XIV. RADIO-ACTIVITY OF THE ATMOSPHERE AND OF ORDINARY MATERIALS

XIV. RADIO-ACTIVITY OF THE ATMOSPHERE AND OF ORDINARY MATERIALS

=273. Radio-activity of the atmosphere.= The experiments of Geitel[384] and C. T. R. Wilson[385] in 1900 showed that a positively or negatively charged conductor placed inside a closed vessel gradually lost its charge. This loss of charge was shown to be due to a small ionization of the air inside the vessel. Elster and Geitel also found that a charged body exposed in the open air lost its charge rapidly, and that the rate of discharge was dependent on the locality and on atmospheric conditions. A more detailed description and discussion of these results will be given later in section 284.

In the course of these experiments, Geitel observed that the rate of discharge increased slightly for some time after the vessel had been closed. He considered that this might possibly be due to the existence of some radio-active substances in the air, which produced excited activity on the walls of the vessel and so increased the rate of dissipation of the charge. In 1901 Elster and Geitel[386] tried the bold experiment of seeing whether it were possible to extract a radio-active substance from the air. The experiments of the writer had shown that the excited radio-activity from the thorium emanation could be concentrated on the negative electrode in a strong electric field. This result indicated that the carriers of the radio-activity had a positive charge of electricity. Elster and Geitel therefore tried an experiment to see whether positively charged carriers, possessing a similar property, were present in the atmosphere. For this purpose a cylinder of wire-netting, charged negatively to 600 volts, was exposed for several hours in the open air. The cylinder was then removed, and quickly placed in a large bell-jar, inside which was placed an electroscope to detect the rate of discharge. It was found that the rate of discharge was increased to a slight extent. In order to multiply the effect a wire about 20 metres in length was exposed at some height from the ground, and was kept charged to a high potential by connecting it to the negative terminal of an influence machine. After exposure for some hours, this wire was removed and placed inside the dissipation vessel. The rate of discharge was found to be increased many times by the presence of the wire. No increase was observed when the wire was charged positively instead of negatively. The results also showed that the radio-active matter could be removed from the wire in the same way as from a wire made active by exposure in the presence of the thorium emanation. A piece of leather moistened with ammonia was rubbed over the active wire. On testing the leather, it was found to be strongly radio-active. When a long wire was used, the amount of activity obtained on the leather was comparable with that possessed by a gram of uranium oxide.

The activity produced on the wire was not permanent, but disappeared to a large extent in the course of a few hours. The amount of activity produced on a wire of given size, exposed under similar conditions, was independent of the material of the wire. Lead, iron and copper wires gave about equal effects.

The amount of activity obtained was greatly increased by exposing a negatively charged wire in a mass of air which had been undisturbed for a long time. Experiments were made in the great cave of Wolfenbüttel, and a very large amount of activity was observed. By transferring the activity to a piece of leather it was found that the rays could appreciably light up a screen of barium platinocyanide in the dark[387]. The rays also darkened a photographic plate through a piece of aluminium 0·1 mm. in thickness.

These remarkable experiments show that the excited radio-activity obtained from the atmosphere is very similar in character to the excited activity produced by the emanations of radium and thorium. No investigators have contributed more to our knowledge of the radio-activity and ionization of the atmosphere than Elster and Geitel. The experiments here described have been the starting-point of a series of researches by them and others on the radio-active properties of the atmosphere, which have led to a great extension of our knowledge of that important subject.

Rutherford and Allan[388] determined the rate of decay of the excited activity produced on a negatively charged wire exposed in the open air. A wire about 15 metres long was exposed in the open air, and kept charged by an influence machine to a potential of about −10,000 volts. An hour’s exposure was sufficient to obtain a large amount of excited activity on the wire. The wire was then rapidly removed and wound on a framework which formed the central electrode in a large cylindrical metal vessel. The ionization current for a saturation voltage was measured by means of a sensitive Dolezalek electrometer. The current, which is a measure of the activity of the wire, was found to diminish according to an exponential law with the time, falling to half value in about 45 minutes. The rate of decay was independent of the material of the wire, of the time of exposure, and of the potential of the wire.

An examination was also made of the nature of the rays emitted by the radio-active wire. For this purpose a lead wire was made radio-active in the manner described, and then rapidly wound into the form of a flat spiral. The penetrating power of the rays was tested in a vessel similar to that shown in Fig. 17. Most of the ionization was found to be due to some very easily absorbed rays, which were of a slightly more penetrating character than the α rays emitted from a wire made active by the radium or thorium emanations. The intensity of the rays was cut down to half value by about 0·001 cm. of aluminium. The photographic action observed by Elster and Geitel through 0·1 mm. of aluminium showed that some penetrating rays were also present. This was afterwards confirmed by Allan, who used the electric method. These penetrating rays are probably similar in character to the β rays from the radio-elements.

=274.= The excited activity produced on the negatively charged wire cannot be due to an action of the strong electric field on the surface of the wire; for very little excited activity is produced if the wire is charged to the same potential inside a closed cylinder.

We have seen that the excited activity produced on the wire can be partially removed by rubbing and by solution in acids, and, in this respect, it is similar to the excited activity produced in bodies by the emanations of radium and thorium. The very close similarity of the excited activity obtained from the atmosphere to that obtained from the radium and thorium emanations suggests the probability that a radio-active emanation exists in the atmosphere. This view is confirmed by a large amount of indirect evidence discussed in sections 276, 277 and 280.

Assuming the presence of a radio-active emanation in the atmosphere, the radio-active effects observed receive a simple explanation. The emanation in the air gradually breaks up, giving rise in some way to positively charged radio-active carriers. These are driven to the negative electrode in the electric field, and there undergo a further change, giving rise to the radiations observed at the surface of the wire. The matter which causes excited activity will thus be analogous to the active deposit of radium and thorium.

Since the earth is negatively electrified with regard to the upper atmosphere, these positive radio-active carriers produced in the air are continuously deposited on the surface of the earth. Everything on the surface of the earth, including the external surface of buildings, the grass, and leaves of trees, must be covered with an invisible deposit of radio-active material. A hill, or mountain peak, or any high mass of rock or land, concentrates the earth’s electric field at that point and consequently will receive more excited radio-activity per unit area than the plain. Elster and Geitel have pointed out that the greater ionization of the air observed in the neighbourhood of projecting peaks receives a satisfactory explanation on this view.

If the radio-active carriers are produced at a uniform rate in the atmosphere, the amount of excited activity I{t}_, produced on a wire exposed under given conditions, will, after exposure for a time t, be given by

$$ I_t = I₀ (1 − e^{–λt} ) $$,

where I₀ is the maximum activity on the wire and λ is the constant of decay of the excited activity. Since the activity of a wire after removal falls to half value in about 45 minutes, the value of λ is 0·92 (hour)⁻¹. Some experiments made by Allan[389] are in rough agreement with the above equation. Accurate comparative results are difficult to obtain on account of the inconstancy of the radio-activity of the open air. After an exposure of a wire for several hours, the activity reached a practical maximum, and was not much increased by continued exposure.

We have seen (section 191) that the carriers of the active deposit of radium and thorium move in an electric field with about the same velocity as the ions. We should expect therefore that a long wire charged to a high negative potential would abstract the active carriers from the atmosphere for a considerable distance. This does not appear to be the case, for Eve (see section 281) has found that the carriers are only abstracted from the air for a radius of less than one metre, for a potential of the wire of −10,000 volts. It seems probable that the carriers of the active matter are deposited on the numerous fine dust particles present in the air and thus move very slowly even in a strong electric field.

The amount of excited activity produced on a wire, supported some distance from the surface of the earth, should increase steadily with the voltage, for the greater the potential, the greater the volume of air from which the radio-active carriers are abstracted.

The presence of radio-active matter in the atmosphere will account for a considerable portion of the ionization of the air observed near the earth. This important question is discussed in more detail in section 281.

=275. Radio-activity of freshly fallen rain and snow.= C. T. R. Wilson[390] tried experiments to see if any of the radio-active material from the air was carried down by rain. For this purpose a quantity of freshly fallen rain was collected, rapidly evaporated to dryness in a platinum vessel, and the activity of the residue tested by placing the vessel in an electroscope. In all cases, the rate of discharge of the electroscope was considerably increased. From about 50 c.c. of rain water, an amount of activity was obtained sufficient to increase the rate of discharge of the electroscope four or five times, after the rays had traversed a thin layer of aluminium or gold-leaf. The activity disappeared in the course of a few hours, falling to half value in about 30 minutes. Rain water, which had stood for some hours, showed no trace of activity. Tap water, when evaporated, left no active residue.

The amounts of activity obtained from a given quantity of rain water were all of the same order of magnitude, whether the rain was precipitated in fine or in large drops, by night or by day, or whether the rain was tested at the beginning or at the end of a heavy rainfall lasting several hours.

The activity obtained from rain is not destroyed by heating the platinum vessel to a red heat. In this and other respects it resembles the excited activity obtained on negatively charged wires exposed in the open air.

C. T. R. Wilson[391] obtained a radio-active precipitate from rain water by adding a little barium chloride and precipitating the barium with sulphuric acid. An active precipitate was also obtained when alum was added to the water, and the aluminium precipitated by ammonia. The precipitates obtained in this way showed a large activity. The filtrate when boiled down was quite inactive, showing that the active matter had been completely removed by precipitation. This effect is quite analogous to the production of active precipitates from a solution containing the active deposit of thorium (see section 185).

The radio-activity of freshly fallen snow was independently observed by C. T. R. Wilson[392] in England, and Allan[393] and McLennan[394] in Canada. In order to obtain a large amount of activity, the surface layer of snow was removed, and evaporated to dryness in a metal vessel. An active residue was obtained with radio-active properties similar to those observed for freshly fallen rain. Both Wilson and Allan found that the activity of rain and snow decayed at about the same rate, the activity falling to half value in about 30 minutes. McLennan states that he found a smaller amount of radio-activity in the air after a prolonged fall of snow.

Schmauss[395] has observed that drops of water falling through air ionized by Röntgen rays acquire a negative charge. This effect is ascribed to the fact that the negative ions in air diffuse faster than the positive. On this view the drops of rain and flakes of snow would acquire a negative charge in falling through the air. They would in consequence act as collectors of the positive radio-active carriers from the air. On evaporation of the water the radio-active matter would be left behind.

=276. Radio-active emanations from the earth.= Elster and Geitel observed that the air in caves and cellars was, in most cases, abnormally radio-active, and showed very strong ionization. This action might possibly be due to an effect of stagnant air, by which it produced a radio-active emanation from itself, or to a diffusion of a radio-active emanation from the soil. To test whether this emanation was produced by the air itself, Elster and Geitel shut up the air for several weeks in a large boiler, but no appreciable increase of the activity or ionization was observed. To see whether the air imprisoned in the capillaries of the soil was radio-active, Elster and Geitel[396] put a pipe into the earth and sucked up the air into a testing vessel by means of a water pump.

The apparatus employed to test the ionization of the air is shown in Fig. 103. C is an electroscope connected with a wire net, Z. The active air was introduced into a large bell-jar of 27 litres capacity, the inside of which was covered with wire netting, MM´. The bell-jar rested on an iron plate AB. The electroscope could be charged by the rod S. The rate of discharge of the electroscope, before the active air was introduced, was noted. On allowing the active air to enter, the rate of discharge increased rapidly, rising in the course of a few hours in one experiment to 30 times the original value. They found that the emanation produced excited activity on the walls of the containing vessel. The air sucked up from the earth was even more active than that observed in caves and cellars. There can thus be little doubt that the abnormal activity observed in caves and cellars is due to a radio-active emanation, present in the earth, which gradually diffuses to the surface and collects in places where the air is not disturbed.

Results similar to those obtained by Elster and Geitel for the air removed from the earth at Wolfenbüttel were also obtained later by Ebert and Ewers[397] at Munich. They found a strongly active emanation in the soil, and, in addition, examined the variation with time of the activity due to the emanation in a sealed vessel. After the introduction of the active air into the testing vessel, the activity was observed to increase for several hours, and then to decay, according to an exponential law, with the time, falling to half value in about 3·2 days. This rate of decay is more rapid than that observed for the radium emanation, which decays to half value in a little less than four days. The increase of activity with time is probably due to the production of excited activity on the walls of the vessel by the emanation. In this respect it is analogous to the increase of activity observed when the radium emanation is introduced into a closed vessel. No definite experiments were made by Ebert and Ewers on the rate of decay of this excited activity. In one experiment the active emanation, after standing in the vessel for 140 hours, was removed by sucking ordinary air of small activity through the apparatus. The activity rapidly fell to about half value, and this was followed by a very slow decrease of the activity with time. This result indicates that about half the rate of discharge observed was due to the radiation from the emanation and the other half to the excited activity produced by it.

The apparatus employed by Ebert and Ewers in these experiments was very similar to that employed by Elster and Geitel, shown in Fig. 103. Ebert and Ewers observed that, when the wire net attached to the electroscope was charged negatively, the rate of discharge observed was always greater than when it was charged positively. The differences observed between the two rates of discharge varied between 10 and 20 per cent. A similar effect has been observed by Sarasin, Tommasina and Micheli[398] for a wire made active by exposure to the open air. This difference in the rates of discharge for positive and negative electricity is probably connected with the presence of particles of dust or small water globules suspended in the gas. The experiments of Miss Brooks (section 181) have shown that the particles of dust present in the air containing the thorium emanation become radio-active. A large proportion of these dust particles acquire a positive charge and are carried to the negative electrode in an electric field. This effect would increase the rate of discharge of the electroscope when charged negatively. In later experiments, Ebert and Ewers noticed that, in some cases, when the air had been kept in the vessel for several days, the effect was reversed, and the electroscope showed a great rate of discharge when charged positively.

J. J. Thomson[399] has observed that the magnitude of the ionization current depends on the direction of the electric field, if fine water globules are suspended in the ionized gas.

In later experiments, Ebert[400] found that the radio-active emanation could be removed from the air by condensation in liquid air. This property of the emanation was independently discovered by Ebert before he was aware of the results of Rutherford and Soddy on the condensation of the emanations of radium and thorium. To increase the amount of radio-active emanation in a given volume of air, a quantity of the active air, obtained by sucking the air from the soil, was condensed by a liquid air machine. The air was then allowed partially to evaporate, but the process was stopped before the point of volatilization of the emanation was reached. This process was repeated with another quantity of air and the residues added together. Proceeding in this way, he was able to concentrate the emanation in a small volume of air. On allowing the air to evaporate, the ionization of the air in the testing vessel increased rapidly for a time and then slowly diminished. Ebert states that the maximum for the emanation which had been liquefied for some time was reached earlier than for fresh air. The rate of decay of activity of the emanation was not altered by keeping it at the temperature of liquid air for some time. In this respect it behaves like the emanations of radium and thorium.

J. J. Thomson[401] found that air bubbled through Cambridge tap water showed much greater conductivity than ordinary air. The air was drawn through the water by means of a water pump into a large gasometer, when the ionization current was tested with a sensitive electrometer. When a rod charged negatively was introduced into this conducting air it became active. After an exposure for a period of 15 to 30 minutes in the conducting gas, the rod, when introduced into a second testing vessel, increased the saturation current in the vessel to about five times the normal amount. Very little effect was produced when the rod was uncharged or charged positively for the same time. The activity of the rod decayed with the time, falling to half value in about 40 minutes. The amount of activity produced on a wire under constant conditions was independent of the material of the wire. The rays from the rod were readily absorbed in a few centimetres of air.

These effects were, at first, thought to be due to the action of the small water drops suspended in the gas, for it was well known that air rapidly drawn through water causes a temporary increase in its conductivity. Later results, however, showed that there was a radio-active emanation present in Cambridge tap water. This led to an examination of the waters from deep wells in various parts of England, and J. J. Thomson found that, in some cases, a large amount of emanation could be obtained from the well water. The emanation was released either by bubbling air through the water or by boiling the water. The gases obtained by boiling the water were found to be strongly active. A sample of air mixed with the radio-active emanation was condensed. The liquefied gas was allowed to evaporate, and the earlier and later portions of the gas were collected in separate vessels. The final portion was found to be about 30 times as active as the first portion.

An examination of the radio-active properties of the active gases so obtained has been made by Adams[402]. He found that the activity of the emanation decayed, according to an exponential law, with the time, falling to half value in about 3·4 days. This is not very different from the rate of decay of the activity of the radium emanation, which falls to half value in a little less than four days. The excited activity produced by the emanation decayed to half value in about 35 minutes. The decay of the excited activity from radium is at first irregular, but after some time falls off, according to an exponential law, diminishing to half value in 28 minutes. Taking into account the uncertainty attaching to measurements of the very small ionization observed in these experiments, the results indicate that the emanation obtained from well water in England is similar to, if not identical with, the radium emanation. Adams observed that the emanation was slightly soluble in water. After well water had been boiled for a while and then put aside, it was found to recover its power of giving off an emanation. The amount obtained after standing for some time was never more than 10 per cent. of the amount first obtained. Thus it is probable that the well water, in addition to the emanations mixed with it, has also a slight amount of a permanent radio-active substance dissolved in it. Ordinary rain water or distilled water does not give off an emanation.

Bumstead and Wheeler[403] have made a very careful examination of the radio-activity of the emanation obtained from the surface water and soil at New Haven, Connecticut. The emanation, obtained from the water by boiling, was passed into a large testing cylinder, and measurements of the current were made by means of a sensitive electrometer. The current gradually rose to a maximum, after the introduction of the emanation, in exactly the same way as the current increases in a vessel after the introduction of the radium emanation. The decay of activity of the emanations obtained from the water and soil was carefully measured, and, within the limits of experimental error, agreed with the rate of decay of activity observed for the radium emanation. The identity of the emanations from the water and soil with the radium emanation was still further established by experiments on the rate of diffusion of the emanation through a porous plate. By comparative tests it was found that the coefficient of diffusion of the emanations from the water and soil was the same as for the radium emanation. Also, by comparison of the rate of diffusion of carbonic acid, it was found that the density of the emanation was about four times that of carbonic acid, a result in good agreement with that found for the radium emanation (sections 161 and 162).

Bumstead[404] has found that a considerable amount of thorium as well as radium emanation exists in the air of New Haven. For a three hour exposure in the open air, 3 to 5 per cent. of the excited activity on the wire is due to thorium. For a twelve hour exposure, the thorium activity was sometimes 15 per cent. of the whole. On account of the comparatively slow decay of the excited activity of thorium, the activity on the wire after removal for three or four hours was due almost entirely to thorium. The rate of decay could then be measured accurately, and was found to be the same as for a wire exposed in the presence of the thorium emanation.

Dadourian[405] has made an examination of the underground air in New Haven, and has found that this too contains a large quantity of the thorium emanation. A circular hole about 50 cms. in diameter and 2 metres deep was dug in the ground. A number of wires were wound on an insulated frame and suspended in the hole, the top of the hole then being covered over. The wire was charged negatively by a Wimshurst machine. After a long exposure the excited activity on the wire diminished at a rate that showed it to be a mixture of the excited activities of thorium and radium.

A very large amount of work has been done in examining various hot and mineral springs for the presence of the radium emanation, and it is not possible here to refer more than briefly to a few of the very numerous papers that have been published on this subject both in Europe and America. H. S. Allen and Lord Blythswood[406] have observed that the hot springs at Bath and Buxton gave off a radio-active emanation. This was confirmed by Strutt[407], who found that the escaping gases contained the radium emanation, and also that the mud deposited from the springs contained a trace of radium salts. These results are of considerable interest, for Lord Rayleigh has observed that helium is contained among the gases evolved by the springs. It appears probable that the helium observed is produced from the radium or radio-active deposits through which the water flows. Many mineral and hot springs which are famous for their curative properties have been found to contain traces of radium and also considerable amounts of radium emanation. It has been suggested that the curative properties may be due to some extent to the presence of these minute quantities of radium.

Himstedt[408] found that the thermal springs at Baden Baden contained the radium emanation, while Elster and Geitel[409] examined the deposits formed by these springs and found them to contain small quantities of radium salts. Results of a similar character were obtained for a number of waters in Germany by Dorn[410], Schenck[411], and H. Mache[412].

Curie and Laborde[413] have tested the waters of a large number of mineral springs and found that the great majority contain the radium emanation. In this connection, it is of interest to note that Curie and Laborde found very little emanation in the waters of Salins-Moutiers, while Blanc[414] observed, on the other hand, that the sediment from the spring was very active. A closer examination of this deposit by Blanc revealed the fact that it contained a considerable quantity of thorium. This was proved by finding that it gave out an emanation, which lost half of its activity in one minute, and produced excited activity, which fell to half value in about 11 hours. Boltwood[415] has tested a number of samples of spring water from different sources in America and has found that many of them contain the radium emanation.

Most of the results upon the amount of radium emanation from different sources have been expressed in arbitrary units without, in many cases, any comparative standard being given. Boltwood (loc. cit.) has described a satisfactory method for collecting and testing the emanation from different waters, and has suggested that the rate of discharge observed by the electroscope or the electrometer should be expressed in terms of the effect due to the emanation liberated on solution of a definite weight of the mineral uraninite. Since in every mineral so far examined, the amount of radium present is proportional to the amount of uranium, such a standard would be sufficiently definite for practical purposes. The emanation liberated from a few centigrams of the mineral is sufficient to give a convenient rate of discharge of an electroscope. Such a method is preferable to using a known quantity of a radium compound as a standard, since it is difficult to know with certainty the activity of the preparations of radium which may be in the possession of the different experimenters.

=277. Radio-activity of constituents of the earth.= Elster and Geitel[416] observed that, although in many cases the conductivity of the air was abnormally high in underground enclosures, the conductivity varied greatly in different places. In the Baumann Cave, for example, the conductivity of the air was nine times the normal, but in the Iberg Cave only three times the normal. In a cellar at Clausthal the conductivity was only slightly greater than the normal, but the excited radio-activity obtained on a negatively charged wire exposed in it was only ¹⁄₁₁ of the excited radio-activity obtained when the wire was exposed in the free air. They concluded from these experiments that the amount of radio-activity in the different places probably varied with the nature of the soil. Observations were then made on the conductivity of the air sucked up from the earth at different parts of the country. The clayey and limestone soils at Wolfenbüttel were found to be strongly active, the conductivity varying from four to sixteen times the normal amount. A sample of air from the shell limestone of Würzburg and from the basalt of Wilhelmshöhe showed very little activity.

Experiments were made to see whether any radio-active substance could be detected in the soil itself. For this purpose some earth was placed on a dish and introduced under a bell-jar, similar to that shown in Fig. 103. The conductivity of the air in the bell-jar increased with the time, rising to three times the normal value after several days. Little difference was observed whether the earth was dry or moist. The activity of the soil seemed to be permanent, for no change in the activity was observed after the earth had been laid aside for eight months.

Attempts were then made to separate the radio-active constituent from the soil by chemical treatment. For this purpose a sample of clay was tested. By extraction with hydrochloric acid all the calcium carbonate was removed. On drying the clay the activity was found to be reduced, but it spontaneously regained its original activity in the course of a few days. It seems probable, therefore, that an active product had been separated from the soil by the acid. Elster and Geitel consider that an active substance was present in the clay, which formed a product more readily soluble in hydrochloric acid than the active material itself. There seemed to be a process of separation analogous to that of Th X from thorium by precipitation with ammonia.

Experiments were also made to see whether substances placed in the earth acquired any radio-activity. For this purpose samples of potter’s clay, whitening, and heavy spar, wrapped in linen, were placed in the earth 50 cms. below the surface. After an interval of a month, these were dug up and their activity examined. The clay was the only substance which showed any activity. The activity of the clay diminished with the time, showing that activity had been excited in it by the emanations present in the soil.

Elster and Geitel[417] have found that a large quantity of the radio-active emanation can be obtained by sucking air through clay. In some cases, the conductivity of the air in the testing vessel was increased over 100 times. They have also found that the so-called “fango”—a fine mud obtained from hot springs in Battaglia, Northern Italy—gives off three or four times as much emanation as clay. By treating the fango with acid, the active substance present was dissolved. On adding some barium chloride to the solution, and precipitating the barium as sulphate, the active substance was removed, and in this way a precipitate was obtained over 100 times as active, weight for weight, as the original fango. Comparisons were made of the rate of decay of the excited activity, due to the emanation from fango, with that due to the radium emanation, and within the limits of error, the decay curves obtained were found to be identical. There can thus be no doubt that the activity observed in fango is due to the presence of a small quantity of radium. Elster and Geitel calculate that the amount of radium, contained in it, is only about one-thousandth of the amount to be obtained from an equal weight of pitchblende from Joachimsthal.

Vincenti and Levi Da Zara[418] have found that the waters and sediments of a number of hot springs in Northern Italy contain the radium emanation. Elster and Geitel observed that natural carbonic acid obtained from great depths of old volcanic soil was radio-active, while Burton[419] found that the petroleum from a deep well in Ontario, Canada, contained a large quantity of emanation, probably of radium, since its activity fell to half value in 3·1 days, while the excited activity produced by the emanation fell to half value in about 35 minutes. A permanently active deposit was left behind after volatilization of the oil, indicating that probably one or more of the radio-elements were present in minute quantity.

Elster and Geitel[420] have found that the active sediments obtained from springs at Nauheim and Baden Baden showed abnormal rates of decay of the excited activity. This was finally traced to the presence in the deposit of both thorium and radium. By suitable chemical methods, the two active substances were separated from each other and were then tested separately.

=278. Effect of meteorological conditions upon the radio-activity of the atmosphere.= The original experiments of Elster and Geitel on the excited radio-activity derived from the atmosphere were repeated by Rutherford and Allan[421] in Canada. It was found that a large amount of excited radio-activity could be derived from the air, and that the effects were similar to those observed by Elster and Geitel in Germany. This was the case even on the coldest day in winter, when the ground was covered deeply with snow and wind was blowing from the north over snow-covered lands. The results showed that the radio-activity present in the air was not much affected by the presence of moisture, for the air during a Canadian winter is extremely dry. The greatest amount of excited activity on a negatively charged wire was obtained in a strong wind. In some cases the amount produced for a given time of exposure was ten to twenty times the normal amount. A cold bright day of winter usually gave more effect than a warm dull day in summer.

Elster and Geitel[422] have made a detailed examination of the effect of meteorological conditions on the amount of excited radio-activity to be derived from the atmosphere. For this purpose a simple portable apparatus was devised by them and used for the whole series of experiments. A large number of observations were taken, extending over a period of twelve months. They found that the amount of excited activity obtained was subject to great variations. The extreme values obtained varied in the ratio of 16 to 1. No direct connection could be traced between the amount of ionization in the atmosphere and the amount of excited activity produced. They found that the greatest amount of excited activity was obtained during a fog, when the amount of ionization in the air was small. This result, however, is not necessarily contradictory to the view that the ionization and activity of the air are to a certain extent connected. From the experiments of Miss Brooks on the effect of dust in acting as carriers of excited activity, more excited activity should be obtained during a fog than in clear air. The particles of water become centres for the deposit of radio-active matter. The positive carriers are thus anchored and are not removed from the air by the earth’s field. In a strong electric field, these small drops will be carried to the negative electrode and manifest their activity on the surface of the wire. On the other hand, the distribution of water globules throughout the air causes the ions in the air to disappear rapidly in consequence of their diffusion to the surface of the drops (see section 31). For this reason the denser the fog, the smaller will be the conductivity observed in the air.

Lowering the temperature of the air had a decided influence. The average activity observed below 0° C. was 1·44 times the activity observed above 0° C. The height of the barometer was found to exert a marked influence on the amount of excited activity to be derived from the air. The lower the barometer the greater was the amount of excited activity in the air. The effect of variation of the height of the barometer is intelligible, when it is considered that probably a large proportion of the radio-activity observed in the air is due to the radio-active emanations which are continuously diffusing from the earth into the atmosphere. Elster and Geitel have suggested that a lowering of the pressure of the air would cause the air from the ground to be drawn up from the capillaries of the earth into the atmosphere. This, however, need not necessarily be the case if the conditions of the escape of the emanation into the atmosphere are altered by the variation of the position of underground water or by a heavy fall of rain.

The amount of excited activity to be derived from the air on the Baltic Coast was only one-third of that observed inland at Wolfenbüttel. Experiments on the radio-activity of the air in mid-ocean would be of great importance in order to settle whether the radio-activity observed in the air is due to the emanations from the soil alone. It is probable that the radio-activity of the air at different points of the earth may vary widely, and may largely depend on the nature of the soil.

Saake[423] has found that the amount of emanation present in the air at high altitudes in the valley of Arosa in Switzerland is much greater than the normal amount at lower levels. Elster and Geitel have observed that there is also a larger number of ions in the air at high altitudes, and suggest that the curative effect of thermal springs and the physiological actions of the air at high levels may be connected with the presence of an unusual amount of radio-active matter in the atmosphere. Simpson[424] made experiments on the amount of excited activity at Karasjoh, Norway, at a height of about 150 feet above sea level. The sun did not rise above the level of the horizon during the time the observations were taken. The average amount of excited activity obtained from the air was considerably greater than the normal amount observed by Elster and Geitel in Germany. This was the more surprising as the ground was frozen hard and covered with deep snow. Allan, working in Montreal, Canada, early observed that the amount of activity to be obtained from the air was about the same in summer as in winter, although, in the latter case, the whole earth was deeply frozen and covered with snow, and the winds blew from the north over snow-covered lands. Under such conditions, a diminution of the amount of activity is to be expected since the diffusion of the emanation must be retarded, if not altogether stopped, by the freezing of the soil. On the other hand, it appears difficult to escape from the conclusion of Elster and Geitel that the emanation present in the atmosphere is evolved from the earth itself.

Some interesting experiments have been made by McLennan[425] on the amount of excited radio-activity to be derived from the air when filled with fine spray. The experiments were made at the foot of the American Fall at Niagara. An insulated wire was suspended near the foot of the Fall, and the amount of excited activity on the wire compared with the amount to be obtained on the same wire for the same exposure in Toronto. The amount of activity obtained from the air at Toronto was generally five or six times that obtained from the air at the Falls. In these experiments it was not necessary to use an electric machine to charge the wire negatively, for the falling spray kept the insulated wire permanently charged to a potential of about −7500 volts. These results indicate that the falling spray had a negative charge and electrified the wire. The small amount of the excited radio-activity at the Falls was probably due to the fact that the negatively charged drops abstracted the positively charged radio-active carriers from the atmosphere, and in falling carried them to the river below. On collecting the spray and evaporating it, no active residue was obtained. Such a result is, however, to be expected on account of the minute proportion of the spray tested compared with that present in the air.

=279. A very penetrating radiation from the earth’s surface.= McLennan[426], and Rutherford and Cooke[427] independently, observed the presence of a very penetrating radiation inside buildings. McLennan measured the natural conductivity of the air in a large closed metal cylinder by means of a sensitive electrometer. The cylinder was then placed inside another and the space between filled with water. For a thickness of water between the cylinders of 25 cms. the conductivity of the air in the inner cylinder fell to about 63 per cent. of its initial value. This result shows that part of the ionization in the inner cylinder was due to a penetrating radiation from an external source, which radiation was partially or wholly absorbed in water.

Rutherford and Cooke observed that the rate of discharge of a sealed brass electroscope was diminished by placing a lead screen around the electroscope. A detailed investigation of the decrease of the rate of discharge in the electroscope, when surrounded by metal screens, was made later by Cooke[428]. A thickness of 5 cms. of lead round the electroscope decreased the rate of discharge about 30 per cent. Further increase of the thickness of the screen had no effect. When the apparatus was surrounded by 5 tons of pig-lead the rate of discharge was about the same as when it was surrounded by a plate about 3 cms. thick. An iron screen also diminished the rate of discharge to about the same extent as the lead. By suitably arranging lead screens it was found that the radiation came equally from all directions. It was of the same intensity by night as by day. In order to be sure that this penetrating radiation did not arise from the presence of radio-active substances in the laboratory, the experiments were repeated in buildings in which radio-active substances had never been introduced, and also on the open ground far removed from any building. In all cases a diminution of the rate of discharge of the electroscope, when surrounded by lead screens, was observed. These results show that a penetrating radiation is present at the surface of the earth, arising partly from the earth itself and partly from the atmosphere.

The result is not surprising when the radio-activity of the earth and atmosphere is taken into account. The writer has found that bodies made active by exposure to the emanations from thorium and radium give out γ rays. We may expect then that the very similar excited radio-activity which is present in the earth and atmosphere should also give rise to γ rays of a similar character. More recent work, however (section 286), indicates that this explanation is not sufficient to explain all the facts observed.

=280. Comparison of the radio-activity of the atmosphere with that produced by the radio-elements.= The radio-active phenomena observed in the earth and atmosphere are very similar in character to those produced by thorium and radium. Radio-active emanations are present in the air of caves and cellars, in natural carbonic acid, and in deep well water, and these emanations produce excited radio-activity on all bodies in contact with them. The question now arises whether these effects are due entirely to known radio-elements present in the earth or to unknown kinds of radio-active matter. The simplest method of testing this point is to compare the rate of decay of the radio-active product in the atmosphere with those of the known radio-active products of thorium and radium. A cursory examination of the facts at once shows that the radio-activity of the atmosphere is much more closely allied to effects produced by radium than to those due to thorium. The activity of the emanation released from well water, and also that sucked up from the earth, decays to half value in about 3·3 days, while the activity of the radium emanation decays to half value in an interval of 3·7 to 4 days. Considering the difficulty of making accurate determinations of these quantities, the rates of decay of the activity of the emanations from the earth and from radium agree within the limits of experimental error. A large number of observers have found that the radium emanation is present in the water of thermal springs and in the sediment deposited by them. Bumstead and Wheeler have shown that the emanation from the soil and surface water of New Haven is identical with that from radium. If the emanations from the earth and from radium are the same, the excited activities produced should have the same rate of decay. The emanation from well water in England approximately fulfils this condition (section 276), but an observation recorded by Ebert and Ewers (section 276) seems to show that the excited activity due to the emanation sucked up from the earth decays at a very slow rate compared with that due to radium.

Bumstead has given undoubted evidence that the thorium as well as the radium emanation is also present in the atmosphere at New Haven, while Dadourian has shown that it is emitted by New Haven soil. Blanc, and Elster and Geitel, have also found that thorium is present in the sediment from some thermal springs.

If the active matter in the atmosphere consists mainly of the radium emanation, the active deposit on a negatively charged wire, exposed in the open air, should initially consist of radium A, B and C. The curve of decay should be identical with the decay curve of the excited activity of radium, measured by the α rays, that is, there should be a rapid initial drop corresponding to the initial 3 minute change, then a slow rate of variation, the activity after several hours decaying to half value in about 28 minutes (see section 222). The rapid initial drop has been observed by Bumstead for the air at New Haven. Allan[429] did not observe this initial drop in Montreal, but found the activity fell to half value in about 45 minutes, reckoning from a time about 10 minutes after the removal of the active wire. This is about the rate of decay to be expected for the active deposit of radium over the same interval. Allan obtained evidence that there were several kinds of active matter deposited on the wire. For example, the activity transferred from the active wire to a piece of leather, moistened with ammonia, fell to half value in 38 minutes; for a piece of absorbent felt treated similarly, the activity fell to half value in 60 minutes, the normal time for the untreated wire being 45 minutes.

It is probable that this variation of the rate of decay is due to the fact that unequal proportions of radium B and C were transferred from the wire to the rubber. If a greater proportion of B than of C were removed, the decay would be slower and vice versa.

The fact that the activity of rain and snow falls to half value in about 30 minutes is a strong indication that the radium emanation is present in the atmosphere. The active matter with the rain and snow after standing some time would consist mainly of radium C and this should decay exponentially with the time, falling to half value in 28 minutes.

On account of the rapid decay of the thorium emanation—half value in one minute—it is not likely that much of the activity of the atmosphere can be ascribed to it. Its effect would be most marked near the surface of the soil.

There can be little doubt, that a large part of the radio-activity of the atmosphere is due to the radium emanation, which is continually diffusing into the atmosphere from the pores of the earth. Since radio-activity has been observed in the atmosphere at all points at which observations have, so far, been made, radio-active matter must be distributed in minute quantities throughout the soil of the earth. The volatile emanations escape into the atmosphere by diffusion, or are carried to the surface in spring water or by the escape of underground gases, and cause the radio-active phenomena observed in the atmosphere. The observation of Elster and Geitel that the radio-activity of the air is much less near the sea than inland is explained at once, if the radio-activity of the atmosphere is due mainly to the diffusion of emanations from the soil into the air above it.

The rare gases helium and xenon which exist in the atmosphere have been tested and found to be non-radio-active. The radio-activity of the air cannot be ascribed to a slight radio-activity possessed by either of these gases.

=281. Amount of the radium emanation in the atmosphere.= It is a matter of great interest to form an estimate of the amount of radium emanation present in the atmosphere, for since it comes from the earth, it indirectly serves as a means of estimating the amount of radium which is distributed over a thin crust of the earth.

Some experiments in this direction have been made by Eve in the laboratory of the writer. The experiments are not yet completed but the results so far obtained allow us to calculate the probable amount of emanation per cubic kilometre of the atmosphere near the earth.

Experiments were first made with a large iron tank 154 cms. square and 730 cms. deep, in a building in which no radium or other radio-active material had ever been introduced. The saturation ionization current for the air in the tank was first measured by means of an electroscope, connected with an insulated electrode passing up the centre of the closed tank. Assuming that the ionization in the tank was uniform, the number of ions produced per c.c. of the air in the tank was found to be 10. This is a considerably lower value than has usually been observed in a small closed vessel (see section 284). Cooke obtained the value 10 for a well cleaned brass electroscope, surrounded by lead, while Schuster obtained a value about 12 for the air in the laboratory of Owens College, Manchester.

In order to measure the amount of the excited activity from the tank, a central insulated wire was charged negatively to about 10,000 volts by a Wimshurst machine. After two hours, the wire was removed and wound on an insulated frame connected with a gold-leaf electroscope. The rate of decay of the activity on the wire was found to be about the same as for the excited activity produced by the radium emanation. In order to estimate the amount of radium emanation present in the large tank, special experiments were made with a smaller tank in which a known quantity of the radium emanation was introduced by employing a solution of pure radium bromide of known concentration. A central wire was made the negative electrode as before, and, after removal, it was wound on the frame and its activity tested. In this way it was found that the amount of radium emanation present in the large tank, in order to produce the excited activity observed, must have been equal to the equilibrium or maximum amount to be obtained from 9·5 × 10⁻⁹ grams of pure radium bromide. The volume of the large tank was 17 cubic metres, so that the amount of emanation present per cubic metre was equivalent to that liberated from 5·6 × 10⁻¹⁰ grams of radium bromide in radio-active equilibrium.

If the amount of the emanation in the tank is taken as the average amount existing in the outside air, the amount of radium emanation present per cubic kilometre of the air is equivalent to that supplied by 0·56 grams of radium bromide.

For the purpose of calculation, suppose the emanation is uniformly distributed over the land portion of the earth (¼ of the total surface), and to extend to an average height of 5 kilometres. The air over the sea is not taken into account as its radio-activity has not been examined. The total amount of emanation present in the atmosphere under these conditions corresponds to that supplied by about 400 tons of radium bromide. In order to maintain this amount of emanation in the atmosphere, it must be supplied at a constant rate from the earth’s surface. Since the greater amount of the emanation probably escapes into the air by transpiration and diffusion through the soil, the emanation cannot reach the surface except from a very thin layer of the earth. The probable thickness of this layer can be estimated if it is assumed that the present loss of heat from the earth is supplied from the radio-active matter contained in it. We have seen (section 271) that, on this hypothesis, there must be an amount of active matter in the earth corresponding to about 300 million tons of radium. If this is supposed to be uniformly distributed, a thickness of layer of about 13 metres will suffice to maintain the calculated amount of emanation in the atmosphere. This thickness of layer is about the order of magnitude to be expected from general considerations.

These results lead indirectly to the conclusion that a large amount of emanation does undoubtedly exist in the surface crust of the earth.

Experiments were also made by Eve with a large zinc cylinder exposed in the open air. Volume for volume, the average amount of excited activity derived from it was only about one-third of that obtained from the large iron tank. This would reduce the amount of emanation, previously deduced, to about one-third.

Before such calculations can be considered at all definite, it will be necessary to make comparative measurements of the amount of emanation in the atmosphere at various parts of the earth. The air at Montreal is not abnormally active, so that the calculations probably give the right order of magnitude of the quantities.

Eve also observed that the amount of activity to be obtained per unit length of the wire in the zinc cylinder of about 70 cms. in diameter was about the same as for a wire ·5 mms. in diameter charged to 10,000 volts in the open air, supported 20 feet from the ground. This shows that such a potential does not draw in the carriers of excited activity which are more than half a metre away, and probably the range is even less.

It is of great importance to find how large a proportion of the number of ions produced in the atmosphere is due to the radio-active matter distributed throughout it. The results of Eve with the large iron tank, already referred to, indicate that a large proportion of the ionization in the tank was due to the radio-active matter contained in it, for the ratio of the excited activity on the central electrode to the total ionization current in the tank was about ⁷⁄₁₀ of the corresponding ratio for a smaller tank into which a supply of the radium emanation had been introduced.

This result requires confirmation by experiments at other parts of the earth, but the results point to the conclusion that a large part, if not all, of the ionization at the earth’s surface is due to radio-active matter distributed in the atmosphere. A constant rate of production of 30 ions per second per c.c. of air, which has been observed in the open air at the surface of the earth in various localities, would be produced by the presence in each c.c. of the air of the amount of emanation liberated from 2·4 × 10⁻¹⁵ grams of radium bromide in radio-active equilibrium. It is not likely, however, that the ionization of the upper part of the atmosphere is due to this cause alone. In order to explain the maintenance of the large positive charge, which generally exists in the upper atmosphere, there must be a strong ionization of the upper air, which may possibly be due to ionizing radiations emitted by the sun.

=282. Ionization of atmospheric air.= A large number of measurements have been made during the last few years to determine the relative amount of ionization in the atmosphere in different localities and at different altitudes. Measurements of this character were first undertaken by Elster and Geitel with a special type of electroscope. A charged body exposed to the air was attached to a portable electroscope, and the rate of loss of charge was observed by the movement of the gold or aluminium leaf. The rates of discharge of the electroscope for positive and negative electricity were generally different, the ratio depending on the locality and the altitude, and on the meteorological conditions. This apparatus is not suitable for quantitative measurements and the deductions to be drawn from the observations are of necessity somewhat indefinite.

Ebert[430] has designed a portable apparatus in which the number of ions per c.c. of the air can be determined easily. A constant current of air is drawn between two concentric cylinders by means of a fan actuated by a falling weight. The inner cylinder is insulated and connected with an electroscope. Knowing the capacity of the apparatus, and the velocity of the current of air, the rate of movement of the gold-leaf affords a measure of the number of ions present in unit volume of the air drawn between the cylinders.

In this way Ebert found that the number of ions in the air was somewhat variable, but on an average corresponded to about 2600 per c.c. in the particular locality where the measurements were made.

This is the equilibrium number of ions present per c.c. when the rate of production balances the rate of recombination. If q is the number of ions produced per second per unit volume of the air and n is the equilibrium number, then q = α_n²_ where α is the constant of recombination (section 30).

By a slight addition to the apparatus of Ebert, Schuster[431] has shown that the constant of recombination for the particular sample of air under investigation can be determined. The value so obtained for air in the neighbourhood of Manchester was variable, and two or three times as great as for dust-free air. The results of some preliminary measurements showed that the number of ions present per c.c. of the air in different localities varied from 2370 to 3660, while the value of q, the number of ions produced per c.c. per second, varied between 12 and 38·5.

Rutherford and Allan and Eberts showed that the ions in the air had about the same mobility as the ions produced in air by Röntgen rays and radio-active substances. In some recent determinations by Mache and Von Schweidler[432], the velocity of the positive ion was found to be about 1·02 cms. per second, and that of the negative 1·25 cms., for a potential gradient of one volt per cm.

Langevin[433] has recently shown that in addition to these swift moving ions, there are also present in the atmosphere some ions which travel extremely slowly in an electric field. The number of these slowly moving ions in the air in Paris is about 40 times as great as the number of the swifter ions. This result is of great importance, for in the apparatus of Ebert these ions escape detection, since the electric field is not strong enough to carry them to the electrodes during the time of their passage between the cylinders.

=283. Radio-activity of ordinary materials.= It has been shown that radio-active matter seems to be distributed fairly uniformly over the surface of the earth and in the atmosphere. The very important question arises whether the small radio-activity observed is due to known or unknown radio-elements present in the earth and atmosphere, or to a feeble radio-activity of matter in general, which is only readily detectable when large quantities of matter are present. The experimental evidence is not yet sufficient to answer this question, but undoubted proof has been obtained that many of the metals show a very feeble radio-activity. Whether this radio-activity is due to the presence of a slight trace of the radio-elements or is an actual property of the metals themselves will be discussed in more detail in section 286.

Schuster[434] has pointed out that every physical property hitherto discovered for one element has been found to be shared by all the others in varying degrees. For example, the property of magnetism is most strongly marked in iron, nickel, and cobalt, but all other substances are found to be either feebly magnetic or diamagnetic. It might thus be expected on general principles that all matter should exhibit the property of radio-activity in varying degrees. On the view developed in chapter X., the presence of this property is an indication that the matter is undergoing change accompanied by the expulsion of charged particles. It does not, however, by any means follow that because the atom of one element in the course of time becomes unstable and breaks up, that, therefore, the atoms of all the other elements pass through similar phases of instability.

It has already been mentioned (section 8), that Mme Curie made a very extensive examination of most of the elements and their compounds for radio-activity. The electric method was used, and any substance possessing an activity of ¹⁄₁₀₀ of that of uranium would certainly have been detected. With the exception of the known radio-elements and the minerals containing uranium and thorium, no other substances were found to be radio-active even to that degree.

Certain substances like phosphorus[435] possess the property of ionizing a gas under special conditions. The air which is drawn over the phosphorus is conducting, but it has not yet been settled whether this conductivity is due merely to ions formed at the surface of the phosphorus or to ions produced by the phosphorus nuclei or emanations, as they have been termed, which are carried along with the current of air. It does not however appear that the ionization of the gas is in any way due to the presence of a penetrating type of radiation such as is emitted by the radio-active bodies. Le Bon (section 8) observed that quinine sulphate, after being heated to a temperature below the melting point and then allowed to cool, showed for a time strong phosphorescence and was able rapidly to discharge an electroscope. The discharging action of quinine sulphate under varying conditions has been very carefully examined by Miss Gates[436]. The ionization could not be observed through thin aluminium foil or gold-leaf, but appeared to be confined to the surface of the sulphate. The current observed by an electrometer was found to vary with the direction of the electric field, indicating that the positive and negative ions had very different mobilities. The discharging action appears to be due either to an ionization of the gas very close to the surface by some short ultra-violet light waves, accompanying the phosphorescence, or to a chemical action taking place at the surface.

Thus, neither phosphorus nor quinine sulphate can be considered to be radio-active, even under the special conditions when they are able to discharge an electrified body. No evidence in either case has been found that the ionization is due to the emission of a penetrating radiation.

No certain evidence has yet been obtained that any body can be made radio-active by exposure to Röntgen rays or cathode rays. A metal exposed to the action of Röntgen rays gives rise to a secondary radiation which is very readily absorbed in a few centimetres of air. It is possible that this secondary radiation may prove to be analogous in some respects to the α rays from the radio-elements. The secondary radiation, however, ceases immediately the Röntgen rays are cut off. Villard[437] stated that a piece of bismuth produced a feeble photographic action after it had been exposed for some time to the action of the cathode rays in a vacuum. It has not however been shown that the bismuth gives out rays of a character similar to those of the radio-active bodies. The experiments of Ramsay and Cooke on the production of apparent activity in inactive matter by the radiations from radium have already been discussed in section 264.

The existence of a very feeble radio-activity of ordinary matter has been deduced from the study of the conductivity of gases in closed vessels. The conductivity is extremely minute, and special methods are required to determine it with accuracy. A brief account will now be given of the gradual growth of our knowledge on this important question.

=284. Conductivity of air in closed vessels.= Since the time of Coulomb onwards several investigators have believed that a charged conductor placed inside a closed vessel lost its charge more rapidly than could be explained by the conduction leak across the insulating support. Matteucci, as early as 1850, observed that the rate of loss of charge was independent of the potential. Boys, by using quartz insulators of different lengths and diameters, arrived at the conclusion that the leakage must in part take place through the air. This loss of charge in a closed vessel was believed to be due in some way to the presence of dust particles in the air.

On the discovery that gases become temporary conductors of electricity under the influence of Röntgen rays and the rays from radio-active substances, attention was again drawn to this question. Geitel[438] and C. T. R. Wilson[439] independently attacked the problem, and both came to the conclusion that the loss of charge was due to a constant ionization of the air in the closed vessel. Geitel employed in his experiments an apparatus similar to that shown in Fig. 103. The loss of charge of an Exner electroscope, with the cylinder of wire netting Z attached, was observed in a closed vessel containing about 30 litres of air. The electroscope system was found to diminish in potential at the rate of about 40 volts per hour, and this leakage was shown not to be due to a want of insulation of the supports.

Wilson, on the other hand, used a vessel of very small volume, in order to work with air which could be completely freed from dust. In the first experiments a silvered glass vessel with a volume of only 163 c.c. was employed. The experimental arrangement is shown in Fig. 104.

The conductor, of which the loss of charge was to be measured, was placed near the centre of the vessel A. It consisted of a narrow strip of metal with a gold-leaf attached. The strip of metal was fixed to the upper rod by means of a small sulphur bead. The upper rod was connected with a sulphur condenser with an Exner electroscope B attached to indicate its potential. The gold-leaf system was initially charged to the same potential as the upper rod and condenser by means of a fine steel wire which was caused to touch the gold-leaf system by the attraction of a magnet brought near it. The rate of movement of the gold-leaf was measured by means of a microscope provided with a micrometer eye-piece. By keeping the upper rod at a slightly higher potential than the gold-leaf system, it was ensured that the loss of charge of the gold-leaf system should not be due in any way to a conduction leakage across the sulphur bead.

The method employed by Wilson in these experiments is very certain and convenient when an extremely small rate of discharge is to be observed. In this respect the electroscope measures with certainty a rate of loss of charge much smaller than can be measured by a sensitive electrometer.

Both Geitel and Wilson found that the leakage of the insulated system in dust-free air was the same for a positive as for a negative charge, and was independent of the potential over a considerable range. The leakage was the same in the dark as in diffuse daylight. The independence of leakage of the potential is strong evidence that the loss of charge is due to a constant ionization of the air. When the electric field acting on the gas exceeds a certain value, all the ions are carried to the electrodes before recombination occurs. A saturation current is reached, and it will be independent of further increase of the electric field, provided, of course, a potential sufficiently high to cause a spark to pass is not applied.

C. T. R. Wilson has recently devised a striking experiment to show the presence of ions in dust-free air which is not exposed to any external ionizing agency. Two large metal plates are placed in a glass vessel connected with an expansion apparatus similar to that described in section 34. On expanding the air, the presence of the ions is shown by the appearance of a slight cloud between the plates. These condensation nuclei carry an electric charge, and are apparently similar in all respects to the ions produced in gases by X rays, or by the rays from active substances.

Wilson found that the loss of charge of the insulated system was independent of the locality. The rate of discharge was unaltered when the apparatus was placed in a deep tunnel, so that it did not appear that the loss of charge was due to an external radiation. From experiments already described, however (section 279), it is probable that about 30 per cent. of the rate of discharge observed was due to a very penetrating radiation. This experiment of Wilson’s indicates that the intensity of the penetrating radiation was the same in the tunnel as at the earth’s surface. Wilson found that the ionization of the air was about the same in a brass vessel as in one of glass, and came to the conclusion that the air was spontaneously ionized.

Using a brass vessel of volume about 471 c.c., Wilson determined the number of ions that must be produced in air per unit volume per second, in order to account for the loss of charge of the insulated system. The leakage system was found to have a capacity of about 1·1 electrostatic units, and lost its charge at the rate of 4·1 volts per hour for a potential of 210 volts, and 4·0 volts per hour for a potential of 120 volts. Taking the charge on an ion as 3·4 × 10⁻¹⁰ electrostatic units, this corresponds to a production of 26 ions per second.

Rutherford and Allan[440] repeated the results of Geitel and Wilson, using an electrometer method. The saturation current was observed between two concentric zinc cylinders of diameter 25·5 and 7·5 cms. respectively and length 154 cms. It was found that the saturation current could practically be obtained with a potential of a few volts. Saturation was however obtained with a lower voltage after the air had remained undisturbed in the cylinders for several days. This was probably due to the gradual settling of the dust originally present in the air.

Later observations of the number of ions produced in air in sealed vessels have been made by Patterson[441], Harms[442], and Cooke[443]. The results obtained by different observers are shown in the following table. The value of the charge on an ion is taken as 3·4 × 10⁻¹⁰ electrostatic units:

Material of Number of ions Observer vessel produced per c.c. per second

Silvered 36 C. T. R. Wilson glass

Brass 26 „ „

Zinc 27 Rutherford and Allan

Glass 53 to 63 Harms

Iron 61 Patterson

Cleaned 10 Cooke brass

It will be shown later that the differences in these results are probably due to differences in the radio-activity of the containing vessel.

=285. Effect of pressure and nature of gas.= C. T. R. Wilson (loc. cit.) found that the rate of leakage of a charged conductor varied approximately as the pressure of the air between the pressures examined, viz. 43 mms. and 743 mms. of mercury. These results point to the conclusion that, in a good vacuum, a charged body would lose its charge extremely slowly. This is in agreement with an observation of Crookes, who found that a pair of gold-leaves retained their charge for several months in a high vacuum.

Wilson[444] at a later date investigated the leakage for different gases. The results are included in the following table, where the ionization produced in air is taken as unity:

Gas Relative (Relative ionization ionization) / (density)

Air 1·00 1·00

Hydrogen 0·184 2·7

Carbon dioxide 1·69 1·10

Sulphur 2·64 1·21 dioxide

Chloroform 4·7 1·09

With the exception of hydrogen, the ionization produced in different gases is approximately proportional to their density. The relative ionization is very similar to that observed by Strutt (section 45) for gases exposed to the influence of the α and β rays from radio-active substances, and points to the conclusion that the ionization observed may be due either to a radiation from the walls of the vessel or from external sources.

Jaffé[445] has made a careful examination of the natural ionization in the very heavy gas nickel-carbonyl, Ni(CO)₄, in a small silvered glass vessel. The ionization of this gas was 5·1 times that of air at normal pressure while its density is 5·9 times that of air. The leak of the electroscope was nearly proportional to the pressures except at low pressure, when the leak was somewhat greater than would be expected if the pressure law held. The fact that a gas of such high density and complicated structure behaves like the simpler and lighter gases is a strong indication that the ionization itself is due to a radiation from the walls of the vessel and not to a spontaneous ionization of the gas.

Patterson[446] examined the variation of the ionization of air with pressure in a large iron vessel of diameter 30 cms. and length 20 cms. The current between a central electrode and the cylinder was measured by means of a sensitive Dolezalek electrometer. He found that the saturation current was practically independent of the pressure for pressures greater than 300 mms. of mercury. Below a pressure of 80 mms. the current varied directly as the pressure. For air at atmospheric pressure, the current was independent of the temperature up to 450° C. With further increase of temperature, the current began to increase, and the increase was more rapid when the central electrode was charged negatively than when it was charged positively. This difference was ascribed to the production of positive ions at the surface of the iron vessel. The results obtained by Patterson render it very improbable that the ionization observed in air is due to a spontaneous ionization of the enclosed air: for we should expect the amount of this ionization to depend on the temperature of the gas. On the other hand, these results are to be expected if the ionization of the enclosed air is mainly due to an easily absorbed radiation from the walls of the vessel. If this radiation had a penetrating power about equal to that observed for the α rays of the radio-elements, the radiation would be absorbed in a few centimetres of air. With diminution of pressure, the radiations would traverse a greater distance of air before complete absorption, but the total ionization produced by the rays would still remain about the same, until the pressure was reduced sufficiently to allow the radiation to traverse the air space in the vessel without complete absorption. With still further diminution of pressure, the total ionization produced by the radiation, and in consequence the current observed, would vary directly as the pressure.

=286. Examination of ordinary matter for radio-activity.= Strutt[447], McLennan and Burton[448], and Cooke[449], independently observed about the same time that ordinary matter is radio-active to a slight degree. Strutt, by means of an electroscope, observed that the ionization produced in a closed vessel varied with the material of the vessel. A glass vessel with a removable base was employed and the vessel was lined with the material to be examined. The following table shows the relative results obtained. The amount of leakage observed is expressed in terms of the number of scale divisions of the eye-piece passed over per hour by the gold-leaf:

Material of Leakage in lining of scale divisions vessel per hour

Tinfoil 3·3

„ another 2·3 sample

Glass coated 1·3 with phosphoric acid

Silver 1·6 chemically deposited on glass

Zinc 1·2

Lead 2·2

Copper (clean) 2·3

„ (oxidized) 1·7

Platinum 2·0, 2·9, 3·9 (various samples)

Aluminium 1·4

There are thus marked differences in the leakage observed for different materials and also considerable differences in different samples of the same metal. For example, one specimen of platinum caused nearly twice the leakage of another sample from a different stock.

McLennan and Burton, on the other hand, measured by means of a sensitive electrometer the ionization current produced in the air in a closed iron cylinder 25 cms. in diameter and 130 cms. in length, in which an insulated central electrode was placed. The open cylinder was first exposed for some time at the open window of the laboratory. It was then removed, the top and bottom closed, and the saturation current through the gas determined as soon as possible. In all cases it was observed that the current diminished for two or three hours to a minimum and then very slowly increased again. In one experiment, for example, the initial current observed corresponded to 30 on an arbitrary scale. In the course of four hours the current fell to a minimum of 6·6, and 44 hours later had risen to a practical maximum of 24. The initial decrease observed is probably due to a radio-activity of the enclosed air or walls of the vessel, which decayed rapidly with the time. The decay of the excited activity produced on the interior surface of the cylinder when exposed to the air was probably responsible for a part of the decrease observed. McLennan ascribes the increase of current with time to a radio-active emanation which is given off from the cylinder, and ionizes the enclosed air. On placing linings of lead, tin, and zinc in the iron cylinder, considerable differences were observed both for the minimum current and also for the final maximum. Lead gave about twice the current due to zinc, while tin gave an intermediate value. These results are similar in character to those obtained by Strutt.

McLennan and Burton also investigated the effect of diminution of pressure on the current. The cylinder was filled with air to a pressure of 7 atmospheres, and allowed to stand until the current reached a constant value. The air was then allowed to escape and the pressure reduced to 44 mms. of mercury. The current was found to vary approximately as the pressure over the whole range. These results are not in agreement with the results of Patterson already described, nor with some later experiments of Strutt. McLennan’s results however point to the conclusion that the ionization was mainly due to an emanation emitted from the metal. Since the air was rapidly removed, a proportionate amount of the emanation would be removed also, and it might thus be expected that the current would vary directly as the pressure. If this is the case the current through the gas at low pressures should increase again to a maximum if time is allowed for a fresh emanation to form.

H. L. Cooke, using an electroscopic method, obtained results very similar to those given by Strutt. Cooke observed that a penetrating radiation was given out from brick. When a brass vessel containing the gold-leaf system was surrounded by brick, the discharge of the electroscope was increased by 40 to 50 per cent. This radiation was of about the same penetrating power as the rays from radio-active substances. The rays were completely absorbed by surrounding the electroscope with a sheet of lead 2 mms. in thickness. This result is in agreement with the observation of Elster and Geitel, already mentioned, that radio-active matter was present in clay freshly dug up from the earth.

Cooke also observed that the ionization of the air in a brass electroscope could be reduced to about one-third of its usual value if the interior surface of the brass was carefully cleaned. By removing the surface of the brass he was able to reduce the ionization of the enclosed air from 30 to 10 ions per c.c. per second. This is an important observation, and indicates that a large proportion of the radio-activity observed in ordinary matter is due to a deposit of radio-active matter on its surface. It has already been shown that bodies which have been exposed in the presence of the radium emanation retain a residual activity which decays extremely slowly. There can be no doubt that the radium emanation is present in the atmosphere, and the exposed surface of matter, in consequence, will become coated with an invisible film of radio-active matter, deposited from the atmosphere. On account of the slow decay of this activity it is probable that the activity of matter exposed in the open air would steadily increase for a long interval. Metals, even if they are originally inactive, would thus acquire a fairly permanent activity, but it should be possible to get rid of this by removing the surface of the metal or by chemical treatment. The rapid increase of activity of all matter left in a laboratory in which a large quantity of emanation has been released has been drawn attention to by Eve[450]. This superficial activity, due to the products radium D, E, and F, was mainly removed by placing the metal in strong acid.

A number of experiments have been made by J. J. Thomson, N. R. Campbell, and A. Wood in the Cavendish laboratory to examine whether the radio-activity observed in ordinary matter is a specific property of such matter or is due to the presence of some radio-active impurity. An account of these experiments was given by Professor J. J. Thomson in a discussion on the Radio-activity of Ordinary Matter at the British Association meeting at Cambridge, 1904. The results[451], as a whole, support the view that each substance gives out a characteristic type or types of radiation and that the radiation is a specific property of the substance. J. J. Thomson[452] has made experiments to observe the action of different substances in cutting off the external very penetrating radiation (section 279) observed by Cooke and McLennan. He found that some substances cut off this external radiation, while others had little if any effect. For example, the ionization in a closed vessel was reduced 17 per cent. by surrounding it with a thick lead envelope; but, on surrounding it with an equivalent absorbing thickness of water, or water mixed with sand, no sensible diminution was observed. In other experiments Wood[453] found that the diminution of the ionization by a given screen depended upon the material of the vessel. For example, the ionization in a lead vessel, surrounded by a lead screen, was reduced 10 per cent., while in an iron vessel it was reduced 24 per cent. He concludes from his experiments that the ionization observed in a closed vessel has a threefold origin. Part of it is due to an external penetrating radiation, part to a secondary radiation set up by it, while the remainder is due to an intrinsic radiation from the walls, altogether independent of the external radiation.

In some experiments of Campbell[454], the variation of the ionization current between two parallel plates was observed for a progressive increase of the distance between them. The effects observed are shown in Fig. 105. The curves at first rise rapidly, then bend over and finally become a straight line. The knee of the curve is at a different distance for the different substances. The shape of these curves indicates that two types of radiation are present, one of which is readily absorbed in the gas while the other, a more penetrating type of radiation, extends over the whole distance between the plates. In another series of experiments, one side of the testing vessel was of thin aluminium, and the ionization current was observed when an exterior screen was brought up to it. Lead gave a considerable increase, but the radiation from it was readily absorbed by an interposed screen. The radiation emitted by carbon and zinc was more than twice as penetrating as from lead.

Attempts were made to see whether a radio-active emanation was given off by dissolving solid substances and then keeping the solutions in a closed vessel and afterwards testing the activity of the air drawn from them. In some cases an emanation was observed, but the amount varied with different specimens of the same material; in others no effect was detected.

When linings of different substances were placed in a closed testing vessel, the ionization current in most cases fell at first, passed through a minimum, and then slowly increased to a maximum. For lead the maximum was reached in 9 hours, for tin in 14 and for zinc in 18 hours. These results indicate that an emanation is given off from the metal, and that the amount reaches a maximum value at different intervals in the various cases. This was confirmed by an examination of a piece of lead which was left in radium-free nitric acid. Twenty times the normal effect was observed after this treatment. This is probably due to the increase of porosity of the lead which allows a greater fraction of the emanation produced in the metal to diffuse out with the gas.

The activity observed in ordinary matter is extremely small. The lowest rate of production of ions yet observed is 10 per cubic centimetre per second in a brass vessel. Suppose a spherical brass vessel is taken of capacity 1 litre. The area of the interior surface would be about 480 sq. cms. and the total number of ions produced per second would be about 10⁴. Now it has been shown, in section 252, that an α particle projected from radium itself gives rise to 8·6 × 10⁴ ions before it is absorbed in the gas. An expulsion of one α particle every 8 seconds from the whole vessel, or of one α particle from each square centimetre of surface per hour would thus account for the minute conductivity observed. Even if it were supposed that this activity is the result of a breaking up of the matter composing the vessel, the disintegration of one atom per second per gram, provided it was accompanied by the expulsion of an α particle, would fully account for the conductivity observed.

While the experiments, already referred to, afford strong evidence that ordinary matter does possess the property of radio-activity to a feeble degree, it must not be forgotten that the activity observed is excessively minute, compared even with a weak radio-active substance like uranium or thorium. The interpretation of the results is complicated, too, by the presence of the radium emanation in the atmosphere, for we have seen that the surface of every body exposed to the open air must become coated with the slowly changing transformation products of the radium emanation. The distribution of radio-active matter throughout the constituents of the earth renders it difficult to be certain that any substance, however carefully prepared, is freed from radio-active impurities. If matter in general is radio-active, it must be undergoing transformation at an excessively slow rate, unless it be supposed (see Appendix A) that changes of a similar character to those observed in the radio-elements may occur without the appearance of their characteristic radiations.

Footnote 384:

Geitel, Phys. Zeit. 2, p. 116, 1900.

Footnote 385:

C. T. R. Wilson, Proc. Camb. Phil. Soc. 11, p. 32, 1900. Proc. Roy. Soc. 68, p. 151, 1901.

Footnote 386:

Elster and Geitel, Phys. Zeit. 2, p. 590, 1901.

Footnote 387:

Elster and Geitel, Phys. Zeit. 3, p. 76, 1901.

Footnote 388:

Rutherford and Allan, Phil. Mag. Dec. 1902.

Footnote 389:

Allan, Phil. Mag. Feb. 1904.

Footnote 390:

C. T. R. Wilson, Proc. Camb. Phil. Soc. 11, p. 428, 1902.

Footnote 391:

C. T. R. Wilson, Proc. Camb. Phil. Soc. 11, p. 428, 1902; 12, p. 17, 1903.

Footnote 392:

C. T. R. Wilson, Proc. Camb. Phil. Soc. 12, p. 85, 1903.

Footnote 393:

Allan, Phys. Rev. 16, p. 106, 1903.

Footnote 394:

McLennan, Phys. Rev. 16, p. 184, 1903.

Footnote 395:

Schmauss, Annal. d. Phys. 9, p. 224, 1902.

Footnote 396:

Elster and Geitel, Phys. Zeit. 3, p. 574, 1902.

Footnote 397:

Ebert and Ewers, Phys. Zeit. 4, p. 162, 1902.

Footnote 398:

Sarasin, Tommasina and Micheli, C. R. 139, p. 917, 1905.

Footnote 399:

J. J. Thomson, Phil. Mag. Sept. 1902.

Footnote 400:

Ebert, Sitz. Akad. d. Wiss. Munich, 33, p. 133, 1903.

Footnote 401:

J. J. Thomson, Phil. Mag. Sept. 1902.

Footnote 402:

Adams, Phil. Mag. Nov. 1903.

Footnote 403:

Bumstead and Wheeler, Amer. Journ. Science, 17, p. 97, Feb. 1904.

Footnote 404:

Bumstead, Amer. Journ. Science, 18, July, 1904.

Footnote 405:

Dadourian, Amer. Journ. Science, 19, Jan. 1905.

Footnote 406:

H. S. Allen and Lord Blythswood, Nature, 68, p. 343, 1903; 69, p. 247, 1904.

Footnote 407:

Strutt, Proc. Roy. Soc. 73, p. 191, 1904.

Footnote 408:

Himstedt, Ann. d. Phys. 13, p. 573, 1904.

Footnote 409:

Elster and Geitel, Phys. Zeit. 5, No. 12, p. 321, 1904.

Footnote 410:

Dorn, Abhandl. d. Natur. Ges. Halle, 25, p. 107, 1904.

Footnote 411:

Schenck, Thesis Univ. Halle, 1904.

Footnote 412:

Mache, Wien. Ber. 113, p. 1329, 1904.

Footnote 413:

Curie and Laborde, C. R. 138, p. 1150, 1904.

Footnote 414:

Blanc, Phil. Mag. Jan. 1905.

Footnote 415:

Boltwood, Amer. Journ. Science, 18, Nov. 1904.

Footnote 416:

Elster and Geitel, Phys. Zeit. 4, p. 522, 1903.

Footnote 417:

Elster and Geitel, Phys. Zeit. 5, No. 1, p. 11, 1903.

Footnote 418:

Vincenti and Levi Da Zara, Atti d. R. Instit. Veneto d. Scienze, 54, p. 95, 1905.

Footnote 419:

Burton, Phil. Mag. Oct. 1904.

Footnote 420:

Elster and Geitel, Phys. Zeit. 6, No. 3, p. 67, 1905.

Footnote 421:

Rutherford and Allan, Phil. Mag. Dec. 1902.

Footnote 422:

Elster and Geitel, Phys. Zeit. 4, p. 138, 1902; 4, p. 522, 1903.

Footnote 423:

Saake, Phys. Zeit. 4, p. 626, 1903.

Footnote 424:

Simpson, Proc. Roy. Soc. 73, p. 209, 1904.

Footnote 425:

McLennan, Phys. Rev. 16, p. 184, 1903, and Phil. Mag. 5, p. 419, 1903.

Footnote 426:

McLennan, Phys. Rev. No. 4, 1903.

Footnote 427:

Rutherford and Cooke, Americ. Phys. Soc. Dec. 1902.

Footnote 428:

Cooke, Phil. Mag. Oct. 1903.

Footnote 429:

Allan, Phil. Mag. Feb. 1904.

Footnote 430:

Ebert, Phys. Zeit. 2, p. 622, 1901. Zeitschr. f. Luftschiffahrt, 4, Oct. 1902.

Footnote 431:

Schuster, Proc. Manchester Phil. Soc. p. 488, No. 12, 1904.

Footnote 432:

Mache and Von Schweidler, Phys. Zeit. 6, No. 3, p. 71, 1905.

Footnote 433:

Langevin, C. R. 140, p. 232, 1905.

Footnote 434:

Schuster, British Assoc. 1903.

Footnote 435:

J. J. Thomson, Conduction of Electricity through Gases, p. 324, 1903.

Footnote 436:

Miss Gates, Phys. Rev. 17, p. 499, 1903.

Footnote 437:

Villard, Société de Physique, July, 1900.

Footnote 438:

Geitel, Phys. Zeit. 2, p. 116, 1900.

Footnote 439:

C. T. R. Wilson, Proc. Camb. Phil. Soc. 11, p. 52, 1900. Proc. Roy. Soc. 68, p. 152, 1901.

Footnote 440:

Rutherford and Allan, Phil. Mag. Dec. 1902.

Footnote 441:

Patterson, Phil. Mag. August, 1903.

Footnote 442:

Harms, Phys. Zeit. 4, No. 1, p. 11, 1902.

Footnote 443:

Cooke, Phil. Mag. Oct. 1903.

Footnote 444:

Wilson, Proc. Roy. Soc. 69, p. 277, 1901.

Footnote 445:

Jaffé, Phil. Mag. Oct. 1904.

Footnote 446:

Patterson, Phil. Mag. Aug. 1903.

Footnote 447:

Strutt, Phil. Mag. June, 1903. Nature, Feb. 19, 1903.

Footnote 448:

McLennan and Burton, Phys. Rev. No. 4, 1903. J. J. Thomson, Nature, Feb. 26, 1903.

Footnote 449:

Cooke, Phil. Mag. Aug. 6, 1903. Rutherford, Nature, April 2, 1903.

Footnote 450:

Eve, Nature, March 16, 1905.

Footnote 451:

See article in Le Radium, No. 3, p. 81, Sept. 15, 1904.

Footnote 452:

J. J. Thomson, Proc. Camb. Phil. Soc. 12, p. 391, 1904.

Footnote 453:

Wood, Phil. Mag. April, 1905.

Footnote 454:

Campbell, Nature, p. 511, March 31, 1904. Phil. Mag. April, 1905.

APPENDIX A. PROPERTIES OF THE α RAYS.

A brief account is given here of some investigations made by the writer on the properties of the α rays from radium—investigations which were not completed in time for the results to be incorporated in the text.

The experiments were undertaken primarily with a view of determining accurately the value of e/m of the α particle from radium, in order to settle definitely whether or not it is an atom of helium. In the previous experiments of the writer, Becquerel, and Des Coudres, on this subject (sections 89, 90, and 91), a thick layer of radium in radio-active equilibrium has been used as a source of α rays. Bragg (section 103) has shown that the rays emitted from radium under such conditions are complex, and consist of particles projected over a considerable range of velocity. In order to obtain a homogeneous pencil of rays it is necessary to use a very thin layer of a simple radio-active substance as a source of rays. In the experiments that follow, this condition was fulfilled by using a fine wire which was made active by exposure for several hours in the presence of a large quantity of radium emanation. By charging the wire negatively the active deposit was concentrated upon the wire, which was made intensely active. The active deposit initially contains radium A, B, and C. The activity of radium A practically disappears in about fifteen minutes, and the α radiation is then due entirely to the single product radium C, since radium B is a rayless product. The activity of radium C decreases to about 15 per cent. of its initial value after two hours.

=Magnetic deflection of the α rays.= The photographic method was employed to determine the deviation of the pencil of rays in a magnetic field. The experimental arrangement is shown in Fig. 106. The rays from the active wire, which was placed in a slot, passed through a narrow slit and fell normally on a photographic plate, placed at a known distance above the slit. The apparatus was enclosed in a brass tube which could be exhausted rapidly to a low pressure by means of a Fleuss pump. The apparatus was placed in a strong uniform magnetic field parallel to the plane of the slit. The magnetic field was reversed every ten minutes, so that on developing the plate two narrow bands were observed, the distance between which represented twice the deviation from the normal of the pencil of rays by the magnetic field. The width of the band was found to be the same whether the magnetic field was applied or not, showing that the pencil of rays was homogeneous and consisted of α particles projected with the same velocity.

By placing the photographic plate at different distances from the slit it was found that the rays, after entering the magnetic field, described the arc of a circle of radius ρ equal to 42·0 cms. The strength of field H was 9470 C.G.S. units, so that the value of _H_ρ for the α particles expelled from radium C is 398,000. This is in good agreement with the maximum values of _H_ρ, previously found for radium rays (see section 92).

The electric deviation of the rays from radium C has not yet been accurately measured, but an approximate determination of e/m for the α particles can be obtained by assuming that the heating effect of radium C is a measure of the kinetic energy of the α particles expelled from it. We have seen in section 246 that the heating effect of the radium C present in one gram of radium in radio-active equilibrium is 31 gram calories per hour, which corresponds to an emission of energy of 3·6 × 10⁵ ergs per second. Now when radio-active equilibrium is reached, the number of α particles expelled from radium C per second is equal to the number of α particles expelled per second from radium at its minimum activity. This number, n, is 6·2 × 10¹⁰ (section 93).

Then ½ mnv² = 3·6 × 10⁵,

or (m/e)v² = 1·03 × 10¹⁶,

substituting the value of n, and the value of the ionic charge e. The value of e in this case has not been assumed, since n = i/e, where i was the measured current due to the charge carried by the α rays.

From the magnetic deflection, it is known that

(m/e)v = 3·98 × 10⁵.

From these two equations we obtain

v = 2·6 × 10⁹ cms. per second. e/m = 6·5 × 10³ electromagnetic units.

These values are in surprisingly good agreement with the previous values of the writer and Des Coudres (section 91). On account of the uncertainty attaching to the value of n, not much weight can be attached to the determination by this method of the constants of the α particles.

=Decrease of velocity of the α particles in passing through matter.= Some experiments were made to determine the velocity of the α particles from radium C after passing through known thicknesses of aluminium. The previous apparatus was employed, and the distance between the photographic bands was observed for successive layers of aluminium foil, each ·00031 cms. thick, placed over the active wire. The photographic plate was placed 2 cms. above the slit, and the magnetic field extended 1 cm. below the slit. The amount of deviation of the rays is inversely proportional to their velocity after traversing the aluminium screens. The impressions on the plate were clear and distinct, and about the same in all cases, showing that the rays were still homogeneous after passing through the aluminium.

A clear photographic impression was obtained for 12 layers of foil, but it was not found possible to obtain any effect through 13 layers. This result shows that the photographic action of the rays, like the ionizing action, ceases very abruptly.

The results obtained are shown in the following table. Assuming that the value of e/m is constant, the third column gives the velocity of the α particles after traversing the aluminium. This is expressed in terms of V₀, the velocity of the α particle when the screens are removed.

Number of Distance Velocity layers of between of α aluminum bands on the particles foil plate

0 1·46 mms. 1·00 V₀

5 1·71 „ ·85 „

8 1·91 „ ·76 „

10 2·01 „ ·73 „

12 2·29 „ ·64 „

13 No photographic effect

The velocity of the α particle is thus reduced only about 36 per cent. of its initial value when it fails to produce any action on the photographic plate.

Now Bragg has shown (section 104) that the α particle produces approximately the same number of ions per cm. of path in air over its whole range. Consequently, the simplest assumption to make is that the energy of the α particle is diminished by a constant amount in traversing each layer of foil. After passing through 12 layers the kinetic energy is reduced to 41 per cent. of the maximum. Each layer of foil thus absorbs 4·9 per cent. of the maximum energy. The observed kinetic energy of the α particle after passing through successive layers of foil, and the value calculated on the above assumptions, are shown in the following table.

Number of Observed Calculated layers of energy energy aluminum foil

0 100 100

5 73 75

8 58 61

10 53 51

12 41 41

The experimental and theoretical values agree within the limits of experimental error. We may thus conclude, as a first approximation, that the same proportion of the total energy is abstracted from the α particles in passing through equal distances of the absorbing screen.

=Range of ionization and photographic action in air.= The abrupt falling off of the photographic impression after the rays had passed through 12 layers of foil suggested that it might be directly connected with the corresponding abrupt falling off of the ionization in air, so clearly brought out by Bragg. This was found to be the case. It was found experimentally that the absorption in each layer of aluminium foil was equivalent to that produced by a distance of ·54 cms. of air. Twelve layers of foil thus corresponded to 6·5 cms. of air. Now Bragg found that the α rays from radium C ionize the air for a distance 6·7 cms., and that the ionization then falls off very rapidly. We may thus conclude that the α rays cease to affect the photographic plate at the same velocity as that at which they cease to ionize the gas. This is a very important result, and, as we shall see later, suggests that the action on the photographic plate is due to an ionization of the photographic salts.

The velocity of the α particles from the different radio-active products can at once be calculated, knowing the maximum range in air of the α rays from each product. The latter have been experimentally determined by Bragg. The velocity is expressed in terms of V₀, the initial velocity of the α particles from radium C. The rays from radium C are projected with a greater velocity than the rays from the other products of radium.

Product Maximum Velocity range of α of α particles particles in air

Radium 3 cms. ·82 V₀

Emanation 3·8 or 4·4 ·87 or ·90 cms. V₀

Rad. A 4·4 or 3·8 ·90 or ·87 „ V₀

Rad. C 6·7 1·00 V₀ „

It is difficult to determine from the experiments whether the range 3·8 cms. belongs to the rays from the emanation or from radium A. The mean velocity of the α particles is thus ·90 V₀, and the maximum variation for the individual products does not vary more than 10 per cent. from the mean value.

The results of Becquerel, discussed in section 92, at once receive an explanation on the above results. The α particles, expelled from radium in radio-active equilibrium, have all ranges lying between 0 and 6·7 cms. of air. The velocity of the α particles which are able to produce a photographic impression varies between ·64 V₀ and V₀. The particles which have only a short range in air are projected with a smaller velocity than those which have a greater range. The former are in consequence more bent by a magnetic field. It is thus to be expected that the apparent curvature of the path of rays in a uniform magnetic field will be greater close to the radium than at some distance away.

=Range of phosphorescent action in air.= Some experiments were also made to see whether the action of the α rays in producing luminosity in substances like zinc sulphide, barium platinocyanide, and willemite, ceased at the same distance as the ionizing action.

A very active wire was placed on a movable plate, the distance of which from a fixed screen of phosphorescent substance could be varied. The distance at which the phosphorescent action ceased could be determined fairly accurately. Different thicknesses of aluminium foil were then placed over the active wire, and the corresponding distance at which the luminosity disappeared was measured. The results are shown graphically in Fig. 107, where the ordinates represent the distance of the phosphorescent screen from the active wire, and the abscissae the number of layers of aluminium foil, each ·00031 cms. thick.

It is seen that the curve joining the points is a straight line. 12·5 thicknesses of foil absorbed the rays to the same extent as 6·8 cms. of air, so that each thickness of aluminium corresponded in absorbing power to ·54 cms. of air. For a screen of zinc sulphide, the phosphorescent action ceased at a distance of air of 6·8 cms., showing that the photographic and phosphorescent ranges of the α rays in air were practically identical.

The experiments with barium platinocyanide and willemite were more difficult, as the β and γ rays from the active wire produced a luminosity comparable with that produced by the α rays. Fairly concordant results, however, were obtained by introducing a thin sheet of black paper between the active wire and the screen. If the luminosity was sensibly changed, it was concluded that the α rays still produced an effect, and in this way the point of cessation of phosphorescent action could be approximately determined. For example, with eight thicknesses of foil over the active wire the additional thickness of air required to cut off the phosphorescent effect of the a rays was 2·5 cms. for willemite, and 2·1 cms. for barium platinocyanide.

The corresponding distance for zinc sulphide was 2·40 cms., a value intermediate between the other two.

Since eight layers of foil are equivalent to 4·3 cms. of air, the ranges in air of phosphorescent action for zinc sulphide, barium platinocyanide, and willemite correspond to 6·7, 6·8, and 6·4 cms. respectively. The differences observed are quite likely to be due to experimental error.

=Discussion of results.= We have seen that the ionizing, phosphorescent, and photographic actions of the α rays emitted from radium C cease after traversing very nearly the same distance of air. This is a surprising result when it is remembered that the α particle, after passing through this depth of air, still possesses a velocity of at least 60 per cent. of its initial value. Taking the probable value of the initial velocity of the α particle from radium C as 2·5 × 10⁹ cms. per sec., the ionizing, phosphorescent, and photographic actions cease when the velocity of the α particle falls below 1·5 × 10⁹ cms. per second, that is, a velocity of about ¹⁄₂₀ of that of light. The particle still possesses nearly 40 per cent. of its initial energy of projection at this stage.

These results show that the property of the α rays of producing ionization in gases, of producing luminosity in some substances, and of affecting a photographic plate, ceases when the velocity of the α particle falls below a certain fixed value which is the same in each case. It seems reasonable, therefore, to suppose that these three properties of the α rays must be ascribed to a common cause. Now the absorption of the α rays in gases is mainly a consequence of the energy absorbed in the production of ions in the gas. When the α particles are completely absorbed in the gas, the same total amount of ionization is produced, showing that the energy required to produce an ion is the same for all gases. On the other hand, for a constant source of radiation, the ionization per unit volume of the gas is approximately proportional to its density. Since the absorption of the α rays in solid matter is approximately proportional to the density of the absorbing medium compared with air, it is probable that this absorption is also a result of the energy used up in producing ions in the solid matter traversed, and that about the same amount of energy is required to produce an ion in matter whether solid, liquid, or gaseous.

It is probable, therefore, that the production of ions in the phosphorescent material and in the photographic film would cease at about the same velocity for which the α particle is unable to ionize the gas. On this view, then, the experimental results receive a simple explanation. The action of the α rays in producing photographic and phosphorescent actions is primarily a result of ionization. This ionization may possibly give rise to secondary actions which influence the effects observed.

This point of view is of interest in connection with the origin of the “scintillations” observed in zinc sulphide and other substances when exposed to the action of the α rays. This effect is ascribed by Becquerel to the cleavage of the crystals under the bombardment of the α particles. These results, however, show that we must look deeper for the explanation of this phenomenon. The effect is primarily due to the production of ions in the phosphorescent material and not to direct bombardment, for we have seen that the α particle produces no scintillations when it still possesses a large amount of kinetic energy. It seems not unlikely that the scintillations produced by the α rays must be ascribed to the recombination of the ions which are produced by the α particle in the crystalline mass. It is difficult to see how this ionization could result in a cleavage of the crystals.

This close connection of the photographic and phosphorescent actions of the α rays with their property of producing ions, raises the question whether photographic and phosphorescent actions in general may not, in the first place, be due to a production of ions in the substance.

=Ionization curve for the α rays from radium C.= Mr McClung, working in the laboratory of the writer, has recently determined the relative ionization per unit path of the α particles projected from radium C, using the method first employed by Bragg and discussed in section 104. An active wire, exposed for several hours to the emanation from radium, was used as a source of rays. The α particles were homogeneous, since the film of radio-active matter was extremely thin.

The relation between the ionization observed over the cross section of the narrow cone of rays and the distance from the source of rays is shown in Fig. 108.

The curve exhibits the same peculiarities as those given by Bragg for a thin film of matter of one kind. The ionization of the α particle per unit path increases slowly for about 4 cms. There is then a more rapid increase just before the α particle ceases to ionize the gas, and then a rapid falling off. The ionization does not appear to end so abruptly as is really the case, since there is a correction to be applied for the angle subtended by the cone of rays. The maximum range of the α rays in air was 6·7 cms., a number in agreement with that obtained by Bragg by measurements on the range of the rays from radium.

These results show that the ionization per unit path of the α particle increases at first slowly and then rapidly with decrease of velocity until the rays cease to ionize the gas.

=Energy required to produce an ion.= From the above results the energy required to produce an ion by collision of the α particle with the gas molecules can readily be deduced. The α particles, emitted from radium itself, are initially projected with a velocity ·88_V₀_ where V₀ is the initial velocity of projection of the α particles from radium C. The α particles cease to ionize the gas at a velocity ·64_V₀_. From this it can at once be deduced that ·48 of the total energy of the α particle, shot out by radium itself, is absorbed when it ceases to ionize the gas. Assuming that the heating effect of radium at its minimum activity—25 gram calories per hour per gram—is a measure of the kinetic energy of the expelled α particles, it can be calculated that the kinetic energy of each α particle is 4·7 × 10⁻⁶ ergs. The amount of energy absorbed when the α particle just ceases to ionize the gas is 2·3 × 10⁻⁶ ergs. Assuming that this energy is used up in ionization, and remembering that the α particle from radium itself produces 86000 ions in its path (section 252), the average energy required to produce an ion is 2·7 × 10⁻¹¹ ergs. This is equivalent to the energy acquired by an ion moving freely between two points differing in potential by 24 volts.

Townsend found that fresh ions were produced by an electron for a corresponding difference of potential of 10 volts. Stark, from other data, obtained a value 45 volts, while Langevin considers that 60 volts is an average value. The value obtained by Rutherford and McClung for ionization by X-rays was 175 volts, and is probably too high.

=Rayless changes.= We have seen that the α particles from the radio-active substances are projected with an average velocity not more than 30 per cent. greater than the minimum velocity, below which the α particles are unable to produce any ionizing, photographic, or phosphorescent action. Such a conclusion suggests that the property of the radio-active substances of emitting α particles has been detected because the α particles were projected slightly above this minimum velocity. A similar disintegration of matter may be taking place in other substances at a rate much greater than in uranium without producing much electrical effect, provided the α particles are projected below the critical velocity.

The α particle, on an average, produces about 100,000 ions in the gas before it is absorbed, so that the electrical effect observed is about 100,000 times as great as that due to the charge carried by the α particles alone.

It is not unlikely that the numerous rayless products which have been observed may undergo disintegration of a similar character to the products which obviously emit α rays. In the rayless product the α particle may be expelled with a velocity less than 1·5 × 10⁹ cms. per second and so fail to produce much electrical effect.

These considerations have an important bearing on the question whether matter in general is radio-active. The property of emitting α particles above the critical velocity may well be a property only of a special class of substances, and need not be exhibited by matter in general. At the same time the results suggest that ordinary matter may be undergoing transformation accompanied by the expulsion of α particles at a rate much greater than that shown by uranium, without producing appreciable electrical or photographic action.

APPENDIX B. RADIO-ACTIVE MINERALS.

Those natural mineral substances which possess marked radio-active properties have been found to contain either uranium or thorium, one of these elements being always present in sufficient proportion readily to permit its chemical separation and identification by the ordinary analytical methods[455].

A large number of uranium and thorium minerals are known at the present time, but they are for the most part found very sparingly, and some of them have been observed to occur only in a single locality. The chief commercial sources of uranium are uraninite, gummite, and carnotite, while thorium is obtained almost exclusively from monazite.

Rutherford and Soddy (Phil. Mag. 65, 561 (1903)), were the first to call attention to the important fact that the relations between the various radio-active substances and the other elements could best be determined from the study of the natural minerals in which these bodies occur, since these minerals represent mixtures of extreme antiquity, which have remained more or less undisturbed for almost countless ages. In dealing with these matters, however, it is highly important that we bring to our aid the data furnished by geology and mineralogy, from which it is often possible to determine the relative ages of the different substances with at least a rough degree of approximation. Thus, for example, if a certain mineral occurs as a primary constituent of a rock of remote geological period, it can safely be assumed that its age is greater than that of a similar or different mineral occurring in a later formation. It is, moreover, quite evident that those minerals which are obviously produced by the decomposition and alteration of the primary minerals, through the action of percolating water and other agencies acting from the surface downward, are of less antiquity than the primary minerals from which they originated. Through the application of these considerations it should, in general, be possible to arrange the various minerals roughly in the order of their probable ages.

The most familiar and widely known uranium mineral is uraninite, commonly called pitchblende, which consists essentially of uranium dioxide (UO₂), uranium trioxide (UO₃), and lead oxide (PbO), present in varying proportions. The uraninites can be distinguished as primary, namely, those which occur as a primary constituent of pegmatitic dikes and coarse granites, and secondary, when they occur in metalliferous veins associated with the sulphides of silver, lead, copper, nickel, iron, and zinc. The former varieties are quite frequently crystalline in character, contain a larger proportion of the rare earths and helium, and have a higher specific gravity than the latter, which are always massive and botryoidal.

The following are the most prominent localities in which primary uraninites occur:

1. North Carolina, U.S.A. (especially in Mitchell and Yancey counties). The uraninite is found in a coarse pegmatitic dike which is mined for the mica constituent. The associated feldspar of the dike is considerably decomposed through the action of meteoric waters and gases, and the uraninite itself is largely altered into the secondary minerals gummite and uranophane through the same agencies. Among the associated primary minerals are allanite, zircon, columbite, samarskite, fergusonite and monazite, while the secondary minerals include gummite, thorogummite, uranophane, autunite, phosphuranylite, hatchettolite, and cyrtolite. The geological period of this formation is difficult to establish with certainty, but is stated to be perhaps Archean, or possibly to correspond with the close of the Ordovician or with the Permian.

2. Connecticut, U.S.A. The best known localities are Glastonbury, where the uraninite is found in the feldspar quarries, and Branchville, where it occurs in an albitic granite. Both of these localities have furnished fine crystals. The geological period probably corresponds with the close of the Ordovician or Carboniferous eras, and is stated to be certainly Post-Cambrian and Pre-Triassic. Among the associated minerals are (primary) columbite, (secondary) torbernite and autunite.

3. Southern Norway, particularly in the neighbourhood of Moss. Here uraninite occurs in the augite-syenite and pegmatite. The varieties found are known as cleveite and bröggerite, and among the primary associated minerals are orthite, fergusonite, monazite, and thorite. The period is stated to be Post-Devonian.

4. Llano County, Texas. The variety of uraninite known as nivenite is found here in a quartzose pegmatite, associated with the primary minerals gadolinite, allanite and fergusonite, and the secondary minerals cyrtolite, yttrialite, gummite, and thorogummite.

Secondary uraninite is found at Johanngeorgenstadt, Marienberg and Schneeberg in Saxony, at Joachimsthal and Pribam in Bohemia, at Cornwall in England, and at Black Hawk, Colorado, and in the Black Hills, South Dakota, in the United States. The exact geological period of most of these secondary occurrences is somewhat uncertain, but they are undoubtedly very much later than the primary occurrences mentioned above.

As a matter of general interest the analysis of a typical primary uraninite (No. 1) and of a typical secondary uraninite (No. 2) is given below[456]:

No. 1 No. 2 Glastonbury, Johanngeorgenstadt, Conn. Saxony

Sp. Gr. 9·59 6·89

UO₃ 26·48 60·05

UO₂ 57·43 22·33

ThO₂ 9·79 ...

CeO₂ 0·25 ...

La₂O 0·13 ...

Y₂O₃ 0·20 ...

PbO 3·26 6·39

CaO 0·08 1·00

He und. und.

H₂O 0·61 3·17

Fe₂O₃ 0·40 0·21

SiO₂ 0·25 0·50

Al₂O₃ ... 0·20

Bi₂O₃ ... 0·75

CuO ... 0·17

MnO ... 0·09

MgO ... 0·17

Na₂O ... 0·31

P₂O₅ ... 0·06

SO₃ ... 0·19

As₂O₃ ... 2·34

Insoluble 0·70 ...

The following list comprises the more important radio-active minerals, with their approximate chemical composition and some notes on their occurrence and probable origin.

Name Composition Remarks

Uraninite, Oxides of Occurs primary Cleveite, uranium and as a Bröggerite, lead. Usually constituent of Nivenite, contains rocks and Pitchblende thorium, secondary in other rare veins with earths and metalliferous helium. sulphides Uranium 50-80%. Thorium 0-10%

Gummite (Pb, Ca) An alteration U₃SiO₁₂ product of . 6H₂O? uraninite. Uranium Formed by the 50-65% action of percolating waters

Uranophane, CaO . 2UO₃ An alteration Uranotil . 2SiO₂ . product of 6H₂O uraninite Uranium through 44-56% gummite

Carnotite A vanadate of Occurs as a uranium and secondary potassium. mineral Uranium impregnating a 42-51% porous, sedimentary sandstone. Found in Colorado and Utah

Uranosphaerite Bi₂O₃ . Alteration 2UO₃ . product of 3H₂O. other uranium Uranium 41% minerals

Torbernite, CuO . 2UO₃ „ „ Cuprouranite . P₂O₅ . 8H₂O. Uranium 44-51%

Autunite, CaO . 2UO₃ „ „ Calciouranite . P₂O₅ . 8H₂O. Uranium 45-51%

Uranocircite BaO . 2UO₃ „ „ . P₂O₅ . 8H₂O. Uranium 46%

Phosphuranylite 3UO₃ . „ „ P₂O₅ . 6H₂O. Uranium 58-64%

Zunerite CuO . 2UO₃ „ „ . As₂O₅ . 8H₂O. Uranium 46%

Uranospinite CaO . 2UO₃ „ „ . As₂O₅ . 8H₂O. Uranium 49%

Walpurgite 5Bi₂O₃ „ „ . 3UO₃ . As₂O₅ . 12H₂O. Uranium 16%

Thorogummite UO₃ . A variety of 3ThO₂ . gummite 3SiO₂ . 6H₂O? Uranium 41%

Thorite, ThSiO₄. A primary Orangite, Uranium constituent of Uranothorite 1-10%. pegmatite Thorium oxide dikes 48-71%

Thorianite Oxide of Occurs as a thorium, primary uranium, the constituent of rare earths a pegmatite and lead. dike in Contains a Ceylon. relatively Geological age large probably proportion of Archean helium. Uranium 9-10%. Thorium oxide 73-77%

Samarskite Niobate and Primary tantalate of constituent of rare earths. pegmatite Uranium 8-10% dikes

Fergusonite Metaniobate „ „ and tantalate of rare earths. Uranium 1-6%

Euxenite Niobate and „ „ titanate of rare earths. Uranium 3-10%

Monazite Phosphate of „ „ the rare earths, chiefly cerium. Uranium 0·3-0·4%

Footnote 455:

An apparent exception has been observed by Danne in the case of certain lead minerals which occur under peculiar conditions at d’Issy-l’Évêque, France. See p. 465.

Footnote 456:

Hillebrand, Am. J. Sci. 40, 384 (1890); ibid. 42, 390 (1891).

INDEX.

The numbers refer to the pages.

α rays discovery of, 141 nature of, 141 magnetic deviation of, 142 et seq. electrostatic deviation of, 146 velocity of, 148 value of e/m for, 148 charge carried by, 151 et seq. number of α particles expelled from one gram of radium, 155 mass and energy of, 156 origin of, in atomic disintegration, 157 scintillations produced by, 158 et seq. absorption of, by matter, 161 et seq. increase of absorption with thickness of matter traversed, 163 relative absorption of α rays from radio-elements, 164 absorption of, by gases, 165 et seq. connection between absorption and density, 169 relation between ionization and absorption, 170 theory of absorption of, 170 et seq. range of ionization of, 172 et seq. complexity of α rays from radium, 174 et seq. effect of thickness of layer of radiating matter on emission of, 195 relative ionization produced by α and β rays, 196 et seq. phosphorescence by α rays, 202 et seq. connection of, with radio-active changes, 235, 444 et seq., 455 from the emanations, 263 emission of energy from radio-elements in form of α rays, 419 _et seq._ connection of heat emission of radium with α rays, 421 et seq. number of ions produced by an α particle, 433 absence of, in rayless changes, 454 emission from active products, 454 et seq. loss of weight due to expulsion of, 473 α particles consist of helium, 479 et seq. magnetic deflection of, from radium C, 543 velocity and e/m for, from radium C, 543 et seq. diminution of velocity of, in passing through matter, 545 diminution in velocity of, in passing through aluminium, 545 velocity of, when ionization ceases, 545 et seq. connection of phosphorescent, photographic, and ionization effects produced by, 546 et seq. energy required to produce an ion by α rays, 551

Abraham apparent mass of moving charged body, 71, 127

Absorption law of, in gases, 64 et seq. relative absorption of α, β and γ rays by matter, 111 connection between absorption and ionization, 134 et seq., 170 _et seq._ of β rays by solids, 134 et seq. connection between absorption and density for β rays, 137 of β rays in radio-active matter, 140 of α rays by solids, 161 et seq. of α rays in gases, 167, 170 et seq. connection between absorption and density for α rays, 169 theory of, 170 et seq. of γ rays by solids, 179 et seq. connection between absorption and density for γ rays, 181 of rays from the emanations, 263 of penetrating rays from the earth, 520, 540

Actinium methods of separation of, 20 et seq. properties of, 21 similarity to “emanating substance” of Giesel, 21 possible connection with radio-activity of thorium, 28 emanation from, 249 excited activity produced by, 311 effect of magnetic field on excited activity from, 324 separation of actinium X, 365 decay of actinium X, 365 source of actinium emanation, 365 analysis of active deposit of, 366 radiations from products of, 368 penetrating power of β and γ rays from, 368 products of, 369 table of products of, 449 possible origin of, 464

Actinium A separation and period of, 367 et seq.

Actinium B period of, 368 properties of, 368

Actinium X separation and decay of, 364 et seq. production of emanation by, 365

Adams decay of activity of emanation from well water, 511 decay of excited activity from the emanation, 511

Age of radium, 457 of sun and earth, 492 et seq.

Allan, S. J. increase with time of excited activity from atmosphere, 505 radio-activity of snow, 506 effect of conditions on decay of activity from air, 519, 523

Allan and Rutherford decay of excited activity from the atmosphere, 503 ionization of air in closed vessels, 534

Allen, H. S. and Lord Blythswood radium emanation in Bath springs, 513

Anderson and Hardy action of radium rays on the eye, 217

Armstrong and Lowry radio-activity and phosphorescence, 444

Arnold rays from phosphorescent substances, 4

Aschkinass and Caspari action of radium rays on microbes, 216

Atmosphere excited activity from, 501 et seq. radio-activity of, due to emanations, 504 diffusion of emanations into, from the earth, 507 effect of temperature, pressure, &c. on radio-activity of, 517 _et seq._ presence of very penetrating radiation in, 520 comparison of radio-activity of, with radio-elements, 521 et seq. amount of radium emanation in, 524 et seq. ionization of, due to radium emanation, 526

Atom number of per c.c., 54 disintegration of, 234 et seq. complex nature of, 235 changing atoms, 444 et seq. possible causes of disintegration of, 486 evolution of, 496

Atomic weight of radium by chemical methods, 17 from spectroscopic evidence, 18 emanations, 273 of radio-elements and connection with radio-activity, 445

β rays discovery of, 113 magnetic deflection of, 114 complexity of, 116 examination by the electrical method, 118 effect of, on a fluorescent screen, 119 charge carried by the, 120 et seq. electrostatic deviation of, 124 velocity of, and value of e/m for, 126 variation of e/m with velocity of, 127 et seq. distribution of velocity amongst β particles, 131 absorption of, 134 et seq. variation of absorption with density, 136 et seq. number of β particles stopped by matter, 137 et seq. variation of intensity of, with thickness of layer, 140 secondary β rays, 189 et seq. relative ionization produced by α and β rays, 196 relative energy emitted in form of α and β rays, 196 et seq. phosphorescent action of, 201 et seq. physical action produced by, 207 et seq. chemical action of, 213 physiological action of, 216 from Ur X, 347 from active deposit of radium, 377 et seq. significance of appearance of, only in last radio-active changes, 455 change of weight due to expulsion of, 473

Barium platinocyanide phosphorescence of, under radium rays, 203 change of colour due to radium rays, 205

Barkla polarization of X rays, 80

Barnes and Rutherford heating effect of radium emanation, 421, 429 connection of heating effect with radio-activity, 421 heating effect of active deposit, 425 heating effect of γ rays, 429 heating effect of emanation, 431 division of heating effect among active products, 433

Bary phosphorescence under radium rays, 202

Baskerville activity of thorium, 29 phosphorescence of kunzite under radium rays, 203

Baskerville and Kunz phosphorescence of substances under radium rays, 204

Beattie, Smolan and Kelvin discharging power of uranium rays, 7

Becquerel rays from calcium sulphide, 4 rays from uranium, 5 et seq. permanence of uranium rays, 6 discharging power of uranium rays, 6 magnetic deflection of radium rays by photographic method, 114 _et seq._ curvature of radium rays in a magnetic field, 115 et seq. complexity of radium rays, 116 et seq. electrostatic deflection of β rays of radium, 124 et seq. value of e/m for β rays of radium, 126 et seq. magnetic deviation of α rays of radium and polonium, 145 trajectory of rays of radium in magnetic field, 148 scintillations due to cleavage of crystals, 160 γ rays from radium, 179 secondary rays produced by active substances, 187 phosphorescence produced by radium rays, 201 conductivity of paraffin under radium radiation, 210 effect of temperature on uranium rays, 210 chemical action of radium rays, 214 removal of activity from uranium by precipitation with barium, 219 theory of radio-activity, 438

Bemont et M. et Mme Curie discovery of radium, 13

Berndt spectrum of polonium, 23

Blanc thorium in sediments from hot springs, 514

Blythswood, Lord and Allen, H. S. radium emanation in Bath springs, 513

Bödlander and Runge evolution of gases from radium, 215

Boltwood origin of radium, 460 amount of radium in minerals, 460 proportionality of uranium and radium in minerals, 461 production of lead by uranium, 484 radium emanation in spring water, 514 method of standardization of amount of emanation in waters, 514

Boys rate of dissipation of charge, 531

Bragg and Kleeman theory of absorption of α rays, 172 et seq. relation between ionization and absorption, 174 et seq. range of α rays in air, 174 four sets of α rays from radium, 174 et seq.

Bronson use of steady deflection method with an electrometer, 104 decay of thorium emanation, 242 decay of excited activity from actinium, 312

Brooks, Miss variation of excited activity from thorium for short exposures, 304 effect of dust on distribution of excited activity, 305 decay curves of excited activity of radium measured by α and β rays, 307 et seq. decay curves of excited activity from actinium, 312

Brooks and Rutherford absorption of α rays by matter, 161 comparison of absorption of α rays from radio-elements, 164 diffusion of radium emanation, 270 decay of excited activity from radium, 306

Bumstead presence of thorium emanation in atmosphere, 512

Bumstead and Wheeler diffusion of radium emanation, 273 emanation from surface water and the soil, 512, 522 identity of emanation from soil with radium emanation, 512, 522

Burton radium emanation in petroleum, 516

Burton and McLennan penetrating radiation from the earth, 520 radio-activity of ordinary materials, 537 emanation from ordinary matter, 538

Campbell radio-activity of ordinary materials, 540

Canal rays discovery of, 78 magnetic and electric deflection of, 78 value of e/m for, 78 similarity of, to α rays, 110

Capacity of electroscopes, 87 of electrometers, 94, 102 standards of, 102

Carbonic acid radio-activity of natural, 516

Caspari and Aschkinass action of radium rays on microbes, 216

Cathode rays discovery of, 73 magnetic and electric deflection of, 74 value of e/m for, 75 radiation of energy from, 79 comparison of, with β rays, 120 absorption of, by matter, 136, 137 see also β rays

Caves radio-active matter present in air of, 514 et seq. radio-activity of air of, due to emanation from the soil, 515

Changes (see Transformations)

Charge carried by the ions, 50 et seq. negative charge carried by β rays, 120 measurement of charge carried by β rays, 121 et seq. positive charge carried by α rays, 145 measurement of charge carried by α rays, 151 et seq.

Chemical nature of emanation, 267 of active deposit, 312

Chemical actions of radium rays production of ozone, 213 coloration of glass and rock-salt, 213 on phosphorus, 214 on iodoform, 214 on globulin, 214 evolution of hydrogen and oxygen, 215

Child potential gradient between electrodes, 65 variation of current with voltage for surface ionization, 66

Clouds formation of, by condensation of water round ions, 46 et seq. difference between positive and negative ions in formation of, 49

Collie and Ramsay spectrum of emanation, 292

Collision ionization by, 39, 57 number of ions produced by β rays per cm. of path, 434 total number of ions produced by collisions of α particles, 434

Coloration of crystals of radiferous barium, 15 of bunsen flame by radium, 15 of glass by radium rays, 213 of rock-salt, fluor-spar and potassium sulphate by radium rays, 213

Concentration of excited activity on negative electrode, 297 activity of radium independent of, 466

Condensation of water round the ions, 46 et seq. of emanations, 277 experimental illustration of, 279 temperature of, 280 difference between point of, for emanations of thorium and radium, 283 from air sucked up from the earth, 510

Conductivity of gases exposed to radiations, 31 et seq. variation of, with pressure, 61 et seq. variation of, with nature of gas, 64 comparison of, for gases exposed to α, β, and γ rays, 64 comparison of, when exposed to γ rays and to hard X rays, 184 of insulators, 209 of air in caves and cellars, 507 et seq. of air in closed vessels, 531 et seq. variation of, in closed vessels with pressure and nature of gas, 534 variation of, with temperature for air in closed vessels, 536 increase of, with time, in a closed vessel, 537

Conservation of radio-activity examples of, 469 et seq.

Cooke, H. L. penetrating rays from the earth, 520 number of ions per c.c. in closed vessels, 534 radio-activity from ordinary matter, 536

Cooke, W. T. and Ramsay radio-activity produced by radiations of radium, 472

Corpuscle (see Electron)

Crookes, Sir W. spectrum of radium, 17 spectrum of polonium, 23 nature of cathode rays, 73 nature of α rays, 142 scintillations produced by radium, 158 spinthariscope, 158 number of scintillations independent of pressure and temperature, 159 phosphorescence of diamond, 204 separation of Ur X, 219 theory of radio-activity, 441

Crookes and Dewar absence of nitrogen spectrum in phosphorescent light of radium at low pressures, 206

Crystallization effect of, on activity of uranium, 349

Curie, Mme permanence of uranium rays, 6 discovery of radio-activity of thorium, 10 radio-activity of uranium and thorium minerals, 11 relative activity of compounds of uranium, 12 coloration of radium crystals, 15 spectrum of radium, 16 discovery of polonium, 22 nature of a rays, 142 absorption of α rays from polonium, 163 secondary radiation tested by electric method, 188 slowly decaying excited activity from radium, 311 recovery of activity of radium, 375 bismuth made active by solution of barium, 417

Curie, P. magnetic deviation of radium rays by electric method, 114 conductivity of dielectrics under radium rays, 209 radio-activity of radium unaffected by temperature, 210 decay of activity of radium emanation, 247 discovery of excited radio-activity from radium, 295 heat emission of radium at low temperature and variation of heat emission with age of radium, 419 nature of the emanation, 439

Curie, M. et Mme discovery of radium, 13 charge carried by β rays, 121 luminosity of radium compounds, 205 production of ozone by radium rays, 213 coloration of glass by radium rays, 213 theory of radio-activity, 439 possible absorption by radio-elements of unknown radiations, 442

Curie, J. et P. quartz piezo-électrique, 105 et seq.

Curie, P. et Danne diffusion of radio-active emanation, 272 decay of excited activity from radium, 309 decay curves of radium and equation, 309 occlusion of radium emanation in solids, 310 changes in radium, 381 effect of temperature on active deposit, 390

Curie, P. and Debierne evolution of gas from radium, 215 active gases evolved from radium, 251 phosphorescence produced by radium emanation, 252 distribution of luminosity, 252 rate of production of emanation independent of pressure, 266 effect of pressure on amount of excited activity, 266, 317

Curie and Dewar production of helium by radium, 479

Curie, P. and Laborde heat emission of radium, 419 origin of heat from radium, 440 radium emanation in waters of hot springs, 514

Current through gases, 31 et seq. variation of, with distance between the plates, 59 et seq. variation of, with pressure of gas, 61 et seq. variation of, with nature of gas, 64 measurement of, by galvanometer, 84 measurement of, by electroscope, 85 et seq. measurement of, by electrometer, 90 et seq. measurement of, by quartz piezo-électrique, 105

Dadourian presence of thorium emanation in the earth, 512

Danne on deposit of radium not containing uranium, 465

Danne et Curie diffusion of radio-active emanation, 272 decay of excited activity from radium, 309 decay curves of radium and equation, 309 occlusion of radium emanation in solids, 310 changes in radium, 381 effect of temperature on active deposit, 390

Danysz action of radium rays on skin, 216

Darwin, G. H. age of sun, 492

Debierne actinium, 21 emanation from actinium, 249 decay of excited activity from actinium, 311 effect of magnetic field on activity excited from actinium, 324 barium made active by actinium, 417

Debierne and Curie evolution of gas from radium, 215 active gases evolved from radium, 251 phosphorescence produced by radium emanation, 252 distribution of luminosity, 252 rate of production of emanation independent of pressure, 266 effect of pressure on amount of excited activity, 266, 317

Decay of activity of Th X, 221 of activity of Ur X, 223 significance of law of, 229 effect of conditions on the rate of, 232 of activity of thorium emanation, 241 of activity of radium emanation, 247 of activity of actinium emanation, 249 of excited activity due to thorium for long exposure, 302 of excited activity due to thorium for short exposure, 304 of excited activity due to radium, 306 et seq. excited activity of slow decay due to radium, 311 of excited activity from actinium, 311 of radium A, B and C, 377 et seq. of radium D, E and F, 397 et seq. of heating effect of emanation, 423 of excited activity from atmosphere, 502 of activity of rain and snow, 506 of emanation from the earth, 508 differences in, of excited activity from atmosphere, 521 et seq.

Demarçay spectrum of radium, 16

Deposit, active connection of, with excited activity, 301 physical and chemical properties of, 312 electrolysis of, 313 effect of temperature on, 315 effect of pressure on distribution of, 317 transmission of, by positive carriers, 318 et seq. nomenclature of, 328 theory of changes in, 331 et seq. theory of activity due to, 337 et seq. theory of rayless change in, 341 et seq. of thorium, 302 et seq., 351 et seq. analysis of, 351 rayless change in, 352 effect of temperature on, 354 period of products of, 355 of actinium, 311 et seq. decay curves of, 311 analysis of, 367 rayless change in, 367 period of products of, 368 radiations from, 368 of radium, 376 et seq. connection of excited activity with, 306 general analysis of, 376 et seq. analysis of, of rapid change, 377 et seq. analysis of α ray curves, 377 α ray curves of, 378 β ray curves of, 379 analysis of β ray curves, 381 equations of activity curves, 389 effect of temperature on, 390 volatility of, 391 of slow transformation, 311, 397 variation of α ray activity of, 398 variation of β ray activity of, 399 separation of constituents of, 401 et seq. successive products in, 402 variation of activity of, for long periods, 407 presence in old radium, 408 effect of, on variation of activity of radium with time, 409 presence in pitchblende, 410 connection with radio-tellurium, 411 connection with polonium, 411, 412 connection with radio-lead, 413 connection of, with radio-active induction, 415 et seq. heat emission of, 425 et seq. use of, to determine number of β particles from radium, 435 use of, as source of α rays, 543

Des Coudres magnetic and electric deviation of α rays, 148 determination of e/m for α rays, 148

Dewar emission of heat from radium in liquid hydrogen, 420

Dewar and Crookes absence of nitrogen spectrum in phosphorescent light of radium at low pressures, 206

Dewar and Curie production of helium by radium, 479

Dielectrics condition of, under radium rays, 209

Diffusion of ions, 51 et seq. of radium emanation into gases, 270 of thorium emanation into gases, 275 of radium emanation into liquids, 276

Discharge action of rays on spark and electrodeless, 208

Disintegration account of theory of, 234, 325, 445 list of products of, 449 rate of, in radio-elements, 457 emission of energy in consequence of, 474 et seq. helium a product of, 476 et seq. possible causes of, 486 et seq. of matter in general, 496 et seq.

Dissipation of charge in caves and cellars, 514 et seq. in closed vessels, 531 effect of pressure and nature of gas on, 534 et seq. effect of material of vessel on, 536 et seq.

Dolezalek electrometer, construction of, 94 et seq.

Dorn charge carried by β rays, 122 electrostatic deflection of β rays from radium, 124 discovery of radium emanation, 246 effect of moisture on emanating power of thorium, 255 electrolysis of radium solution, 313 loss of weight of radium, 474 radium emanation in springs, 513

Dreyer and Salomonsen coloration of quartz by radium rays, 213

Dunston analysis of thorianite, 486

Durack ionization by collision of electrons of great velocity, 171

Dust effect of, on recombination of ions, 42 effect of, on distribution of excited activity, 305

Earth amount of radium in, 493 et seq. age of, 496 excited activity deposited on, 504 activity concentrated on peaks of, 504 emanation from, 507 very penetrating radiation from, 520

Ebert condensation of emanation from the earth, 510 apparatus for determining number of ions per c.c. in air, 527 velocity of ions in air, 528

Ebert and Ewers emanation from the earth, 508

Electrolysis separation of radio-tellurium by, 25 of solutions of active deposit, 313 of radium solutions, 313 of thorium solutions, 314

Electrometer description of, 90 et seq. use of, in measurements, 90 construction of, 91 et seq. Dolezalek, 94 adjustment and screening of, 95 special key for, 97 application of, to measurements of radio-activity, 97 et seq. measurement of current by, 100 capacity of, 101 use with steady deflection, 103 use with quartz piezo-électrique, 105

Electron definition of, 56 production of, under different conditions, 76 et seq. identity of β rays with electrons, 120 et seq. variation of apparent mass of electron with velocity, 127 et seq. evidence that mass of electron is electromagnetic, 129 et seq. diameter of, 131

Electroscope description of, used by Curie, 85 construction of, for accurate measurements, 86 use of, in measurements of minute currents, 86 of C. T. R. Wilson, 89 use of, in measuring conductivity of air in closed vessels, 531 _et seq._ use of, for determining radio-activity of ordinary matter, 537

Elster and Geitel radio-active lead, 27 effect of magnetic field on conductivity produced in air by β rays, 113 scintillations produced by active substances, 158 action of radium rays on spark, 208 photo-electric action of body, coloured by radium rays, 214 radio-active matter in earth, 494 discovery of excited activity in atmosphere, 501 emanations from the earth, 507 radio-activity of air in caves, 507 radio-activity of the soil, 515 radio-activity of fango, 516 variation of radio-activity in atmosphere with meteorological conditions, 517 effect of temperature and pressure on atmospheric radio-activity, 518

Emanation of thorium, discovery and properties of, 238 methods of measurement of, 240 decay of activity of, 241 effect of thickness of layer on amount of, 243 increase of, with time to a maximum, 245 of radium, 246 decay of activity of, 247 of actinium, properties of, 249 of radium, phosphorescence produced by, 251 rate of emission of, 254 effect of conditions on rate of emission of, 255 regeneration of emanating power, 256 continuous rate of production of, 257 source of thorium emanation, 261 source of radium and actinium emanation, 263 radiations from, 263 effect of pressure on production of, 265 chemical nature of, 267 experiments to illustrate gaseous nature of, 268 rate of diffusion of radium emanation, 269 rate of diffusion of thorium emanation, 275 diffusion of, into liquids, 276 condensation of, 277 temperature of condensation of, 280 volume of, from one gram of radium and thorium, 288 measurement of volume of, from radium, 289 diminution of volume of, 290 spectrum of emanation, 292 connection between emanation and excited activity, 298 effect of removal of, on activity of radium, 371 et seq. fraction of activity of radium due to, 374 effect of rate of escape of, on activity of radium, 374 heat emission of, 420, 431 variation of heat emission with time, 421 et seq. enormous emission of energy from emanation, 431 radio-activity of atmosphere due to emanations, 504 sucked up from the earth, 507 in caves, 507 et seq. rate of decay of activity of, from the earth, 508 condensation of, from the atmosphere, 510 in well water and springs, 510 et seq. from “fango,” 516 effect of meteorological conditions on amount of, in atmosphere, 517 et seq. from metals, 538

Emanating power measurement of, 254 effect of conditions on, 255 regeneration of, 256

Emanium or “emanating substance” of Giesel (see Actinium) discovery of, 21 separation and properties of, 21 similarity of, to actinium, 21 emanation from, 249 excited activity produced by, 311 action of an electric field on, 323

Energy of α particle, 156 of β particle, 196 comparison of, for α and β particles, 196 emitted from radium in form of heat, 419 et seq. emission of, from the emanation, 431 emission of, from radio-active products of radium, 433 total emission of, from 1 gram of radio-elements, 474 et seq. latent store of, in matter, 475

Eve conductivity of gases exposed to X rays, 64 conductivity of gases exposed to X rays and γ rays, 183, 184 secondary rays produced by β and γ rays, 189 et seq. magnetic deflection of secondary rays from γ rays, 193 variation of activity of radium with concentration, 467 amount of radium emanation in the atmosphere, 524 et seq. ionization due to emanation in atmosphere, 526 et seq.

Evolution of matter evidence of, 497

Ewers and Ebert emanation from the earth, 508

Excited radio-activity discovery and properties of, 295 et seq. concentration of, on negative electrode, 297 connection of, with the emanations, 298 removal of, by acids, 300 decay of, due to thorium, 302 decay of, for short exposure to thorium, 304 effect of dust on distribution of, 305 decay curves for different times of exposure, 306 et seq. decay of, from radium, 306 decay curves of, measured by α rays, 308 decay curves of, measured by β rays, 309 decay curves of, from actinium, 311 of radium, of very slow decay, 311 effect of solution on, 312 electrolysis of active solutions, 313 effect of temperature on, 315 variation with electric field, of amount of, 316 effect of pressure on distribution of, 317 transmission of, 318 from actinium and emanium, 323 heat emission due to, 425 et seq. from the atmosphere, 501 et seq. decay of, 502 due to emanation in atmosphere, 504 distribution of, on surface of the earth, 504 concentration of, on prominences of the earth, 504 of rain and snow, 506 produced by emanation from tap water, 510 effect of meteorological conditions on amount of, 517 et seq. amount of, at Niagara Falls, 520 rate of decay of, dependent on conditions, 522 et seq.

Exner and Haschek spectrum of radium, 17

Eye action of radium rays on, 217

Fehrle distribution of excited activity on a plate in electric field, 318

Fluorescence produced in substances by radium rays, 18 produced in substances by radium and polonium rays, 201 et seq.

Fog large amount of excited activity during, 518

Forch loss of weight of radium, 474

γ rays relative conductivity of gas exposed to γ and hard X rays, 64, 184 discovery of, 179 absorption of, by matter, 179 et seq. connection between absorption of, and density, 182 discussion of nature of rays, 182 et seq. secondary rays produced by γ rays, 189 measurement of radio-activity by means of, 442, 467 conservation of radio-activity measured by, 471

Gases evolved by radium, 215 presence of helium in gases from radium, 216

Gates, Miss F. effect of temperature on excited activity, 315 discharge of quinine sulphate, 530

Geitel natural conductivity of air in closed vessels, 501, 531

Geitel and Elster radio-active lead, 27 effect of magnetic field on conductivity produced by radium rays, 113 scintillations produced by active substances, 158 action of radium rays on spark, 208 photo-electric action of bodies coloured by radium rays, 214 radio-active matter in earth, 494 discovery of radio-active matter in atmosphere, 501 emanations from the earth, 507 radio-activity of air in caves, 507 radio-activity of the soil, 515 radio-activity of fango, 516 variation of radio-activity of air with meteorological conditions, 517 effect of temperature and pressure on radio-activity in atmosphere, 518

Giesel coloration of bunsen flame by radium, 15 separation of radium by crystallization of bromide, 15 emanating substance, 21 radio-active lead, 27 magnetic deviation of β rays, 113 decrease with time of luminosity of radio-active screen, 205 spectrum of phosphorescent light of emanium due to didymium, 206, 207 coloration of bodies by radium rays, 213 evolution of gases from radium, 215 action of radium rays on the eye, 217 emanation from the emanating substance, 250 luminosity produced by radium emanation, 251 decay of excited activity of emanium, 312 activity of radium dependent on age, 371 bismuth made active by radio-active solution, 417 temperature of radium bromide above air, 420

Gimingham and Rossignol decay of thorium emanation, 242

Glass coloration produced in, by radium rays, 213 phosphorescence produced in, by emanation, 252

Glew simple form of spinthariscope, and scintillations, 159

Globulin action of radium rays on, 214

Godlewski effect of crystallization on activity of uranium, 349 diffusion of uranium X, 350 separation of actinium X, 365 source of actinium emanation, 365 recovery and decay curves of actinium, 366 penetrating power of β and γ rays from actinium, 368 radiations from active products, 368

Goldstein canal rays, 78 coloration of bodies by radium rays, 213

Gonder, Hofmann, and Wölfl properties of radio-active lead, 27, 413

Grier and Rutherford magnetic deviation of β rays of thorium, 114 relative current due to α and β rays, 195 nature of rays from Ur X, 347

Hardy coagulation of globulin by radium rays, 214

Hardy and Miss Willcock coloration of iodoform solutions by radium rays, 214

Hardy and Anderson action of radium rays on the eye, 217

Harms number of ions per c.c. in closed vessel, 534

Hartmann spectrum of phosphorescent light of emanium, 206

Haschek and Exner spectrum of radium, 17

Heat rate of emission of, from radium, 419 et seq. emission of, from radium at low temperatures, 420 connection of heat emission with the radio-activity, 421 et seq. source of heat energy, 421 et seq. rate of emission of, after removal of the emanation, 422 et seq. rate of emission of, by emanation, 423, 431 variation with time of heat emission of radium, and of its emanation, 423 heating effect of the emanation, 423, 431 heating effect of active deposit, 425 proportion of heating effect due to radio-active products, 433 origin of, in radium, 442 et seq. total heat emission during life of radio-elements, 474 et seq. heating of earth by radio-active matter, 493

Heaviside apparent mass of moving charged body, 71, 127

Helium produced by radium and its emanation, 476 et seq. amount of, from radium, 480 origin of, 480

Helmholtz and Richarz action of ions on steam jet, 47

Hemptinne action of rays on spark and electrodeless discharge, 208

Henning resistance of radium solutions, 208 effect of voltage on amount of excited activity, 316

Henning and Kohlrausch conductivity of solutions of radium bromide, 208

Hertz electric deviation of cathode rays, 73

Heydweiler loss of weight of radium, 474

Himstedt action of radium rays on selenium, 208 radium emanation in springs of Baden, 513

Himstedt and Meyer production of helium by radium, 479

Himstedt and Nagel action of radium rays on the eye, 217

Hofmann, Gonder, and Wölfl properties of radio-active lead, 27, 413

Hofmann and Strauss radio-active lead, 27

Hofmann and Zerban connection of activity of thorium with uranium, 29

Huggins, Sir W. and Lady spectrum of phosphorescent light of radium bromide, 205

Hydrogen production of, by radium rays, 215

Induced radio-activity (see Excited radio-activity)

Induction radio-active, 24 meaning and examples of, 415 et seq.

Insulators conduction of, under radium rays, 209

Iodoform coloration produced in, by radium rays, 214

Ionization theory of, to explain conductivity of gases, 31 et seq. by collision, 39, 57 variation of, with pressure of gas, 61 et seq. variation of, with nature of gas, 64 comparison of, produced by rays, 111, 194 production of, in insulators, 209 total, produced by 1 gram of radium, 433 et seq. natural ionization of gases, 531 et seq. connection of, with phosphorescent and photographic actions, 549

Ions in explanation of conductivity of gases, 31 et seq. production of, by collision, 39, 57 rate of recombination of, 40 et seq. mobility of, 42 et seq. difference between mobility of positive and negative, 43 et seq. condensation of water around, 46 et seq. difference between positive and negative, 49 charge carried by, 50 diffusion of, 51 et seq. charge on an ion same as on hydrogen atom, 54 number of, produced per c.c., 54 size and nature of, 55 et seq. definition of, 56 et seq. velocity acquired by, between collisions, 58 energy required to produce, 58, 551 comparative number of, produced in gases, 65 disturbance of potential gradient by movement of, 65 production of, in insulators, 209 number of, produced by α particle, 433 number produced per c.c. in closed vessels, 533 et seq.

Joly motion of radium in an electric field, 211 absorption of radium rays by atmosphere, 492 (see foot-note)

Kaufmann velocity of cathode rays, 75 variation of e/m with velocity of electron, 127 et seq.

Kelvin theory of radio-activity, 441 age of sun and earth, 492, 493

Kelvin, Smolan and Beattie discharging power of uranium rays, 7

Kleeman and Bragg theory of absorption of α rays, 172 et seq. relation between ionization and absorption, 174 et seq. range of α rays in air, 174 four sets of α rays from radium, 174 et seq.

Kohlrausch conductivity of water altered by radium rays, 208

Kohlrausch and Henning conductivity of solutions of radium bromide, 208

Kunz phosphorescence of willemite and kunzite, 203

Kunz and Baskerville phosphorescence of substance under radium rays, 204

Kunzite phosphorescence of, under radium rays, 203

Laborde and Curie heat emission of radium, 419 origin of heat from radium, 440 radium emanation in waters of hot springs, 514

Langevin coefficient of recombination of ions, 41 velocity of ions, 45 et seq. energy required to produce an ion, 58 secondary radiation produced by X rays, 187 slow moving ions in air, 528

Larmor radiation theory, 77 radiation of energy from moving electron, 79 structure of the atom, 157

Lead, radio-active preparation of, 26 radiations from, 26

Le Bon rays from bodies exposed to sunlight, 5 discharging power of quinine sulphate, 9, 530

Lenard ionization of gases by ultra-violet light, 9 action of ions on a steam jet, 47 penetrating power of cathode rays, 73 negative charge carried by Lenard rays, 120 absorption of cathode rays proportional to density, 136, 137

Lerch, von chemical properties of active deposit of thorium, 313 electrolysis of solution of active deposit, 313 effect of temperature on excited activity, 315 temporary activity of active deposit from thorium, 415

Lockyer inorganic evolution, 499

Lodge, Sir Oliver electronic theory, 69 instability of atoms, 487

Lorentz structure of atoms, 157

Lowry and Armstrong radio-activity and phosphorescence, 444

Luminosity of radium compounds, 205 change of, in radium compounds with time, 205 spectrum of phosphorescent light from radium bromide, 206 of radium compounds unaffected by temperature, 210

Mache radium emanation in hot springs, 513

Mache and von Schweidler velocity of ions in air, 528

Makower diffusion of radium emanation, 274 diffusion of thorium emanation, 276

Marckwald preparation of radio-tellurium, 25 rate of decay of radio-tellurium, 411

Mass apparent mass of electron, 71, 127 variation of mass of electron with speed, 127 et seq. of α particle, 147 et seq.

Materials radio-activity of ordinary, 528, 536 et seq.

Matteucci rate of dissipation of charge in closed vessels, 531

McClelland absorption of γ rays, 181 secondary rays from β and γ rays from radium, 192

McClung coefficient of recombination of ions, 41 conductivity of gases exposed to X rays, 64 ionization by α rays from radium C, 550

McClung and Rutherford energy required to produce an ion, 58 variation of current with thickness of layer of uranium, 195 estimate of energy radiated from radio-elements, 418 radiation of energy from radium, 438

McLennan absorption of cathode rays, 65 radio-activity of snow, 506 excited radio-activity at Niagara Falls, 519

McLennan and Burton penetrating radiation from the earth, 520 radio-activity of ordinary materials, 537 emanation from ordinary matter, 538

Metabolon definition of, 446 table of metabolons, 449 radio-elements as metabolons, 457

Meteorological conditions effect of, on radio-activity of atmosphere, 517

Methods of measurement in radio-activity, 82 et seq. comparison of photographic and electrical, 83 et seq. description of electrical, 84 et seq.

Meyer and Himstedt production of helium by radium, 479

Meyer and Schweidler magnetic deviation of β rays by electrical method, 113 absorption of β rays of radium by matter, 136 activity proportional to amount of uranium, 195 emanation from uranium, 348 effect of crystallization on activity of uranium, 349 rate of decay of radio-tellurium, 411

Minerals, radio-active constant ratio of radium to uranium, 459 et seq. list of minerals, 461 age of, 485 composition of, 554 et seq.

Mobility of ions, 43 et seq.

Moisture effect of, on velocity of ions, 43, 45 effect of, on emanating power, 255

Molecule number of, in 1 c.c. of hydrogen, 54

Molecular weight of radium emanation, 273 of thorium emanation, 275

Nagel and Himstedt action of radium rays on the eye, 217

Niewenglowski rays from sulphide of calcium, 4

Nomenclature of successive products, 328 et seq.

Number of molecules per c.c. of hydrogen, 54 of ions produced in gas by active substances, 55 of β particles expelled from 1 gram of radium, 124 of α particles emitted per gram of radium, 155 of ions produced per c.c. in closed vessels, 534

Occlusion of emanation in thorium and radium, 258 of radium emanation by solids, 310

Owens saturation current affected by dust, 42 penetrating power of rays independent of compound, 164 absorption of α rays varies directly as the pressure of gas, 169 effect of air currents on conductivity produced by thorium, 238

Oxygen change into ozone, by radium rays, 213 production of, from radium solutions, 215

Ozone production of, by radium rays, 213

Paraffin objection to, as an insulator, 96 conductivity of, under radium rays, 210

Paschen distribution of velocity amongst β particles, 131 et seq. absence of magnetic deflection of γ rays, 183 γ rays and electrons, 185 heating effect of γ rays, 186, 429

Patterson number of ions per c.c. in closed vessel, 534 natural conductivity of air due to an easily absorbed radiation, 536 effect of temperature on natural conductivity of air, 536

Peck and Willows action of radium rays on spark, 208

Pegram electrolysis of thorium solutions, 314 temporary activity of substances separated from thorium, 415

Penetrating power comparison of, for α, β and γ rays, 111 variation in, of β rays, 134 et seq. variation of, with density for β rays, 137 comparison of, for α rays from radio-elements, 164 variation of, with density for α rays, 169 variation of, with density for γ rays, 182

Penetrating radiation from the earth and atmosphere, 520

Perrin charge carried by cathode rays, 73 theory of radio-activity, 437

Phosphorescence production of, by radium, 19 production of, by radium and polonium rays, 201 et seq. comparison of, produced by α and β rays, 202 of zinc sulphide, 202 of barium platinocyanide, 203 of willemite and kunzite, 203 produced by radium emanation in substances, 203, 252 diminution of, with time, 205 of radium compounds, 205 spectrum of phosphorescent light of radium bromide, 205 spectrum of phosphorescent light of “emanium,” 206 production of by heat (thermo-luminescence), 207 use of, to illustrate condensation of emanations, 279 connection of with ionization, 547 et seq.

Phosphorus action of radium rays on, 214 ionization produced by, 529

Photo-electric action produced by radium rays in certain substances, 214

Photographic method, advantages and disadvantages of, 83 relative photographic action of rays, 83 connection of photographic action with ionization, 546

Physical action of radium rays on sparks, 208 on electrodeless discharge, 208 on selenium, 208 on conductivity of insulators, 209

Physiological action of radium rays production of burns, 216 effect on bacteria, 216 effect on eye, 217

Piezo-électrique of quartz description of, 105

Pitchblendes comparison of radio-activity of, 11 radio-elements separated from, 13 et seq. radium continually produced from, 459 constitution of, 557

Polarization of uranium rays absence of, 7

Polonium methods of separation of, 22 rays from, 23 decay of activity of, 23 discussion of nature of, 24 similarity to radio-tellurium, 26 magnetic deviation of α rays from, 146, 150 slow moving electrons, 153 increase of absorption with thickness of matter traversed, 163 connection of, with radium F, 411

Potential required to produce saturation, 32 et seq. fall of potential needed to produce ions at each collision, 58 gradient due to movement of ions, 65

Precht and Runge spectrum of radium, 17 atomic weight of radium, 18 heating effect of radium, 420

Pressure effect of, on velocity of ions, 46 effect of, on current through gases, 61 et seq. production of emanation independent of, 265 effect of, on distribution of excited activity, 317 effect of, on natural conductivity of air in closed vessels, 534

Products, radio-active list of, from radio-elements, 449 properties of, 449 amount of in radium, 452 et seq. radiations from, 455

Quartz piezo-électrique use of, in measurement of current, 105

Quinine sulphate discharging power of, 530 phosphorescence of, 530

Radiations emitted by uranium, 8 emitted by thorium, 10 emitted by radium, 18 emitted by actinium, 21 emitted by polonium, 23 method of measurement of, 82 et seq. methods of comparison of, 108 three kinds of, 109 analogy to rays from a Crookes tube, 110 relative ionizing and penetrating power of, 111 difficulties of comparative measurement of, 112 β rays, 113 α rays, 141 γ rays, 179 secondary rays, 187 comparison of ionization of α and β rays, 194 phosphorescent effect of, 201 et seq. physical actions of, 207 et seq. chemical actions of, 213 et seq. physiological actions of, 216 from the emanation, 263 from Ur X, 347 connection of, with heat emission, 421 et seq. from different active products, 455 conservation of energy of each specific type of, 469 et seq.

Radio-lead connection of, with polonium, 411 et seq. connection of, with radium D, 413

Radio-tellurium rate of decay of, 411 connection of with radium F, 411

Radium discovery of, 13 separation of, 13 spectrum of, 16 atomic weight of, 17 radiations from, 18 compounds of, 19 nature of radiations from, 109 β rays from, 113 α rays from, 141 γ rays from, 179 secondary rays from, 187 production of phosphorescence by, 201 et seq. spectrum of phosphorescent light of, 206 physical actions of, 207 et seq. chemical actions of, 213 et seq. physiological actions of, 216 emanation from, 246 properties of emanation from, 247 et seq. chemical nature of emanation from, 267 diffusion of emanation from, 269 condensation of emanation from, 277 amount of emanation from, 288 volume of emanation from, 289 spectrum of emanation from, 292 excited radio-activity from, 295 et seq. decay of excited activity from, 306 et seq. difference in properties of radium and the emanation, 327 nomenclature of products, 328 theory of successive changes in, 330 alteration of activity of, by removal of emanation, 371 et seq. recovery of activity of, after removal of emanation, 372 effect of escape of emanation on recovery of activity of, 374 non-separable activity of, 375 period and properties of radium A, B and C, 376 et seq. analysis of active deposit of rapid changes of radium, 377 analysis of β ray curves, 381 et seq. analysis of α ray curves, 386 et seq. equations of activity curves, 389 effect of temperature on active deposit of, 390 relative activity due to products of, 395 active deposit of slow transformation, 397 physical and chemical properties of radium D, E and F, 398 et seq. effect of temperature on active deposit of slow change, 401 separation of radium F by bismuth, 402 products of, 402 et seq. rate of transformation of radium D, 404 et seq. variation of the activity of the active deposit over long periods of time, 407 amounts of radium D, E and F in old radium, 408 variation of activity of, with time, 409 products of in pitchblende, 410 origin of radio-tellurium, 411 origin of polonium, 411, 412 origin of radio-lead, 413 temporary activity of inactive matter separated from pitchblende, 415 et seq. heat emission of, 419 et seq. heat emission of emanation from, 420, 431 heating effects due to products of, 433 theories of radio-activity of, 437 et seq. discussion of theories of radio-activity of, 441 et seq. energy of radiations, not derived from external source, 442 et seq. theory of radio-active change, 444 et seq. list of active products of, 449 amount of products of, 452 rate of change of, 457 life of radium, 457 origin of, 459 et seq. production of, by uranium, 459 et seq. amount of in 1 gram of uranium, 461 amount of, in minerals, 461 radio-activity of, independent of concentration, 466 et seq. disappearance of, 467 life of, independent of concentration, 468 conservation of radio-activity of, 469 et seq. loss of weight of, 473 experiments to determine loss of weight of, 474 total emission of energy from 1 gram of, 474 et seq. production of helium from, 476 helium, disintegration product of, 479 et seq. amount of helium from, 480 possible causes of disintegration of, 486 et seq. amount of, to account for heat of sun, 491 possible connection of with heat of sun, 491 possible connection of with heat of earth, 493 probable amount of, in earth, 495 amount of, in atmosphere, 495, 524 presence of, in atmosphere, 521 et seq.

Radium A decay curve of, 378 radiation from, 381 effect of, on activity curves, 386 et seq. connection with later changes, 392 activity supplied by, 393

Radium B absence of rays in, 381 effect of, on activity curves, 381 et seq. effect of temperature on, 390 volatility of, 390 absence of heating effect of, 433 nature of rayless change in, 454, 552

Radium C radiations from, 381 analysis of β ray curves of, 381 et seq. analysis of α ray curves of, 386 et seq. effect of temperature on, 390 activity supplied by, 394 et seq. heating effect of, 425 use of, as a source of β rays, 435 explosive nature of change in, 456 magnetic deflection of rays from, 543 velocity and value of e/m for rays from, 544

Radium D origin of name of, 376 connection of, with active deposit, 403 period of transformation of, 406 effect of, on variation of activity, 407 presence in old radium, 408 effect of, on activity of old radium, 409 presence in pitchblende, 410 connection with radio-lead, 413 amount of, in 1 ton of uranium, 454

Radium E effect of temperature on, 401 connection of, with β ray activity active deposit, 403, 400 connection with radio-lead, 413

Radium F variation of activity due to, 398 effect of temperature on, 401 separation of, on bismuth plate, 402 connection with active deposit, 403 variation of activity of, over long periods of time, 407 presence in old radium, 409 effect of, on activity of old radium, 409 presence in pitchblende, 410 connection with radio-tellurium, 411 connection with polonium, 411, 412 connection with radio-lead, 413

Rain radio-activity of, 505 decay of activity of, 506

Ramsay, Sir W. amount of helium in thorianite, 486

Ramsay and Collie spectrum of emanation, 292

Ramsay and Cooke radio-activity produced by radiation from radium, 472

Ramsay and Soddy evolution of gas from radium, 215 production of hydrogen and oxygen from radium, 215 chemical nature of the emanation, 268 gaseous nature of the emanation, 268 volume of emanation, and change with time, 289 helium from radium emanation, 291 amount of helium produced by radium, 480

Ramsay and Travers amount of helium in fergusonite, 486

Rayless changes discussion of, 454, 552

Re, F. theory of radio-activity, 441

Recombination of ions, 40 et seq. constant of, 42

Recovery of activity of thorium after removal of Th X, 221 of activity of uranium after removal of Ur X, 223 significance of law of, 224 effect of conditions on rate of, 232 of activity of radium after removal of emanation, 372 of heating effect of radium, 423

Reflection no evidence of direct reflection for uranium rays, 7 diffuse reflection of rays, 7

Refraction no evidence of, for uranium rays, 7

Regeneration of emanating power, 256

Richarz and von Helmholtz action of ions on steam jet, 47

Richarz and Schenck theory of radio-activity, 441

Rossignol and Gimingham decay of thorium emanation, 242

Runge spectrum of radium, 17

Runge and Bödlander evolution of gas from radium, 215

Runge and Precht spectrum of radium, 17 atomic weight of radium, 18 heating effect of radium, 420

Russel photographic action of substances, 83

Saake amount of emanation in air at high altitudes, 519

Salomonsen and Dreyer coloration of quartz by radium rays, 213

Saturation current meaning of, 33 et seq. application of, to measurements of radio-activity, 84 measurement of, 100 et seq.

Schenck radium emanation in springs, 513

Schenck and Richarz theory of radio-activity, 441

Schmidt discovery of radio-activity of thorium, 10

Schmidt and Wiedemann thermo-luminescence, 207

Schuster number of ions per c.c. in air of Manchester, 528 radio-activity of matter, 529

Schweidler and Mache velocity of ions in air, 528

Schweidler and Meyer magnetic deviation of β rays by electrical method, 113 absorption of β rays of radium by matter, 136 activity proportional to amount of uranium, 195 emanation from uranium, 348 effect of crystallization on activity of uranium, 349 rate of decay of radio-tellurium, 411

Scintillations discovery of, in zinc sulphide screen, 158 connection of, with α rays, 158 illustration of, by spinthariscope, 158 cause of, 160 production of, by action of electric field, 160

Searle apparent mass of moving charged body, 71, 127

Secondary rays examination of, by photographic method, 187 examination of, by electrical method, 188 production of, by β and γ rays, 189 et seq. from different materials, 191 amount of, depends upon atomic weight, 192 magnetic deflection of, 193

Seitz absorption of electrons by matter, 137 et seq.

Selenium action of radium rays on, 208

Simon value of e/m for cathode rays, 75, 129

Simpson amount of excited activity in north of Norway, 519

Slater, Miss effect of temperature on active deposit of thorium, 354

Smolan, Beattie and Kelvin discharging power of uranium rays, 7

Snow radio-activity of, 506 decay of activity of, 507

Soddy comparison of photographic and electrical action of uranium rays, 83 nature of rays from Ur X, 347 production of radium from uranium, 463

Soddy and Ramsay evolution of gas from radium, 215 production of hydrogen and oxygen from radium, 215 chemical nature of the emanation, 268 gaseous nature of the emanation, 268 volume of the emanation, and change with time, 289 helium from radium emanation, 291 amount of helium produced by radium, 480

Soddy and Rutherford separation of Th X, 220 decay of activity of Th X, 221 recovery of activity of thorium freed from Th X, 221 decay of activity of Ur X, 223 recovery of activity of uranium freed from Ur X, 223 explanation of decay and recovery curves, 224 rate of production of Th X, 227 theory of decay of activity, 229 influence of conditions on rate of decay and recovery of activity, 233 disintegration hypothesis, 234 decay of activity of radium emanation, 247 measurements of emanating power, 254 effect of temperature, moisture, and solution, on emanating power, 255 regeneration of emanating power, 256 constant rate of production of emanation of radium and thorium, 257 source of thorium emanation, 261 radiations from the emanation, 264 chemical nature of emanation, 267 condensation of emanations of radium and thorium, 277 temperature of condensation of emanation, 278 effect of successive precipitations on activity of thorium, 358 recovery of activity of radium, 372 theory of radio-activity, 439 theory of radio-active change, 445 conservation of radio-activity, 469

Soil radio-activity of, 507 et seq. difference in activity of, 508 et seq.

Solution coloration of, by radium, 15 of active deposit in acids, 312 electrolysis of active, 313

Source of thorium emanation, 261 of radium and actinium emanations, 263

Spark action of radium rays on, 208

Spectrum spark spectrum of radium, 15, 16 flame spectrum of radium, 17 effect of a magnetic field on spectrum of radium, 17 of polonium, 23 of phosphorescent light of radium bromide, 206 of emanation, 292 of helium in radium gases and emanation, 477

Spinthariscope description of, 158

Springs emanation from water of, 513

Stark energy to produce an ion, 58

Stoney, Johnstone use of term electron, 76

Strauss and Hofmann radio-active lead, 27

Strutt conductivity of gases for radiation, 63, 64 conductivity of gases produced by γ rays, 64, 183 negative charge carried by β rays, 122 et seq. absorption of β rays proportional to density, 136 nature of α rays, 142 attempt to measure charge of α rays, 153 constant ratio of uranium to radium in minerals, 462 connection of thorium with helium, 483 absorption of radium rays from sun by atmosphere, 492 presence of radium in Bath waters, 513 radio-activity of ordinary matter, 536

Sun effect of radium in, 491 age of, 492

Temperature effect of, on intensity of radiations from uranium and radium, 210 effect of, on luminosity, 210 rate of decay of radium emanation unaffected by, 249 of condensation of emanations, 283 rate of decay of thorium emanation unaffected by, 287 effect of, on excited activity, 315 effect of, on active deposit of thorium, 354 effect of, on active deposit of actinium, 368 effect of, on active deposit of rapid change of radium, 390 effect of, on active deposit of slow change, 401 of radium above surrounding space, 419 effect of, on amount of excited activity in atmosphere, 518 effect of, on natural ionization of air, 536

Theories of radio-activity, review of, 437 et seq. discussion of, 441 et seq. disintegration theory, 445 et seq.

Thermo-luminescence, 207

Thomson, J. J. relation between current and voltage for ionized gases, 34 difference between ions as condensation nuclei, 49 charge on ion, 50 magnetic field produced by an ion in motion, 69 apparent mass of electron, 71 action of magnetic field on moving ion, 72 determination of e/m for cathode stream, 73 origin of X rays, 80 slow velocity electrons from radio-tellurium, 153 charge carried by α rays, 154 theory of radio-activity, 440 cause of heat emission from radium, 442 structure of atom, 487 possible causes of disintegration of radium, 487 nature of electrons, 496 emanation from tap-water and deep wells, 510 radio-activity of ordinary materials, 539

Thomson, J. J. and Rutherford ionization theory of gases, 31 et seq.

Thorium discovery of radio-activity of, 10 emanation from, 11 preparation of non-radio-active thorium, 29 nature of radiations from, 109 β rays from, 114 α rays from, 141 γ rays from, 180 separation of Th X from, 220 recovery of activity of, 221 disintegration of, 234 emanation from, 238 properties of emanation from, 239 diffusion of emanation from, 275 condensation of emanation from, 277 excited radio-activity from, 295 et seq. analysis of active deposit of, 351 et seq. rayless change in, 352 explanation of initial portion of decay curve, 358 explanation of initial portion of recovery curve, 358 effect of successive precipitations on, 358 recovery curve after large number of precipitations, 359 products of, 363 non-separable activity of, 363 radiations from active products of, 363 division of activity amongst active products of, 363 rate of emission of energy by, 432 theories of radio-activity of, 438 discussion of theories of radio-activity, 441 et seq. source of energy of radiations, 442 et seq. theory of radio-active change, 444 et seq. table of radio-active products of, 449 rate of change of, 458 life of, 458 conservation of radio-activity of, 469 total emission of energy from 1 gram of, 475 possible causes of disintegration of, 486 et seq.

Thorium A period and properties of, 352 et seq. absence of rays in, 352 effect of temperature on, 354

Thorium B period and properties of, 352 et seq. effect of temperature on, 354 radiations from, 363

Thorium X methods of separation of, 220 law of decay of activity of, 221 law of recovery of activity of, 221 theory to explain production of, 224 material nature of, 226 continuous production of, 227 explanation of decay of activity of, 229 effect of conditions on the rate of change of, 233 disintegration hypothesis to explain production of, 234 minute amount of, produced, 237 effect of successive separations of, on activity of thorium, 358 _et seq._ analysis of decay and recovery curves of, 358 radiations from, 363

Tommasina scintillations produced by electrification, 160

Townsend ions by collision, 39, 57 coefficient of recombination, 41 diffusion of ions, 51 et seq. comparison of charge on ion with that on hydrogen atom in electrolysis, 53 number of molecules per c.c. of gas, 54 ionization by collision for different speeds, 171

Transformations, successive theory of, 325 et seq. nomenclature of, 328 activity due to, 337 detection of a rayless change in, 341 in uranium, 346 et seq. in thorium, 351 et seq. in actinium, 364 et seq. in radium, 371 et seq. list of, 449 origin of radium in, 459 helium, a result of, 476 et seq. possible cause of, 486 et seq. application of, to evolution of matter, 497 et seq.

Transmission of excited radio-activity of radium and thorium, 318 et seq. of excited radio-activity of actinium, 323

Travers and Ramsay amount of helium in fergusonite, 486

Troost rays from hexagonal blende, 4

Uranium discovery of radio-activity of, 5 persistence of radiations of, 6 discharging power of rays, 7 absence of reflection, refraction and polarization, 7 examination of uranium minerals, 11 et seq. relative activity of compounds of uranium, 12 nature of radiations from, 109 β rays from, 114 α rays from, 141 γ rays from, 180 separation of Ur X from, 219 recovery of activity of, 219 changes in, 346 et seq. non-separable activity of, 347 radiations from Ur X, 347 et seq. method of measurement of activity of Ur X, 347 emission of energy by, 418 theories of radio-activity of, 437 et seq. discussion of theories of radio-activity, 441 et seq. source of energy of radiation, 442 et seq. theory of radio-active change, 444 et seq. table of active products, 449 rate of change of, 458 life of, 458 radium probable product of, 459 et seq. amount of radium in, 460 et seq. amount of, in radio-active minerals, 461 growth of radium in, 463 conservation of radio-activity of, 469 total emission of energy from 1 gram of, 475 possible causes of disintegration of, 486 et seq.

Uranium X separation of, by Crookes, 219 separation of, by Becquerel, 219 decay of activity of, 223 recovery of activity of, 223 theory to explain production of, 224 et seq. material nature of, 226 explanation of decay of activity of, 229 changes in, 346 et seq. radiations from, 347 et seq. method of measurement of radiations from, 347 effect of crystallization on activity of, 349 diffusion of, 350

Velocity of ions in electric field, 42 et seq. difference between, of positive and negative ions, 43 et seq. of β particle or electron, 126 et seq. variation of mass of electron with, 127 of α particle, 148 of transmission of carriers of excited activity, 320 et seq. of ions in atmosphere, 528

Villard discovery of γ rays from radium, 179 alteration of X ray screen with time, 205 activity produced by cathode rays, 530

Vincenti and Levi Da Zara radium emanation in spring waters, 516

Voller variation of activity of radium with concentration, 467

Volume of radium emanation, calculation of, 289 decrease of, of radium emanation, 290

Walker, G. W. theory of electrometer, 90

Walkhoff action of radium rays on skin, 216

Wallstabe diffusion of radium emanation into liquids, 276

Water emanation from, 510 et seq. decay of activity of emanation from, 511 et seq.

Water-falls amount of excited activity produced at Niagara, 520 electrification produced near, 520

Watts, Marshall atomic weight of radium, 18

Weichert velocity of cathode rays, 76

Weight loss of by radio-elements, 473 attempts to measure loss of in radium, 474

Wheeler and Bumstead diffusion of radium emanation, 273 emanation from surface water and the soil, 512, 522 identity of emanation from soil with radium emanation, 512, 522

Whetham effect of valency of ion on colloidal solutions, 215 production of radium from uranium, 463

Wiedemann thermo-luminescence, 207

Wiedemann and Schmidt thermo-luminescence, 207

Wien value of e/m for canal rays, 78 positive charge of canal rays, 78 amount of charge carried by β rays, 124

Willcock, Miss and Hardy coloration of iodoform solution by radium rays, 214

Willemite phosphorescence of, under radium rays, 203 use of, to show condensation of emanation, 279

Willows and Peck action of radium rays on spark, 208

Wilson, C. T. R. ions as nuclei of condensation, 47 et seq. difference between positive and negative ions as condensation nuclei, 49 equality of charges carried by positive and negative ions, 50 construction of electroscope, 86, 88 natural ionization of air in vessels, 501 radio-activity of rain and snow, 505, 506 loss of charge in closed vessels, 531 et seq., 534 presence of ions in dust-free air shown by condensation, 533 number of ions produced per c.c., 533 effect of pressure and nature of gas on ionization in sealed vessels, 534

Wilson, H. A. charge on ion, 51

Wilson, W. E. radium in sun, 491

Wölfl, Hofmann and Gonder properties of radio-active lead, 27, 413

Wood, A. radio-activity of ordinary materials, 540

Zara, Levi Da and Vincenti radium emanation in spring waters, 516

Zeeman action of magnetic field on light, 77

Zeleny velocity of ions, 42 et seq. difference of velocity of ions, 45 potential gradient between electrodes, 65

Zerban and Hofmann connection of activity of thorium with uranium, 29

Zinc Sulphide scintillations produced in by α rays, 158 cause of luminosity of, 160, 549

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● Transcriber’s Notes: ○ Inconsistent spelling and hyphenation were made consistent only when a predominant form was found in this book. ○ Text that was in italics is enclosed by underscores (italics).

Text that was in bold face is enclosed by equals signs (=bold=).

○ Footnotes have been moved to follow the chapters in which they are referenced. ○ Formulae in the text have been rendered in text form, and in some cases there are unusual conventions used. The convention name^{expression} means that the expression is a subscript to the name, and name_{expression} means that expression is a subscript to the name. Many parentheses are added to allow an expression to be transcribed on one line.

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