce great changes in emanating power. The regeneration of the emanating power of de-emanated thoria and radium by solution and chemical treatment made it evident that the original power of thorium and radium of producing the emanation still persisted in an unaltered degree.
The question whether the emanation was produced at the same rate in emanating as in non-emanating compounds can be put to a sharp quantitative test. If the rate of production of emanation goes on at the same rate in the solid compound where very little escapes, as in the solution where probably all escapes, the emanation must be occluded in the compound, and consequently there must be a sudden release of this emanation on solution of the compound. On account of the very slow decay of the activity of the emanation of radium, the effects should be far more marked in that compound than in thorium.
From the point of view developed in section 133, the exponential law of decay of the emanation expresses the result that N{t}_ the number of particles remaining unchanged at the time t is given by
$$ \frac {N_t} {N₀} = e^{–λt} $$ (1).
where N₀ is the initial number of particles present. When a steady state is reached, the rate of production q₀ of fresh emanation particles is exactly balanced by the rate of change of the particles N₀ already present, i.e.
q₀ = λ_N₀_,
N₀ in this case represents the amount of emanation “occluded” in the compound. Substituting the value of λ found for the radium emanation in section 145,
N₀ ----- = 1/λ = 463,000. q₀
The amount of emanation stored in a non-emanating radium compound should therefore be nearly 500,000 times the amount produced per second by the compound. This result was tested in the following way[247].
A weight of ·03 gr. of radium chloride of activity 1000 times that of uranium was placed in a Drechsel bottle and a sufficient amount of water drawn in to dissolve it. The released emanation was swept out by a current of air into a small gas holder and then into a testing cylinder. The initial saturation current was proportional to N₀. A rapid current of air was then passed through the radium solution for some time in order to remove any slight amount of emanation which had not been removed initially. The Drechsel bottle was closed air-tight, and allowed to stand undisturbed for a definite time t. The accumulated emanation was then swept out as before into the testing vessel. The new ionization current represents the value of N{t}_ the amount of emanation formed in the compound during the interval t.
In the experiment t = 105 minutes, and the observed value _N__{t} ----- = ·0131. N₀
Assuming that there is no decay during the interval,
N{t}_ = 105 × 60 × q₀.
N₀ Thus --- = 480,000. q₀
Making the small correction for the decay of activity during the interval,
N₀ ----- = 477,000. q₀
We have previously shown that from the theory
N₀ 1 ----- = ---- = 463,000. q₀ λ
The agreement between theory and experiment is thus as close as could be expected from the nature of the experiments. This experiment proves conclusively that the rate of production of emanation in the solid compound is the same as in the solution. In the former case it is occluded, in the latter it escapes as fast as it is produced.
It is remarkable how little emanation, compared with the amount stored up in the compound, escapes from solid radium chloride in a dry atmosphere. One experiment showed that the emanating power in the dry solid state was less than ½% of the emanating power of the solution. Since nearly 500,000 times as much emanation is stored up in the solid compound as is produced per second, this result showed that the amount of emanation which escaped per second was less than 10⁻⁸ of that occluded in the compound.
If a solid radium chloride compound is kept in a moist atmosphere, the emanating power becomes comparable with the amount produced per second in the solution. In such a case, since the rate of escape is continuous, the amount occluded will be much less than the amount for the non-emanating material.
The phenomenon of occlusion of the radium emanation is probably not connected in any way with its radio-activity, although this property has here served to measure it. The occlusion of helium by minerals presents almost a complete analogy to the occlusion of the radium emanation. Part of the helium is given off by fergusonite, for example, when it is heated and all of it when the mineral is dissolved.
=153.= Similar results hold for thorium, but, on account of the rapid loss of activity of the emanation, the amount of emanation occluded in a non-emanating compound is very small compared with that observed for radium. If the production of the thorium emanation proceeds at the same rate under all conditions, the solution of a solid non-emanating compound should be accompanied by a rush of emanation greater than that subsequently produced. With the same notation as before we have for the thorium emanation,
N₀ 1 ----- = ----- = 87. q₀ λ
This result was tested as follows: a quantity of finely powdered thorium nitrate, of emanating power ¹⁄₂₀₀ of ordinary thoria, was dropped into a Drechsel bottle containing hot water and the emanation rapidly swept out into the testing vessel by a current of air. The ionization current rose quickly to a maximum, but soon fell again to a steady value; showing that the amount of emanation released when the nitrate dissolves, is greater than the subsequent amount produced from the solution.
The rapid loss of the activity of the thorium emanation makes a quantitative comparison like that for radium very difficult. By slightly altering the conditions of the experiment, however, a definite proof was obtained that the rate of production of emanation is the same in the solid compound as in the solution. After dropping in the nitrate, a rapid air stream was blown through the solution for 25 seconds into the testing vessel. The air stream was stopped and the ionization current immediately measured. The solution was then allowed to stand undisturbed for 10 minutes. In that time the accumulation of the emanation again attained a practical maximum and again represented a steady state. The stream of air was blown through, as before, for 25 seconds, stopped and the current again measured. In both cases, the electrometer recorded a movement of 14·6 divisions per second. By blowing the same stream of air continuously through the solution the final current corresponded to 7·9 divisions per second or about one-half of that observed after the first rush.
Thus the rate of production of emanation is the same in the solid nitrate as in the solution, although the emanating power, i.e. the rate of escape of the emanation, is over 600 times greater in the solution than in the solid.
It seems probable that the rate of production of emanation by thorium, like the rate of production of Ur X and Th X, is independent of conditions. The changes of emanating power of the various compounds by moisture, heat, and solution must therefore be ascribed solely to an alteration in the rate of escape of the emanation into the surrounding gas and not to an alteration in the rate of its production in the compound.
On this view, it is easy to see that slight changes in the mode of preparation of a thorium compound may produce large changes in emanating power. Such effects have been often observed, and must be ascribed to slight physical changes in the precipitate. The fact that the rate of production of the emanation is independent of the physical or chemical conditions of the thorium, in which it is produced, is thus in harmony with what had previously been observed for the radio-active products Ur X and Th X.
Source of the Thorium Emanation.
=154.= Some experiments of Rutherford and Soddy[248] will now be considered, which show that the thorium emanation is produced, not directly by the thorium itself, but by the active product Th X.
When the Th X, by precipitation with ammonia, is removed from a quantity of thorium nitrate, the precipitated thorium hydroxide does not at first possess appreciable emanating power. This loss of emanating power is not due, as in the case of the de-emanated oxide, to a retardation in the rate of escape of the emanation produced; for the hydroxide, when dissolved in acid, still gives off no emanation. On the other hand, the solution, containing the Th X, possesses emanating power to a marked degree. When the precipitated hydroxide and the Th X is left for some time, it is found that the Th X decreases in emanating power, while the hydroxide gradually regains its emanating power. After about a month’s interval, the emanating power of the hydroxide has nearly reached a maximum, while the emanating power of the Th X has almost disappeared.
The curves of decay and recovery of emanating power with time are found to be exactly the same as the curves of decay and recovery of activity of Th X and the precipitated hydroxide respectively, shown in Fig. 47. The emanating power of Th X, as well as its activity, falls to half value in four days, while the hydroxide regains half its final emanating power as well as half its lost activity in the same interval.
It follows from these results that the emanating power of Th X is directly proportional to its activity, i.e. that the rate of production of emanating particles is always proportional to the number of α particles, projected from the Th X per second. The radiation from Th X thus accompanies the change of the Th X into the emanation. Since the emanation has chemical properties distinct from those of the Th X, and also a distinctive rate of decay, it cannot be regarded as a vapour of Th X, but it is a distinct chemical substance, produced by the changes occurring in Th X. On the view advanced in section 136, the atom of the emanation consists of the part of the atom of Th X left behind after the expulsion of one or more α particles. The atoms of the emanation are unstable, and in turn expel α particles. This projection of α particles constitutes the radiation from the emanation, which serves as a measure of the amount of emanation present. Since the activity of the emanation falls to half value in one minute while that of Th X falls to half value in four days, the emanation consists of atoms which disintegrate at intervals nearly 6000 times shorter than those of the atoms of Th X.
Source of the Radium and Actinium Emanation.
=155.= No intermediate stage—Radium X—between radium and its emanation, corresponding to the Th X for thorium, has so far been observed. The emanation from radium is probably produced directly from that element. In this respect, the radium emanation holds the same position in regard to radium as Th X does to thorium, and its production from radium can be explained on exactly similar lines. It will be shown later in chapter X, that the emanation of actinium, like that of thorium, does not arise directly from the parent element but from an intermediate product actinium X, which is very analogous in physical and chemical properties to Th X.
Radiations from the Emanations.
=156.= Special methods are necessary to examine the nature of the radiation from the emanations, for the radiations arise from the volume of the gas in which the emanations are distributed. Some experiments to examine the radiations from the thorium emanation were made by the writer in the following way.
A highly emanating thorium compound wrapped in paper was placed inside a lead box B about 1 cm. deep, shown in Fig. 55. An opening was cut in the top of the box, over which a very thin sheet of mica was waxed. The emanation rapidly diffused through the paper into the vessel, and after ten minutes reached a state of radio-active equilibrium. The penetrating power of the radiation from the emanation which passed through the thin mica window was examined by the electrical method in the usual way by adding screens of thin aluminium foil. The results are expressed in the following table:
Thickness of mica window ·0015 cm. Thickness of aluminium foil ·00034 cm.
Layers of Current foil
0 100
1 59
2 30
3 10
4 3·2
The greater proportion of the conductivity is thus due to α rays, as in the case of the radio-active elements. The amount of absorption of these α rays by aluminium foil is about the same as that of the rays from the active bodies. No direct comparison can be made, for the α rays from the emanation show the characteristic property of increased rate of absorption with thickness of matter traversed. Before testing, the rays have been largely absorbed by the mica window, and the penetrating power has consequently decreased.
No alteration in the radiation from the emanation was observed on placing an insulated wire inside the emanation vessel, and charging it to a high positive or negative potential. When a stream of air through the vessel carried away the emanation as fast as it was produced, the intensity of the radiation fell to a small fraction of its former value.
No evidence of any β rays in the radiations was found in these experiments, although a very small effect would have been detected. After standing some hours, however, β rays began to appear. These were due to the excited activity deposited on the walls of the vessel from the emanation, and not directly to the emanation itself.
The radium emanation, like that of thorium, only gives rise to α rays. This was tested in the following way[249]:
A large amount of emanation was introduced into a cylinder made of sheet copper ·005 cm. thick, which absorbed all the α rays but allowed the β and γ rays, if present, to pass through with but little loss. The external radiation from the cylinder was determined at intervals, commencing about two minutes after the introduction of the emanation. The amount observed at first was extremely small, but increased rapidly and practically reached a maximum in three or four hours. Thus the radium emanation only gives out α rays, the β rays appearing as the excited activity is produced on the walls of the vessel. On sweeping out the emanation by a current of air, there was no immediately appreciable decrease of the radiation. This is another proof that the emanation does not emit any β rays. In a similar way it can be shown that the emanation does not give out γ rays; these rays always make their appearance at the same time as the β rays.
The method of examination of the radiations from the emanations has been given in some detail, as the results are of considerable importance in the discussion, which will be given later in chapters X and XI, of the connection between the changes occurring in radio-active products and the radiations they emit. There is no doubt that the emanations, apart from the excited activity to which they give rise, only give out α rays, consisting most probably of positively charged bodies projected with great velocity.
Effect of pressure on the rate of production of the Emanation.
=157.= It has already been mentioned that the conductivity due to the thorium emanation is proportional to the pressure of the gas, pointing to the conclusion that the rate of production of the emanation is independent of the pressure, as well as of the nature of the surrounding gas. This result was directly confirmed with the apparatus of Fig. 55. When the pressure of the gas under the vessel was slowly reduced, the radiation, tested outside the window, increased to a limit, and then remained constant over a wide range of pressure. This increase, which was far more marked in air than in hydrogen, is due to the fact that the α rays from the emanation were partially absorbed in the gas inside the vessel when at atmospheric pressure. At pressures of the order of 1 millimetre of mercury the external radiation decreased, but experiment showed that this must be ascribed to a removal of the emanation by the pump, and not to a change in the rate of production. The thorium compounds very readily absorb water-vapour, which is slowly given off at low pressures, and in consequence some of the emanation is carried out of the vessel with the water-vapour.
Curie and Debierne[250] found that both the amount of excited activity produced in a closed vessel containing active samples of radium, and also the time taken to reach a maximum value, were independent of the pressure and nature of the gas. This was true in the case of a solution down to the pressure of the saturated vapour, and in the case of solid salts to very low pressures. When the pump was kept going at pressures of the order of ·001 mm. of mercury, the amount of excited activity was much diminished. This was probably not due to any alteration of the rate of escape of the emanation, but to the removal of the emanation by the action of the pump as fast as it was formed.
Since the amount of excited activity, when in a state of radio-active equilibrium, is a measure of the amount of emanation producing it, these results show that the amount of emanation present when the rate of production balances the rate of decay is independent of the pressure and nature of the gas. It was also found that the time taken to reach the point of radio-active equilibrium was independent of the size of the vessel or the amount of active matter present. This proves that the state of equilibrium cannot in any way be ascribed to the possession by the emanation of any appreciable vapour pressure; for if such were the case, the time taken to reach the equilibrium value should depend on the size of the vessel and the amount of active matter present. The results are, however, in agreement with the view that the emanation is present in minute quantity in the tube, and that the equilibrium is governed purely by the radio-active constant λ, the constant of decay of activity of the emanation. This has been seen to be the same under all conditions of concentration, pressure and temperature, and, provided the rate of supply of the emanation from the active compound is not changed, the time-rate of increase of activity to the equilibrium value will always be the same, whatever the size of the vessel or the nature and pressure of the surrounding gas.
Chemical Nature of the Emanations.
=158.= We shall now consider some experiments on the physical and chemical properties of the emanations themselves, without reference to the material producing them, in order to see if they possess any properties which connect them with any known kind of matter.
It was soon observed that the thorium emanation passed unchanged through acid solutions, and later the same result was shown to hold true in the case of both emanations for every reagent that was tried. Preliminary observations[251] showed that the thorium emanation, obtained in the usual way by passing air over thoria, passed unchanged in amount through a platinum tube heated electrically to the highest temperature obtainable. The tube was then filled with platinum-black, and the emanation passed through it in the cold, and with gradually increasing temperatures, until the limit was reached. In another experiment, the emanation was passed through a layer of red-hot lead-chromate in a glass tube. The current of air was replaced by a current of hydrogen, and the emanation was sent through red-hot magnesium-powder and red-hot palladium-black, and, by using a current of carbon dioxide, through red-hot zinc-dust. In every case the emanation passed through without sensible change in the amount. If anything, a slight increase occurred, owing to the time taken for the gas-current to pass through the tubes when hot being slightly less than when cold, the decay en route being consequently less. The only known gases capable of passing in unchanged amount through all the reagents employed are the recently discovered members of the argon family.
But another possible interpretation might be put upon the results. If the emanation were the manifestation of a type of excited radio-activity on the surrounding atmosphere, then, since from the nature of the experiments it was necessary to employ in each case as the atmosphere, a gas not acted on by the reagent employed, the result obtained might be expected. Red-hot magnesium would not retain an emanation consisting of radio-active hydrogen, nor red-hot zinc-dust an emanation consisting of radio-active carbon dioxide. The incorrectness of this explanation was shown in the following way. Carbon dioxide was passed over thoria, then through a =T=-tube, where a current of air met and mixed with it, both passing on to the testing-cylinder. But between this and the =T=-tube a large soda-lime tube was introduced, and the current of gas was thus freed from its admixed carbon dioxide, before being tested in the cylinder for the emanation. The amount of emanation found was quite unchanged, whether carbon dioxide was sent over thoria in the manner described, or whether, keeping the other arrangements as before, an equally rapid current of air was substituted for it. The theory that the emanation is an effect of the excited activity on the surrounding medium is thus excluded.
Experiments of a similar kind on the radium emanation were made later. A steady stream of gas was passed through a radium chloride solution and then through the reagent to be employed, into a testing-vessel of small volume, so that any change in the amount of emanation passing through could readily be detected. The radium emanation, like that of thorium, passed unchanged in amount through every reagent used.
In later experiments by Sir William Ramsay and Mr Soddy[252], the emanation from radium was exposed to still more drastic treatment. The emanation in a glass tube was sparked for several hours with oxygen over alkali. The oxygen was then removed by ignited phosphorus and no visible residue was left. When, however, another gas was introduced, mixed with the minute amount of emanation in the tube and withdrawn, the activity of emanation was found to be unaltered. In another experiment, the emanation was introduced into a magnesium lime tube, which was heated for three hours at a red heat. The emanation was then removed and tested, but no diminution in its discharging power was observed.
The emanations of thorium and radium thus withstand chemical treatment in a manner hitherto unobserved except in gases of the argon family.
=159.= Ramsay and Soddy (loc. cit.) record an interesting experiment to illustrate the gaseous nature of the emanation. A large amount of the radium emanation was collected in a small glass tube. This tube phosphoresced brightly under the influence of the rays from the emanation. The passage of the emanation from point to point was observed in a darkened room by the luminosity excited in the glass. On opening the stop-cock connecting with the Töpler pump, the slow flow through the capillary tube was noticed, the rapid passage along the wider tubes, the delay in passing through a plug of phosphorous pentoxide, and the rapid expansion into the reservoir of the pump. When compressed, the luminosity of the emanation increased, and became very bright as the small bubble containing the emanation was expelled through the fine capillary tube.
Diffusion of the Emanations.
=160.= It has been shown that the emanations of thorium and radium behave like radio-active gases, distributed in minute amount in the air or other gas in which they are tested. With the small quantities of active material so far investigated, the emanations have not yet been collected in sufficient amount to determine their density. Although the molecular weight of the emanations cannot yet be obtained by direct chemical methods, an indirect estimate of it can be made by determining the rate of their inter-diffusion into air or other gases. The coefficients of inter-diffusion of various gases have long been known, and the results show that the coefficient of diffusion of one gas into another is, for the simpler gases, approximately inversely proportional to the square root of the product of their molecular weights. If, therefore, the coefficient of diffusion of the emanation into air is found to have a value, lying between that of two known gases A and B, it is probable that the molecular weight of the emanation lies between that of A and B.
Although the volume of the emanation given off from radium is very small, the electrical conductivity produced by the emanation in the gas, with which it is mixed, is often very large, and offers a ready means of measuring the emanation present.
Some experiments have been made by Miss Brooks and the writer[253] to determine the rate of the diffusion of the radium emanation into air, by a method similar to that employed by Loschmidt[254] in 1871, in his investigations of the coefficient of inter-diffusion of gases.
Fig. 56 shows the general arrangement. A long brass cylinder AB, of length 73 cms., and diameter 6 cms., was divided into two equal parts by a movable metal slide S. The ends of the cylinder were closed with ebonite stoppers. Two insulated brass rods, a and b, each half the length of the tube, passed through the ebonite stoppers and were supported centrally in the tube. The cylinder was insulated and connected with one pole of a battery of 300 volts, the other pole of which was earthed. The central rods could be connected with a sensitive quadrant electrometer. The cylinder was covered with a thick layer of felt, and placed inside a metal box filled with cotton wool in order to keep temperature conditions as steady as possible.
In order to convey a sufficient quantity of emanation into the half-cylinder A, it was necessary to heat the radium slightly. The slide S was closed and the side tubes opened. A slow current of dry air from a gasometer was passed through a platinum tube, in which a small quantity of radium compound was placed. The emanation was carried with the air into the cylinder A. When a sufficient quantity had been introduced, the stream of air was stopped. The side tubes were closed by fine capillary tubes. These prevented any appreciable loss of gas due to the diffusion, but served to keep the pressure of the gas inside A at the pressure of the outside air. The three entrance tubes into the cylinder, shown in the figure, were for the purpose of initially mixing the emanation and gas as uniformly as possible.
After standing several hours to make temperature conditions steady, the slide was opened, and the emanation began to diffuse into the tube B. The current through the tubes A and B was measured at regular intervals by an electrometer, with a suitable capacity in parallel. Initially there is no current in B, but after the opening of the slide, the amount in A decreased and the amount in B steadily increased. After several hours the amount in each half is nearly the same, showing that the emanation is nearly uniformly diffused throughout the cylinder.
It can readily be shown[255] that if
K = coefficient of diffusion of the emanation into air, t = duration of diffusion experiments in secs., a = total length of cylinder, S₁ = partial pressure of emanation in tube A at end of diffusion, S₂ = partial pressure of emanation in tube B at end of diffusion,
then
$$ \frac {S_1 − S_2} {S_1 + S_2} = \frac {8} {\pi^2} (e^{\frac {\pi^2 Kt} {a^2}} + \frac {1}{9}e^{-\frac {9 \pi^2 Kt} {a^2}} + ...) $$ .
Now the values of S₁ and S₂ are proportional to the saturation ionization currents due to the emanations in the two halves of the cylinder. From this equation K can be determined, if the relative values of S₁ and S₂ are observed after diffusion has been in progress for a definite interval t.
The determination of S₁ and S₂ is complicated by the excited activity produced on the walls of the vessel. The ionization due to this must be subtracted from the total ionization observed in each half of the cylinder, for the excited activity is produced from the material composing the emanation, and is removed to the electrodes in an electric field. The ratio of the current due to excited activity to the current due to the emanation depends on the time of exposure to the emanation, and is only proportional to it for exposures of several hours.
The method generally adopted in the experiments was to open the slide for a definite interval, ranging in the experiments from 15 to 120 minutes. The slide was then closed and the currents in each half determined at once. The central rods, which had been kept negatively charged during the experiments, had most of the excited activity concentrated on their surfaces. These were removed, new rods substituted and the current immediately determined. The ratio of the currents in the half cylinders under these conditions was proportional to S₁ and S₂, the amounts of emanation present in the two halves of the cylinder.
The values of K, deduced from different values of t, were found to be in good agreement. In the earlier experiments the values of K were found to vary between ·08 and ·12. In some later experiments, where great care was taken to ensure that temperature conditions were very constant, the values of K were found to vary between ·07 and ·09. The lower value ·07 is most likely nearer the true value, as temperature disturbances tend to give too large a value of K. No certain differences were observed in the value of K whether the air was dry or damp, or whether an electric field was acting or not.
=161.= Some experiments on the rate of diffusion of the radium emanation into air were made at a later date by P. Curie and Danne[256]. If the emanation is contained in a closed reservoir, it has been shown that its activity, which is a measure of the amount of emanation present, decreases according to an exponential law with the time. If the reservoir is put in communication with the outside air through a capillary tube, the emanation slowly diffuses out, and the amount of emanation in the reservoir is found to decrease according to the same law as before, but at a faster rate. Using tubes of different lengths and diameters, the rate of diffusion was found to obey the same laws as a gas. The value of K was found to be 0·100. This is a slightly greater value of K than the lowest value 0·07 found by Rutherford and Miss Brooks. No mention is made by Curie and Danne of having taken any special precautions against temperature disturbances, and this may account for the higher value of K obtained by them.
They also found that the emanation, like a gas, always divided itself between two reservoirs, put in connection with one another, in the proportion of their volumes. In one experiment one reservoir was kept at a temperature of 10° C. and the other at 350° C. The emanation divided itself between the two reservoirs in the same proportion as would a gas under the same conditions.
=162.= For the purpose of comparison, a few of the coefficients of inter-diffusion of gases, compiled from Landolt and Bernstein’s tables, are given below.
Gas or vapour Coefficient of Molecular diffusion into weight air
Water vapour 0·198 18
Carbonic acid 0·142 44 gas
Alcohol vapour 0·101 46
Ether vapour 0·077 74
Radium 0·07 ? emanation
The tables, although not very satisfactory for the purpose of comparison, show that the coefficient of inter-diffusion follows the inverse order of the molecular weights. The value of K for the radium emanation is slightly less than for ether vapour, of which the molecular weight is 74. We may thus conclude that the emanation is of greater molecular weight than 74. It seems likely that the emanation has a molecular weight somewhere in the neighbourhood of 100, and is probably greater than this, for the vapours of ether and alcohol have higher diffusion coefficients compared with carbonic acid than the theory would lead us to anticipate. Comparing the diffusion coefficients of the emanation and carbonic acid into air, the value of the molecular weight of the emanation should be about 176 if the result observed for the simple gases, viz. that the coefficient of diffusion is inversely proportional to the square root of the molecular weights, holds true in the present case. Bumstead and Wheeler[257] compared the rates of diffusion of the radium emanation and of carbon dioxide through a porous plate, and concluded that the molecular weight of the emanation was about 180. On the disintegration theory, the atom of the emanation is derived from the radium atom by the expulsion of one α particle. Thus, it is to be expected that its molecular weight would be over 200.
It is of interest to compare the value of K = ·07 with the value of K determined by Townsend (section 37) for the gaseous ions produced in air at ordinary pressure and temperature, by Röntgen rays or by the radiations from active substances. Townsend found that the value of K in dry air was ·028 for the positive ions and ·043 for the negative ions. The radium emanation thus diffuses more rapidly than the ions produced by its radiation in the gas, and behaves as if its mass were smaller than that of the ions produced in air, but considerably greater than that of the air molecules with which it is mixed.
It is not possible to regard the emanation as a temporarily modified condition of the gas originally in contact with the active body. Under such conditions a much larger value of K would be expected. The evidence derived from the experiments on diffusion strongly supports the view that the emanation is a gas of heavy molecular weight.
Makower[258] has recently attacked the question of the molecular weight of the radium emanation by another method. The rate of diffusion of the emanation through a porous plug of plaster-of-Paris was compared with that of the gases oxygen, carbon dioxide, and sulphur dioxide. It was found that Graham’s law, viz. that the coefficient of diffusion K is inversely proportional to the square root of its molecular weight M, was not strictly applicable. The value of K √M was not found to be constant for these gases, but decreased with increase of molecular weight of the gas. If, however, a curve was plotted with K √M as ordinate and K as abscissa, the points corresponding to the values of O, CO₂ and SO₂ were found to lie on a straight line. By linear extrapolation, the molecular weight of the emanation was estimated. The value obtained from experiments on three different porous plugs was 85·5, 97, and 99 respectively. This method indicates that the molecular weight of the radium emanation is about 100; but in all the experiments on diffusion, it must be remembered that the emanation, whose rate of inter-diffusion is being examined, exists in minute quantity mixed with the gas, and is compared with the rate of inter-diffusion of gases which are present in large quantity. For this reason, deductions of the molecular weight of the emanation may be subject to comparatively large errors, for which it is difficult to make correction.
Diffusion of the Thorium Emanation.
=163.= On account of the rapid decay of the activity of the thorium emanation, it is not possible to determine the value of K its coefficient of diffusion into air by the methods employed for the radium emanation. The value of K has been determined by the writer in the following way. A plate C, Fig. 57, covered with thorium hydroxide, was placed horizontally near the base of a long vertical brass cylinder P. The emanation released from the thorium compound diffuses upwards in the cylinder.
Let p be the partial pressure of the emanation at a distance x from the source C. This will be approximately uniform over the cross section of the cylinder. From the general principles of diffusion we get the equation
d²p dp K ----- = − ---- . dx² dt
The emanation is continuously breaking up and expelling α particles. The emanation-residue gains a positive charge, and, in an electric field, is removed at once from the gas to the negative electrode.
Since the activity of the emanation at any time is always proportional to the number of particles which have not broken up, and since the activity decays with the time according to an exponential law,
$$ p = p_1 e^{–λt} $$,
where p₁ is the value of p when t = 0 and λ is the radio-active constant of the emanation.
Then
dp ---- = -λ_p_, dt
and
d²p K ----- = λ_p_. dx²
Thus
$$ p = Ae^{-\sqrt {\frac {λ} {K} x}} + Be^{\sqrt {\frac {λ} {K} x}} $$ .
Since p = 0 when x = infinity, B = 0. If p = p₀ when x = 0, A = p₀.
Thus
$$ p = p₀ e^{-\sqrt {\frac {λ} {K} x}} $$ .
It was not found convenient in the experiments to determine the activity of the emanation along the cylinder, but an equivalent method was used which depends upon measuring the distribution of “excited activity,” produced along a central rod AB, which is charged negatively.
It will be shown later (section 177) that the amount of excited activity at any point is always proportional to the amount of emanation at that point. The distribution of “excited activity” along the central rod from the plate C upwards thus gives the variation of p for the emanation along the tube.
In the experiments, the cylinder was filled with dry air at atmospheric pressure and was kept at a constant temperature. The central rod was charged negatively and exposed from one to two days in the presence of the emanation. The rod was then removed, and the distribution of the excited activity along it determined by the electric method. It was found that the amount of excited activity fell off with the distance x according to an exponential law, falling to half value in about 1·9 cms. This is in agreement with the above theory.
Since the activity of the emanation falls to half value in 1 minute, λ = ·0115. The value K = ·09 was deduced from the average of a number of experiments. This is a slightly greater value than K = ·07, obtained for the radium emanation, but the results show that the two emanations do not differ much from one another in molecular weight.
Makower (loc. cit.) compared the rates of diffusion of the thorium and radium emanation through a porous plate, and concluded that the two emanations were of about the same molecular weight, thus confirming the results obtained by the above method.
Diffusion of the Emanation into Liquids.
=164.= Experiments have been made by Wallstabe[259] on the coefficient of diffusion of the radium emanation into various liquids. The radium emanation was allowed to diffuse into a closed reservoir, containing a cylinder of the liquid under observation. The cylinder was provided with a tube and a stop-cock extending beyond the closed vessel, so that different layers of the liquid could be removed. The liquid was then placed in a closed testing vessel, where the ionization current due to the escape of the emanation from the liquid was observed to rise to a maximum after several hours, and then to decay. This maximum value of the current was taken as a measure of the amount of emanation absorbed in the liquid.
The coefficient of diffusion K of the emanation into the liquid can be obtained from the same equation used to determine the diffusion of the thorium emanation into air,
$$ p = p₀ e^{-\sqrt {\frac {λ} {K} x}} $$ .
where λ is the constant of decay of activity of the radium emanation and x the depth of the layer of water from the surface.
Putting
$$ a = \sqrt {\frac {λ} {K}} $$,
it was found that
for water α = 1·6, for toluol α = ·75.
The value of λ expressed in terms of a day as the unit of time is about ·17.
Thus the value of K for the diffusion of the radium emanation into water = ·066 cm.² / day.
The value of K found by Stefan[260] for the diffusion of carbon dioxide into water was 1·36 cm.²/day. These results are thus in harmony with the conclusion drawn from the diffusion of the radium emanation into air, and show that the radium emanation behaves as a gas of high molecular weight.
Condensation of the Emanations.
=165. Condensation of the emanations.= During an investigation of the effect of physical and chemical agencies on the thorium emanation, Rutherford and Soddy[261] found that the emanation passed unchanged in amount through a white-hot platinum tube and through a tube cooled to the temperature of solid carbon dioxide. In later experiments the effects of still lower temperatures were examined, and it was then found that at the temperature of liquid air both emanations were condensed[262].
If either emanation is conveyed by a slow stream of hydrogen, oxygen, or air through a metal spiral immersed in liquid air, and placed in connection with a testing vessel as in Fig. 51, no trace of emanation escapes in the issuing gas. When the liquid air is removed and the spiral plunged into cotton-wool, several minutes elapse before any deflection of the electrometer needle is observed, and then the condensed emanation volatilizes rapidly, and the movement of the electrometer needle is very sudden, especially in the case of radium. With a fairly large amount of radium emanation, under the conditions mentioned, a very few seconds elapse after the first sign of movement before the electrometer needle indicates a deflection of several hundred divisions per second. It is not necessary in either case that the emanating compound should be retained in the gas stream. After the emanation is condensed in the spiral, the thorium or radium compound may be removed and the gas stream sent directly into the spiral. But in the case of thorium, under these conditions, the effects observed are naturally small owing to the rapid loss of the activity of the emanation with time, which proceeds at the same rate at the temperature of liquid air as at ordinary temperatures.
If a large amount of radium emanation is condensed in a glass =U= tube, the progress of the condensation can be followed by the eye, by means of the phosphorescence which the radiations excite in the glass. If the ends of the tube are sealed and the temperature allowed to rise, the glow diffuses uniformly throughout the tube, and can be concentrated at any point to some extent by local cooling of the tube with liquid air.
=166. Experimental arrangements.= A simple experimental arrangement to illustrate the condensation and volatilization of the emanation and some of its characteristic properties is shown in Fig. 58. The emanation obtained from a few milligrams of radium bromide by solution or heating is condensed in the glass =U= tube T immersed in liquid air. This =U= tube is then put into connection with a larger glass tube V, in the upper part of which is placed a piece of zinc sulphide screen Z, and in the lower part of the tube a piece of the mineral willemite. The stop-cock A is closed and the =U= tube and the vessel V are partially exhausted by a pump through the stop-cock B. This lowering of the pressure causes a more rapid diffusion of the emanation when released. The emanation does not escape if the tube T is kept immersed in liquid air. The stop-cock B is then closed, and the liquid air removed. No luminosity of the screen or the willemite in the tube V is observed for several minutes, until the temperature of T rises above the point of volatilization of the emanation. The emanation is then rapidly carried into the vessel V, partly by expansion of the gas in the tube T with rising temperature, and partly by the process of diffusion. The screen Z and the willemite W are caused to phosphoresce brilliantly under the influence of the rays from the emanation surrounding them.
If the end of the vessel V is then plunged into liquid air, the emanation is again condensed in the lower end of the tube, and the willemite phosphoresces much more brightly than before. This is not due to an increase of the phosphorescence of willemite at the temperature of the liquid air, but to the effect of the rays from the emanation condensed around it. At the same time the luminosity of the zinc sulphide gradually diminishes, and practically disappears after several hours if the end of the tube is kept in the liquid air. If the tube is removed from the liquid air, the emanation again volatilizes and lights up the screen Z. The luminosity of the willemite returns to its original value after the lapse of several hours. This slow change of the luminosity of the zinc sulphide screen and of the willemite is due to the gradual decay of the “excited activity” produced by the emanation on the surface of all bodies exposed to its action (chapter VIII). The luminosity of the screen is thus due partly to the radiation from the emanation and partly to the excited radiation caused by it. As soon as the emanation is removed from the upper to the lower part of the tube, the “excited” radiation gradually diminishes in the upper and increases in the lower part of the tube.
The luminosity of the screen gradually diminishes with the time as the enclosed emanation loses its activity, but is still appreciable after an interval of several weeks.
An apparatus of a similar character to illustrate the condensation of the radium emanation has been described by P. Curie[263].
=167. Determination of the temperature of condensation.= A detailed investigation was made by Rutherford and Soddy (loc. cit.) of the temperatures at which condensation and volatilization commenced for the two emanations. The experimental arrangement of the first method is shown clearly in Fig. 59. A slow constant stream of gas, entering at A, was passed through a copper spiral S, over 3 metres in length, immersed in a bath of liquid ethylene. The copper spiral was made to act as its own thermometer by determining its electrical resistance. The resistance temperature curve was obtained by observation of the resistances at 0°, the boiling point of liquid ethylene −103·5°, the solidification point of ethylene −169° and in liquid air. The temperature of the liquid air was deduced from the tables given by Baly for the boiling point of liquid air for different percentages of oxygen. The resistance-temperature curve, for the particular spiral employed, was found to be nearly a straight line between 0° and −192°C., cutting the temperature axis if produced nearly at the absolute zero. The resistance of the spiral, deduced from readings on an accurately calibrated Weston millivoltmeter, with a constant current through the spiral, was thus very approximately proportional to the absolute temperature. The liquid ethylene was kept vigorously stirred by an electric motor, and was cooled to any desired temperature by surrounding the vessel with liquid air.
The general method employed for the radium emanation was to pass a suitable amount of emanation, mixed with the gas to be used, from the gas holder B into the spiral, cooled below the temperature of condensation. After the emanation was condensed in the spiral, a current of electrolytic hydrogen or oxygen was passed through the spiral. The temperature was allowed to rise gradually, and was noted at the instant when a deflection of the electrometer, due to the presence of emanation in the testing vessel T, was observed. The resistance, subject to a slight correction due to the time taken for the emanation to be carried into the testing vessel, gave the temperature at which some of the emanation commenced to volatilize. The ionization current in the testing vessel rose rapidly to a maximum value, showing that, for a small increase of temperature, the whole of the radium emanation was volatilized. The following table gives an illustration of the results obtained for a current of hydrogen of 1·38 cubic centimetres per second.
Temperature Divisions per second of the electrometer
−160° 0
−156° 0
−154°·3 1
−153°·8 21
−152°·5 24
The following table shows the results obtained for different currents of hydrogen and oxygen.
Current of Gas T₁ T₂
Hydrogen ·25 c.c. per sec. −151·3 −150 „ ·32 „ „ −153·7 −151 „ ·92 „ „ −152 −151 „ 1·38 „ „ −154 −153 „ 2·3 „ „ −162·5 −162 Oxygen ·34 „ „ −152·5 −151·5 „ ·58 „ „ −155 −153
The temperature T₁ in the above table gives the temperature of initial volatilization, T₂ the temperature for which half of the condensed emanation had been released. For slow currents of hydrogen and oxygen, the values of T₁ and T₂ are in good agreement. For a stream of gas as rapid as 2·3 cubic centimetres per second the value of T₁ is much lower. Such a result is to be expected; for, in too rapid a stream, the gas is not cooled to the temperature of the spiral, and, in consequence, the inside surface of the spiral is above the mean temperature, and some of the emanation escapes at a temperature apparently much lower. In the case of oxygen, this effect appears for a gas stream of 0·58 cubic centimetres per second.
In the experiments on the thorium emanation, on account of the rapid loss of activity, a slightly different method was necessary. The steady stream of gas was passed over the thorium compound, and the temperature was observed at the instant when an appreciable movement of the electrometer appeared. This gave the temperature at which a small fraction of the thorium emanation escaped condensation, and not the value T₁ observed for the radium emanation, which gave the temperature for which a small fraction of the previously condensed emanation was volatilized.
The following table illustrates the results obtained.
Current of Gas Temperature
Hydrogen ·71 c.c. per −155° C. sec.
„ 1·38 „ „ −159° C.
Oxygen ·58 „ „ −155° C.
On comparing these results with the values obtained for the radium emanation, it will be observed that with equal gas streams the temperatures are nearly the same.
A closer examination of the thorium emanation showed, however, that this apparent agreement was only accidental, and that there was, in reality, a very marked difference in the effect of temperature on the two emanations. It was found experimentally that the radium emanation was condensed very near the temperature at which volatilization commenced, and that the points of condensation and volatilization were defined fairly sharply.
On the other hand, the thorium emanation required a range of over 30° C. after condensation had started in order to ensure complete condensation. Fig. 60 is an example of the results obtained with a steady gas stream of 1·38 c.c. per sec. of oxygen. The ordinates represent the percentage proportion of the emanation uncondensed at different temperatures. It will be observed that condensation commences about −120°, and that very little of the emanation escapes condensation at −155° C.
To investigate this difference of behaviour in the two emanations, a static method was employed, which allowed an examination of the two emanations to be made under comparable conditions. The emanation, mixed with a small amount of the gas to be used, was introduced into the cool spiral, which had been exhausted previously by means of a mercury pump. The amount of emanation remaining uncondensed after definite intervals was rapidly removed by means of the pump, and was carried with a constant auxiliary stream of gas into the testing vessel.
Tested in this way, it was found that the volatilization point of the radium emanation was very nearly the same as that obtained by the blowing method, viz. −150° C. With thorium, on the other hand, the condensation started at about −120° C., and, as in the blowing method, continued over a range of about 30° C. The proportion of the emanation condensed at any temperature was found to depend on a variety of conditions, although the point at which condensation commenced, viz. −120° C., was about the same in each case. It depended on the pressure and nature of the gas, on the concentration of the emanation, and on the time for which it was left in the spiral. For a given temperature a greater proportion of the emanation was condensed, the lower the pressure and the longer the time it was left in the spiral. Under the same conditions, the emanation was condensed more rapidly in hydrogen than in oxygen.
=168.= Thus there is no doubt that the thorium emanation begins to condense at a temperature higher than that at which the radium emanation condenses. The explanation of the peculiar behaviour of the thorium emanation is clear when the small number of emanation particles present in the gas are taken into consideration. It has been shown that both emanations give out only α rays. It is probable that the α particles from the two emanations are similar in character and produce about the same number of ions in their passage through the gas. The number of ions produced by each α particle before its energy is dissipated is probably about 70,000. (See section 252.)
Now, in the experiment, the electrometer readily measured a current of 10⁻³ electrostatic units. Taking the charge on an ion as 3·4 × 10⁻¹⁰ electrostatic units, this corresponds to a production in the testing vessel of about 3 × 10⁶ ions per sec., which would be produced by about 40 expelled α particles per second. Each radiating particle cannot expel less than one α particle and may expel more, but it is likely that the number expelled by an atom of the thorium emanation is not greatly different from that expelled by an atom of the radium emanation.
In section 133 it has been shown that, according to the law of decay, λ_N_ particles change per second when N are present. Thus, to produce 40 α particles, λ_N_ cannot be greater than 40. Since for the thorium emanation λ is ¹⁄₈₇, it follows that N cannot be greater than 3500. The electrometer thus detected the presence of 3500 particles of the thorium emanation, and since in the static method the volume of the condensing spiral was about 15 c.c., this corresponded to a concentration of about 230 particles per c.c. An ordinary gas at atmospheric pressure and temperature probably contains about 3·6 × 10¹⁹ molecules per c.c. Thus the emanation would have been detected on the spiral if it had possessed a partial pressure of less than 10⁻¹⁷ of an atmosphere.
It is not surprising then that the condensation point of the thorium emanation is not sharply defined. It is rather a matter of remark that condensation should occur so readily with so sparse a distribution of emanation particles in the gas; for, in order that condensation may take place, it is probable that the particles must approach within one another’s sphere of influence.
Now in the case of the radium emanation, the rate of decay is about 5000 times slower than that of the thorium emanation, and consequently the actual number of particles that must be present to produce the same ionization per second in the two cases must be about 5000 times greater in the case of radium than in the case of thorium. This conclusion involves only the assumption that the same number of rays is produced by a particle of emanation in each case, and that the expelled particles produce in their passage through the gas the same number of ions. The number of particles present, in order to be detected by the electrometer, in this experiment, must therefore have been about 5000 × 3500, i.e. about 2 × 10⁷. The difference of behaviour in the two cases is well explained by the view that, for equal electrical effects, the number of radium emanation particles must be far larger than the number of thorium emanation particles. The probability of the particles coming into each other’s sphere of influence will increase very rapidly as the concentration of the particles increases, and, in the case of the radium emanation, once the temperature of condensation is attained, all but a small proportion of the total number of particles present will condense in a very short time. In the case of the thorium emanation, however, the temperature might be far below that of condensation, and yet a considerable portion remain uncondensed for comparatively long intervals. On this view the experimental results obtained might reasonably be expected. A greater proportion of emanation condenses the longer the time allowed for condensation under the same conditions. The condensation occurs more rapidly in hydrogen than in oxygen, as the diffusion is greater in the former gas. For the same reason the condensation occurs faster the lower the pressure of the gas present. Finally, when the emanation is carried by a steady stream of gas, a smaller proportion condenses than in the other cases, because the concentration of emanation particles per unit volume of gas is less under these conditions.
It is possible that the condensation of the emanations may not occur in the gas itself but at the surface of the containing vessel. Accurate observations of the temperature of condensation have so far only been made in a copper spiral, but condensation certainly occurs in tubes of lead or glass at about the same temperature as in tubes of copper.
=169.= In experiments that were made by the static method with a very large quantity of radium emanation, a slight amount of escape of the condensed emanation was observed several degrees below the temperature at which most of the emanation was released. This is to be expected, since, under such conditions, the electrometer is able to detect a very minute proportion of the whole quantity of the emanation condensed.
Special experiments, with a large quantity of emanation, that were made with the spiral immersed in a bath of rapidly boiling nitric oxide, showed this effect very clearly. For example, the condensed emanation began to volatilize at −155° C. In 4 minutes the temperature had risen to −153·5°, and the amount volatilized was four times as great as at −155°. In the next 5½ minutes the temperature had increased to −152·3° and practically the whole quantity, which was at least fifty times the amount at the temperature of −153·5°, had volatilized.
It thus seems probable that, if the temperature were kept steady at the point at which volatilization was first observed, and the released emanation removed at intervals, the whole of the emanation would in course of time be liberated at that temperature. Curie and Dewar and Ramsay have observed that the emanation condensed in a =U= tube, immersed in liquid air, slowly escapes if the pump is kept steadily working. These results point to the probability that the condensed emanation possesses a true vapour pressure, but great refinements in experimental methods would be necessary before such a conclusion could be definitely established.
The true temperature of condensation of the thorium emanation is probably about −120° C., and that of radium about −150° C. Thus there is no doubt that the two emanations are quite distinct from each other in this respect, and also with regard to their radio-activity, although they both possess the property of chemical inertness. These results on the temperatures of condensation do not allow us to make any comparison of the condensation points of the emanations with those of known gases, since the lowering of the condensation points of gases with diminution of pressure has not been studied at such extremely minute pressures.
=170.= It has been found[264] that the activity of the thorium emanation, when condensed in the spiral at the temperature of liquid air, decayed at the same rate as at ordinary temperatures. This is in accord with results of a similar kind obtained by P. Curie for the radium emanation (section 145), and shows that the value of the radio-active constant is unaffected by wide variations of temperature.
Amount of Emanation from Radium and Thorium.
=171.= It has been shown in section 93 from experimental data that 1 gram of radium bromide at its minimum activity emits about 3·6 × 10¹⁰ α particles per second. Since the activity due to the emanation stored up in radium, when in a state of radio-active equilibrium, is about one quarter of the whole and about equal to the minimum activity, the number of α particles projected per second by the emanation from 1 gram of radium bromide is about 3·6 × 10¹⁰. It has been shown in section 152 that 463,000 times the amount of emanation produced per second is stored up in the radium. But, in a state of radio-active equilibrium, the number of emanation particles breaking up per second is equal to the number produced per second. Assuming that each emanation particle in breaking up expels one α particle, it follows that the number of emanation particles present in 1 gram of radium bromide in radio-active equilibrium is 463,000 × 3·6 × 10¹⁰, i.e. 1·7 × 10¹⁶. Taking the number of hydrogen molecules in 1 c.c. of gas at atmospheric pressure and temperature as 3·6 × 10¹⁹ (section 39), the volume of the emanation from 1 gram of radium bromide is 4·6 × 10⁻⁴ cubic centimetres at atmospheric pressure and temperature. Assuming the composition of radium bromide as RaBr₂, the amount from 1 gram of radium in radio-active equilibrium is 0·82 cubic millimetres. Quite independently of any method of calculation it was early evident that the volume of the emanation was very small, for all the earlier attempts made to detect its presence by its volume were unsuccessful. It will be seen, however, that, when larger quantities of radium were available for experiment, the emanation has been collected in volume sufficiently large to measure.
In the case of thorium, the maximum quantity of emanation to be obtained from 1 gram of the solid is very minute, both on account of the small activity of thorium and of the rapid break up of the emanation after its production. Since the amount of emanation, stored in a non-emanating thorium compound, is only 87 times the rate of production, while in radium it is 463,000 times, and the rate of production of the emanation by radium is about 1 million times faster than by thorium, it follows that the amount of emanation to be obtained from 1 gram of thorium is not greater than 10⁻¹⁰ of the amount from an equal weight of radium, i.e. its volume is not greater than 10⁻¹³ c.c. at the ordinary pressure and temperature. Even with large quantities of thorium, the amount of emanation is too small ever to be detected by its volume.
=172. Volume of the emanation from radium.= The evidence already considered points very strongly to the conclusion that the emanation possesses all the properties of a chemically inert gas of high molecular weight.
Since the emanation continuously breaks up, and is transformed into a solid type of matter, which is deposited on the surface of bodies, the volume of the emanation, when separated from radium, should contract at the same rate as it loses its activity, i.e. it should decrease to half value in about four days. The amount of emanation to be obtained from a given quantity of radium is a maximum when the rate of production of new emanation balances its rate of change. This condition is practically attained when the emanation has been allowed to collect for an interval of one month. The probable volume of the emanation to be obtained from 1 gram of radium was early calculated on certain assumptions, and from data then available the writer[265] deduced that the volume of the emanation from 1 gram of radium lay between ·06 and ·6 cubic millimetre at atmospheric pressure and temperature, and was probably nearer the latter value. The volume to be expected on the latest data has been discussed in the preceding section and shown to be about ·82 cubic mm. The volume of the emanation is thus very small, but not too small to be detected if several centigrams of radium are available. This has been proved to be the case by Ramsay and Soddy[266] who, by very careful experiment, finally succeeded in isolating a small quantity of the emanation and in determining its volume. The experimental method employed by them will now be briefly described.
The emanation from 60 milligrams of radium bromide in solution was allowed to collect for 8 days and then drawn off through the inverted siphon E (Fig. 61) into the explosion burette F. This gas consisted for the most part of hydrogen and oxygen, produced by the action of the radiations on the water of the solution. After explosion, the excess of hydrogen mixed with emanation was left some time in contact with caustic soda, placed in the upper part of the burette, in order to remove all trace of carbon dioxide. In the meantime the upper part of the apparatus had been completely evacuated. The connection C to the pump was closed, and the hydrogen and emanation were allowed to enter the apparatus, passing over a phosphorous pentoxide tube D. The emanation was condensed in the lower part of the capillary tube A, by surrounding it with the tube B filled with liquid air. The process of condensation was rendered manifest by the brilliant luminosity of the lower part of the tube. The mercury from the burette was then allowed to run to G, and the apparatus again completely evacuated. The connection of the pump was again closed, the liquid air was removed and the volatilized emanation forced into the fine capillary tube A. Observations were then made, from day to day, of the volume of the emanation. The results are given in the table below.
Time Volume Time Volume
Start 0·124 cub. 7 days 0·0050 cub. mm. mm.
1 day 0·027 „ 9 „ 0·0041 „
3 „ 0·011 „ 11 „ 0·0020 „
4 „ 0·0095 „ 12 „ 0·0011 „
6 „ 0·0063 „ 28 „ 0·0004 „
The volume contracted with the time, and was very small after a month’s interval, but the minute bubble of the emanation still retained its luminosity to the last. The tube became deep purple in colour, which rendered readings difficult except with a strong light. There was a sudden decrease in the first day, which may have been due to the mercury sticking in the capillary tube.
The experiments were repeated with another capillary tube and the volume of gas observed at normal pressure was 0·0254 c. mm. The gas obtained was found to obey Boyle’s law within the limit of experimental error over a considerable range of pressure. But, unlike in the first experiment, the gas did not contract but expanded rapidly during the first few hours, and then more slowly, finally reaching a volume after 23 days of 0·262 c. mm. or about 10 times the initial volume. The measurements were complicated by the appearance of bubbles of gas in the top of the mercury column. The differences observed in these two experiments are difficult to account for. We shall see, later, that the emanation always produces helium, and, in the first experiment, the decrease of the volume to zero indicates that the helium was buried or absorbed in the walls of the tube. In the second case, probably owing to some difference in the glass of the capillary tube, the helium may have been released. This suggestion is confirmed by the observation that the volume of gas, after the experiment ended, gave a brilliant spectrum of helium.
We shall see later that there is considerable evidence that the α particles expelled from radio-active substances consist of helium atoms. Since the particles are projected with great velocity, they will first be buried in the walls of the tube, and then may gradually diffuse out into the gas again under conditions probably depending on the kind of glass employed. Since α particles are projected from the emanation and also from two of the rapidly changing products which arise from it, the volume of helium should, on this view, be three times the initial volume of the emanation. If the helium produced escaped from the walls of the tube into the gas, the apparent volume of the gas in the capillary should increase to three times the initial volume in a month’s interval, for during that time the emanation itself has been transformed into a solid type of matter deposited on the walls of the tube.
Ramsay and Soddy concluded from their experiments that the maximum volume of emanation to be obtained from 1 gram of radium was about 1 cubic millimetre at standard pressure and temperature, and that the emanation was produced from 1 gram of radium at the rate of 3 × 10⁻⁶ c. mm. per second. This amount is in very good agreement with the calculated value, and is a strong indication of the general correctness of the theory on which the calculations are based.
=173. Spectrum of the emanation.= After the separation of the emanation and the determination of its volume, Ramsay and Soddy made numerous attempts to obtain its spectrum. In some of the earlier experiments several bright lines were seen for a short time, but these lines were soon masked by the appearance of the hydrogen lines. In later experiments Ramsay and Collie[267] succeeded in obtaining a spectrum of the emanation, which persisted for a short time, during which a rapid determination of the wave-lengths was made. They state that the spectrum was very brilliant, consisting of very bright lines, the spaces between being perfectly dark. The spectrum bore a striking resemblance in general character to the spectrum of the gases of the argon family.
The spectrum soon faded, and the spectrum of hydrogen began to appear. The following table shows the wave-length of the lines observed in the spectrum. The degree of coincidence of the lines of known wave-lengths shows that the error is probably less than five Ångström units.
Wave-length Remarks
6567 Hydrogen C; true wave-length, 6563; observed each time.
6307 Observed only at first; evanescent.
5975 „ „ „
5955 „ „ „
5805 Observed each time; persistent.
5790 Mercury; true wave-length, 5790.
5768 „ „ 5769.
5725 Observed only at first; evanescent.
5595 Observed each time; persistent and strong.
5465 Mercury; true wave-length, 5461.
5105 Not observed at first; appeared after some seconds; persisted and was visible during the second examination.
4985 Observed each time; persistent and strong.
4865 Hydrogen F; true wave-length, 4861.
4690 Observed only at first.
4650 Not observed when the emanation was examined again.
4630 „ „ „
4360 Mercury: true wave-length, 4359.
The experiments were repeated with a new supply of emanation, and some of the stronger lines were observed again, while some new lines made their appearance. Ramsay and Collie suggest that the strong line 5595 may be identical with a line which was observed by Pickering[268] in the spectrum of lightning, and was not identified with the spectrum of any known gas.
Until large quantities of radium are available for the experimenter it would appear difficult to make sure how many of these lines must be ascribed to the spectrum of the emanation or to measure the wave-lengths with accuracy.
The results are of great interest, as showing that the emanation has a definite and new spectrum of the same general character as the argon group of gases to which, as we have seen, it is chemically allied.
Summary of Results.
=174.= The investigations into the nature of the radio-active emanations have thus led to the following conclusions:—The radio-elements thorium, radium and actinium continuously produce from themselves radio-active emanations at a rate which is constant under all conditions. In some cases, the emanations continuously diffuse from the radio-active compounds into the surrounding gas; in other cases, the emanations are unable to escape from the material in which they are produced, but are occluded, and can only be released by solution or by the action of heat.
The emanations possess all the properties of radio-active gases. They diffuse through gases, liquids, and porous substances, and can be occluded in some solids. Under varying conditions of pressure, volume, and temperature, the emanations distribute themselves in the same way and according to the same laws as does a gas.
The emanations possess the important property of condensation under the influence of extreme cold, and by that means can be separated from the gases with which they are mixed. The radiation from the emanation is material in nature, and consists of a stream of positively charged particles projected with great velocity.
The emanations possess the property of chemical inertness, and in this respect resemble the gases of the argon family. The emanations are produced in minute amount; but a sufficient quantity of the radium emanation has been obtained to determine its volume and its spectrum. With regard to their rates of diffusion, the emanations of both thorium and radium behave like gases of high molecular weight.
These emanations have been detected and their properties investigated by the property they possess of emitting radiations of a special character. These radiations consist entirely of α rays, i.e. particles, projected with great velocity, which carry a positive charge and have a mass about twice that of the hydrogen atom. The emanations do not possess the property of permanently radiating, but the intensity of the radiations diminishes according to an exponential law with the time, falling to half value, from actinium in 4 seconds, from thorium in one minute, and from radium in about four days. The law of decay of activity does not seem to be influenced by any physical or chemical agency.
The emanation particles gradually break up, each particle as it breaks up expelling a charged body. The emanation after it has radiated ceases to exist as such, but is transformed into a new kind of matter, which is deposited on the surface of bodies and gives rise to the phenomena of excited activity. This last property, and the connection of the emanation with it, are discussed in detail in the next chapter.
Footnote 231:
Owens, Phil. Mag. p. 360, Oct. 1899.
Footnote 232:
Rutherford, Phil. Mag. p. 1, Jan. 1900.
Footnote 233:
Rossignol and Gimingham, Phil. Mag. July, 1904.
Footnote 234:
Bronson, Amer. Journ. Science, Feb. 1905.
Footnote 235:
Phil. Mag. April, 1904.
Footnote 236:
Dorn, Abh. der. Naturforsch. Ges. für Halle-a-S., 1900.
Footnote 237:
P. Curie, C. R. 135, p. 857, 1902.
Footnote 238:
Rutherford and Soddy, Phil. Mag. April, 1903.
Footnote 239:
P. Curie, C. R. 136, p. 223, 1903.
Footnote 240:
Debierne, C. R. 136, p. 146, 1903.
Footnote 241:
Giesel, Ber. D. deutsch. Chem. Ges. p. 3608, 1902.
Footnote 242:
Curie and Debierne, C. R. 132, pp. 548 and 768, 1901.
Footnote 243:
Curie and Debierne, C. R. 133, p. 931, 1901.
Footnote 244:
Rutherford and Soddy, Trans. Chem. Soc. p. 321, 1902. Phil. Mag. Sept. 1902.
Footnote 245:
Rutherford, Phys. Zeit. 2, p. 429, 1901.
Footnote 246:
Rutherford and Soddy, Phil. Mag. Nov. 1902.
Footnote 247:
Rutherford and Soddy, Phil. Mag. April, 1903.
Footnote 248:
Rutherford and Soddy, Phil. Mag. Nov. 1902.
Footnote 249:
Rutherford and Soddy, Phil. Mag. April, 1903.
Footnote 250:
Curie and Debierne, C. R. 133, p. 931, 1901.
Footnote 251:
Rutherford and Soddy, Phil. Mag. Nov. 1902.
Footnote 252:
Ramsay and Soddy, Proc. Roy. Soc. 72, p. 204, 1903.
Footnote 253:
Rutherford and Miss Brooks, Trans. Roy. Soc. Canada 1901, Chem. News 1902.
Footnote 254:
Loschmidt, Sitzungsber. d. Wien. Akad. 61, II. p. 367, 1871.
Footnote 255:
See Stefan, Sitzungsber. d. Wien. Akad. 63, II. p. 82, 1871.
Footnote 256:
P. Curie and Danne, C. R. 136, p. 1314, 1903.
Footnote 257:
Bumstead and Wheeler, Amer. Jour. Science, Feb. 1904.
Footnote 258:
Makower, Phil. Mag. Jan. 1905.
Footnote 259:
Wallstabe, Phys. Zeit. 4, p. 721, 1903.
Footnote 260:
Stefan, Wien. Ber. 2, p. 371, 1878.
Footnote 261:
Rutherford and Soddy, Phil. Mag. Nov. 1902.
Footnote 262:
Phil. Mag. May, 1903.
Footnote 263:
P. Curie, Société de Physique, 1903.
Footnote 264:
Rutherford and Soddy, Phil. Mag. May, 1903.
Footnote 265:
Nature, Aug. 20, 1903.
Footnote 266:
Proc. Roy. Soc. 73, No. 494, p. 346, 1904.
Footnote 267:
Proc. Roy. Soc. 73, No. 495, p. 470, 1904.
Footnote 268:
Pickering, Astrophys. Journ. Vol. 14, p. 368, 1901.