The phenomena and laws of heat

Front matter

! UNITED STATES OP AMERICA, i ectruiD showing eke absorptive power of Sodium vapour Pig.6

THE

PHENOMENA AND LAWS

Chapter I. General Phenomena Of Heat

PAGE

1. Our ' ordinary perceptions of Heat distinguished from its

proper phenomena . . • . . . I

2. Heat produced by the atomic movement which produces

chemical action * 2

3. Transference of Heat by radiation ..... 4

4. Transference of Heat by conduction . . . . . 5

5. The combustion of bodies determined by Heat ... 5

6. Change of bulk effected by Heat 7

7. Fusion and solidification 8

8. Evaporation, ebullition, and condensation of vapours . 9

9. Mechanical effects of Heat . . . . . . . 1 1

10. A fire-machine spends Heat in effecting work . . . 15

11. Relation between animal Heat and mechanical work . 19

12. Electricity produced by Heat 22

13. Fire-worshippers 24

Chapter II. On The Experimental Method And The Thermometer

1. Aim of physics

2. Hypothesis as to the nature of Heat

3. Invention of the thermometer

4. Graduation and use of the thermometer

5. The measurement of Heat .

26 28 32 36 39

VI CONTENTS.

Chapter III. Sources Of Heat

PAGE

1. Solar Heat. — Terrestrial Heat 42

2. Heat produced by chemical action. — Combustion . . 44

3. Heat produced by mechanical motion 54

4. Heat produced by mechanical work is equivalent to the work

done . 56

5. Mechanical equivalent of Heat 60

6. On some grand natural phenomena in which mechanical force

is converted into Heat . . 63

7. Heat developed by electricity 69

8. Heat developed by animal and vegetable life . 70

•

Chapter IV. ' The Radiation Of Heat

i. Reflection of Heat. — Burning-mirrors . c •

2. Refraction of Heat. — Burning-glasses ....

3. Physical identity of a ray of light with a ray of Heat-

Luminous and calorific spectra ....

4. Sifting of the rays

5. Influence of temperature on the emission of Heat

6. Influence of the nature of the source on emission. — Corre

lation between emission and absorption

7. Influence of distance on radiant Heat

8. Various applications of the preceding principles. — Dew. —

Atmospheric water vapour 100

85 92

97 99

Chapter V. Conduction Of Heat

1. Bodies which are good conductors . 107

2. Substances which are bad conductors . . , .114

3. - Convection of Heat in liquids and gases . . , .117

4. Effect* of convection in the ocean. — Marine currents , , 122

5. Effects of convection in the atmosphere. — Winds . . .129

CONTENTS. Vli

Chapter VI. Change Of The Volume Of Bodies

PAGE

1. Action of Heat on gases. — Sensible Heat. — Exterior work . 133

2. Action of Heat on solids and liquids. — Grandeur of mole-

cular forces. — Interior work . . . . . .141

3. How the expansion of bodies is measured. — Maximum density

of water . 146

4. Explanation of various phenomena . . . . .152

5. On specific Heat 155

CHAPTER VII/

ON FUSION AND SOLIDIFICATION.

1. Law concerning the temperature at which substances fuse. —

Heat consumed during fusion and produced by solidification 158

2. Interior work. — Crystals. — Ice-flowers . . . ' . 163

3. Change in volume and exterior resistance. — Expansive force

oMce 170

4. Regelation. — Glaciers 175

Chapter VIII. Concerning Evaporation And Ebullition

1. Superficial evaporation of solids and liquids . . . 1 81

2. Elastic force of vapours. — Papin's Digester . . . 184

3. Evaporation is accompanied bv a disappearance of sensible

4. Ebullition under a constant pressure. — Law of temperature . 194

5. Interior and exterior work. — Heat of evaporation . . 198

6. The utilization 01 Heat in the steam-engine and hot-air engine 202

7. How a liquid may be made to boil by cold .... 204

8. The Geysers 208

9. The spheroidal state of liquids. — How the human body may

be incombustible 212

Vffi CONTENTS.

Chapter IX

ON THE THREE STATES OF MATTER, AND ON THE ARTIFICIAL METHODS OF PRODUCING COLD.

PAGE

1. Liquefaction of gases, and solidification of liquids . . 220

2. Artificial production of cold. — Manufacture of ice. — Solid

carbonic acid . ........ 225

3. Solution. — Crystallisation 239

Chapter X. Heat Upon The Terrestrial Globe

1. Equilibrium of Heat on the surface of the globe. — Law of the

conservation of energy . . . . . . 243

2. Distribution of temperature on the earth's surface. — Climates,

— Effects of atmospheric moisture . . . . 249

3. The changes which have occurred in the distribution of Heat

previous to our epoch. — Geological revolutions . . 257

4. Future of the terrestrial globe •••••• 264

LIST OF ILLUSTRATIONS

KG. PAGff

1. Recoiling ^Eolypile II

2. Laubereau's Hot-air Engine . . . • . . 13 2<z.Section of the same Engine ..••.. 15

3. Haiiy's Tourmaline Apparatus ...... 22

4. Thermo-electric Pile 23

5. Pile and Galvanometer 23

6. Air Thermometers 33

7. Alcohol Thermometer ....... 34

8. Apparatus for graduating the Thermometer . • . 37

9. Ordinary Thermometers . . . . . . . 37

10. Brongniart's Pyrometer 38

11. Pyrheliometer . . . 43

12 Combustion of Phosphorus in Chlorine .... 46

13. Gas Flame 48

14. Bunsen's Gas Burner ........ 5°

15. Candle Flame 50

16. Oxy-hydiogen Blowpipe . .' 53

17. Compressibility of liquids • 55

18. Compression Syringe . ...... 56

19. Joule's Apparatus for producing Heat by the friction of

20. Theory of Tides 66

21. Voltaic Pile and Arc . . . . . . . 71

22. Spathe of the Arum ...... • • 73

24. Two opposed Mirrors . ....... 78

25. Converging Lens 81

25<?.Bernieres' Immense Burning-glass 83

28. Savart's Wheels 89

LIST OF ILLUSTRATIONS.

FIG. PAGE

29. Solar Spectrum {Frontispiece.)

30. Successive Prisms showing the action of the Prism on simple

rays {Frontispiece.)

31. Inverted Prisms ......... 95

33. Voltaic Arc . . 98

34. Spectrum of Sodium {Frontispiece.)

35. Conductibility of solid bodies 10S

36. Ingenhous's Apparatus 109

37. Conductibility of Iron and Bismuth . . ...... 1 10

38. Double Cylinder of copper and wood . , . . .Ill

39. Properties of Metallic Gauze . . . . .112

40. Sir Humphrey Davy's Safety Lamp 113

42. Convection of water 117

43. Apparatus to demonstrate the conductibility of liquids . 119

44. Water boiling over ice . ,. . . . . .. 120

45. Apparatus for showing the cooling powers of gases . . 121

46. Theory of Marine Currents . . . . 123

47. Map of Marine Currents 125

48. Theory of Polar Currents .129

49. Expansion of air under constant pressure . . . 134

50. Montgolfier's Balloon . . . . 135

51. Heating of a gas, its volume remaining the same . . 138

52. Straightening of the walls at the Conservatoire des arts et

Metiers 144

53. Apparent Expansion of Liquids 147

54. Apparatus for measuring the Expansion of Solids . . 1 50

55. Double bar (iron and copper) . . . . . .152

56. Trevelyan's Experiment . . . . . . 153

57. Gore's Experiment 154

58. Fusion of Sulphur 159

59. Crystallization of Sulphur 164

60. Various forms of Snow . . . . . .166

61. Dissection of Ice by the Solar Rays . . . . . 166

63. Exhibition of Ice Flowers by projection . . . .170

64. Expansive force of Ice 173

65. Freezing of Water in an open vessel 174

66. Freezing of Water above an uneven surface . . .175

67. Moulding Ice . .176

68. Air-cells in Ice 178

69. Apparatus for the vaporisation of liquids . . . .185

70. Papin's Digester . . . ."" 188

71. Ordinary Ebullition 194

72. Donny's experiment in Ebullition . . . . .196

LIST OF ILLUSTRATIONS, XI

FIG.

74. Ebullition in vacuo

75. Ebullition of water by cold

76. Theory of Geysers .......

77. Spheroidal State of Water ......

78. Flame seen between the hot surface and the globule ,

79. Explosion produced by the Cooling of Water in the sphe

roidal state .... ....

80. Liquefaction of Sulphurous Acid by Cold .

81. Liquefaction of Sulphurous Acid by pressure . .

82. Liquefaction of Gases by the process of Davy and Faraday

83. Thilorier's Apparatus for liquefying Carbonic Acid

84. Cold produced by a Jet of Air .....

85. Condensation of water vapour by rarefaction of the air

86. Freezing Water by Evaporation ....

87. Freezing Water by the Evaporation of Ether

88. Carre's Freezing Apparatus

89. Family Ice-machine

90. Box for collecting solid Carbonic Acid

91. Crystallization of Alum

92. Maximum and Minimum Thermometers

205 207 210 213 214

231 232 236

237 241

WEIGHTS AND MEASURES,

In general, throughout the following treatise, the figures of the French metric system have been placed in juxtaposition with their English equivalents, or the latter have been substituted for them. In some instances, this could not be done without an entire reconstruction of the author's argument. The following short table of equivalent weights and measures may therefore be found useful : —

I kilogramme igo* do.

I gramme loo da.

I metre

I decimetre

I centimetre

I millimetre-

I kilometre

* cubic metre

I ir decimetre

I ,, centimetre

I litre

I decilitre

I. centilitre

2*205 lbs., avoirdupois.

I cwt. 3 qrs. 24*462 lbs. 0544 drams avoirdupois. 3 oz. 8 438 drams avoirdupois, 39/37°79 inches,.

61027*051 + cubic inches.

61*02705. +

0*06112 + ,, „

1 cubic decimetre — 1*760 + pints.

THE THERMOMETER.

The range of temperature from the freezing to the boiling point of water is divided by Fahrenheit's scale inf.c 180 portions or degrees, numbered from 32 to.212 .

In the Centigrade scale the same range of temperature is divided into 100 parts, numbered from o to ioo°.

Hence, to convert degrees of Fahrenheit into Centigrade, subtract 32, and multiply the remainder by {, and —

To convert Centigrade into Fahrenheit, multiply the Centigrade by f> and add 32 to the product

THE

PHENOMENA AND LAWS OF HEAT

Chapter I. General Phenomena Of Heat

I. Our ordinary perceptions of Heat distinguished from its proper phenomena.

When we touch any of the bodies that surround us, we generally recognise a difference in their state : some communicate to us the sensation of heat, others that of cold. This difference is merely relative to our sensations ; it depends on the fact that the sense of touch undergoes a particular modification, which causes a sensation to be experienceu, and a judgment to be formed concerning it, practically at the same moment. This special sensibility is given to us that we may always be on the watch for our preservation. But it is not this kind of action with which we are concerned in the present work. We have to investigate heat and cold apart from ourselves as exhibited by inanimate bodies. We have to discover what phenomena occur when such bodies act one upon another ; what modifications they undergo in becoming hot or cold. And in the degree that these modifications impress our senses we shall make those impressions also contribute towards the judgments we have to form. In most cases the sense of sight will reveal what is taking place ; but it may happen that our hearing or our sense of smell may also be called upon to

A t THE PHENOMENA AND LAWS OF HEAl".

assist in the observation. It is thus that the human mind collects tfre various data furnished by the senses, and makes such an orderly arrangement of observed facts as to obtain a knowledge of nature.

The following striking example will teach us to be ever on our guard against the sense of touch when heat is in question. Plunge the right hand into a vessel of tepid water and the left into one of iced water. Then put them both into water of the ordinary temperature. The latter will now appear to be cold, if we decide according to the sensation experienced by the right hand, but warm if we judge by our left ; and yet in both cases it is the same water, having the same uniform temperature. The different manner in which we prepared our hands is the sole reason why we experience different sensations.

The same thing takes place when we pass immediately from a hot bath into the open air — the temperature feels cold. But if we pass out of a cool room or vault into the same atmosphere as before, it will seem to be warm.

It thus appears that there is no essential difference between heat and cold when we abstract our sensations, and consider only the body which impresses us. The action of heating is simply the inverse of that of cooling, and the word heat designates the cause of this kind of action.

Let us now prepare ourselves to study this subject by separating from amongst the thousand phenomena presented by nature those which belong to heat. It is certain that our first view will be very incomplete ; but as we advance we shall become better observers, and at every step a new discovery will recompense our toil, and urge us to further advances. We may not succeed in seeing and understanding all that we desire ; but we may, at least, derive profit from the study in proportion to our means, and the intelligence we bring to bear upon them.

2. Heat produced by the atomic movement which produces chemical action.

We are in the midst of winter ; the surface of the earth is white with snow, the rivers are frozen over ; but the room

GENERAL PHENOMENA OF HEAT. 3

we inhabit is warmed by a brilliant fire. These conditions are admirably well adapted for the commencement of our investigations.

What is a fire ? What, for example, takes place in the grate ? We must here have recourse for a moment to the science of chemistry ; but at present our object is only to obtain from it some two or three very simple ideas, requiring a very slight effort of the intellect to comprehend.

The combustible substance burns in a current of air, which enters the room through the interstices of the doors and windows, and rises in the chimney after passing over the coals. This air becomes modified in the act of supporting combustion. Air is formed of two parts — the one called nitrogen, the other oxygen. It is the latter which gives rise to the phenomena of combustion whilst uniting itself to the carbon; and by this union carbonic acid gas is produced, which escapes up the chimney in company with the nitrogen. We need not enter into a detailed explanation of this phenomenon, our object being to concentrate our attention on the subject of heat

The action which takes place in this chemical combination may be somewhat understood by imagining oxygen and carbon to be formed of particles called atoms, which rush together and unite in one body when they are brought sufficiently near each other. It is this sudden rush or shock of the particles which causes combustion. It produces at once heat and light; for the flame that we see is only a phenomenon which takes place over the coals, and which we judge of according to the impression it makes on our eyes.

Are we then to understand that there can be no chemical combination producing heat without the simultaneous production of light? Take a single example from a thousand that might be cited to prove the contrary. Powdered sulphur is mixed with iron filings, and a hole being made in the ground, it is filled with the mixture, which is afterwards covered with earth and watered ; in a little time the mass heats spontaneously, swells up, raises the earth, and disengages vapours. This experiment was first made by Nicolas Lemery, a chemist of the seventeenth century, who was educated in a very humble way at Rouen, his native place, but lived to acquire an immense reputation as a scientific chemist. His object having been to explain volcanic action, this little experiment came to be known as " Lemery's Volcano." His explanation of that wonderful natural phenomenon is not now admitted, but the experiment remains as a curious example of chemical combination ; iron and sulphur under the influence of water unite to form a solid brown body, called sulphide of iron, and at the instant of this union there is a production of heat without light. Heat, then, is a consequence of the motion of the atoms of matter. If we pass its effects in review, we shall always discover in them motion as the correlative of heat.

And first let us inquire into the transference of heat from the fire into neighbouring bodies.

3. Transference of Heat by radiation.

The heat which warms us, even at a distance, through the air interposed between our bodies and the burning coal, may be intercepted by a common wooden screen. The matter of which wood is composed is a barrier to heat, as any opaque body is to light. If, however, we substitute for the wooden screen a pane of glass, we shall feel the heat at the same time that we see the light from the coals through the pane. It is not because the glass gets warm, and afterwards acts upon us; for the effect is immediate, and a considerable time is necessary to make the glass hot. When the sun throws its rays on our windows the light and heat instantly traverse the glass, whereas both are completely stopped by the walls. Now, the sun is an immense fire, more than ninety millions of miles distant from our globe — so far distant, in fact, that, if we travelled at the rate often miles an hour, it would take more than a thousand years to reach the sun ; and yet its rays require only eight minutes to reach us. To conceive this, imagine a wave of motion commencing at the sun, and spreading itself step by step across the celestial space, much like those familiar circles produced by the splash of a stone on the tranquil surface of water : how gradually it extends

TRANSFERENCE OF HEAT. 5

farther and farther from its centre, until at last it strikes the shore, where it dies out. Now, it is obvious that we can follow the liquid wave with the eye, because its speed is not too great. We have, then, but to imagine a speed two or three million times greater in order to grasp the idea of the propagation of the solar rays. Our ordinary fires act in the same manner ; they are centres of motion ; they resemble little suns of brief duration ; they radiate heat around them in obedience to a general law, and every ray is the seat of motion.

4. Transference of Heat by conduction.

Heat extends itself by still another method. Let the extremity of a bar of iron be surrounded by red-hot coals : very soon the other end becomes too hot to be held. The bar will have been progressively heated by a communication of motion from stratum to stratum, from particle to particle. This phenomenon has been called conduction. In wood this property is scarcely to be recognised ; a stick of wood can be very easily held by one end whilst the other end is burning : therefore wood is said to be a bad conductor of heat, whilst iron is a good conductor.

Thus the reader has obtained a general notion of two kinds of calorific phenomena, radiation and conduction. It will now be necessary to fix our attention on the bodies themselves which receive heat, in order that we may classify its effects.

5. The combustion of bodies determined by Heat.

A phosphorus match brought near the fire, without touching the coals, will catch light, to use the common expression. Hence heat-rays, when brought in contact with certain substances, are said to determine their combustion.

In this case the phosphorus, which covers the end of the match, is heated in the open air. The union of oxygen with the phosphorus has formed a white substance (phosphoric acid), which disappears in smoke. The phenomenon is analogous to that which occurs in the combustion of coal. The chemical combination which takes place is accompanied with the disengagement of heat and light ; thus heat causes the sulphur, which the phosphorus covered, to burn, so that it also combines with the oxygen of the air to form sulphurous acid gas. Finally, this combustion, first of the phosphorus and afterwards of the sulphur, determines that of the wood of which the match is composed.

It will be sufficient for our purpose to analyse what happens in the case of the phosphorus, as the same reasoning applies to the sulphur and the wood. The atoms of phosphorus are united by a force called cohesion, which force opposes itself to their combination with the atoms of oxygen. The rays of heat which come from the fire put these atoms in motion, acting like some actual force capable of destroying cohesion, and rending the atoms apart. When this happens, the atoms rush to unite themselves with the atoms of oxygen, and phosphoric acid results.

We have thus briefly studied a case of combustion determined by heat in concurrence with the air. There are other cases of combustion in which air plays no part, and which may be effected when the combustible is placed under the exhausted receiver of an air-pump.

Soak a bit of cotton in a mixture of equal weights of sulphuric acid and fuming nitric acid ; afterwards wash it with abundance of water, and you will obtain gun-cotton ; that is to say, cotton combined with a certain quantity of oxygen and nitrogen. If brought near a fire, it will instantly ignite and disappear, without leaving any visible traces. Evidently the atoms of which the gun-cotton was composed have been separated from one another by the action of heat, and they have recombined in another form as gases, which the air has forthwith carried away. If a more complete chemical explanation is desired, the case may be stated as follows : — Gun-cotton is a combination of carbon, hydrogen, oxygen, and nitrogen. When the heat of the fire has disassociated these four kinds of atoms, they recombine in the following order : the carbon and a part of the oxygen form carbonic acid gas, the hydrogen and the rest of the oxygen form vapour of water, and the nitrogen remains free. This is all done so quickly, that gun-cotton can be

CHANGE OF BULK EFFECTED BY HEAT. 7

burnt in the hand without causing the sensation of heat; a great flame is produced, which instantly disappears, leaving no trace if the gun-cotton has been properly prepared.

The action which takes place in the combination of gunpowder is essentially the same. Gunpowder is a mixture of saltpetre, charcoal, and sulphur. Decomposed by heat, its atoms enter into a fresh arrangement, producing carbonic acid gas and nitrogen gas, and leaving a brown compound of sulphur and potassium. It is the great bulk of gas which, developed in a fire-arm, presses against its sides, and drives out the cannon-ball.

As the facts we have enumerated belong to chemistry, we need not dwell upon them any longer. They are sufficient to give us a notion of the constitution of bodies, which may be defined as assemblages of particles bound together by cohesion, and which heat tends to separate, acting like a force contrary to cohesion. In the phenomena specially treated by physics, the particles are not supposed to change their nature; they simply separate from or come nearer to each other, or change their respective positions, always in themselves remaining identically the same. They are called molecules, and they must not be confounded with atoms, which may be of different kinds in the same body, and which go to form molecules. Thus water is an assemblage of like molecules ; each molecule is composed of two atoms of hydrogen and one atom of oxygen. Here we only study those phenomena in which there is no separation between the atoms, and in which the molecules alone act a part.

6. Change of bulk effected by Heat.

Take a copper cylinder or ball, and a ring through which it can pass freely. Then let the cylinder or ball be heated for some time in a fire, and it will be found that it can no longer pass through the ring. After being cooled, however, it will again pass freely as before ; from which the inference is plain — that copper expands by heat and contracts by cold. Hence one of the physical effects of heat is to change the volume of substances. We can form an idea of the rationale of this change by imagining the mole* cules to recede from, or to approach to, one another. It follows, also, that they do not really touch each other, a certain force keeping them always at a proper distance, in the same way that gravitation keeps the earth and othes planets at determinate distances from the sun, and uniter the innumerable globes which fill the universe ; for each star is a sun, which has its attendant planets, and is the centre of a movement which follows the plan traced by the Creator. Do not forget this general resemblance between the world of molecules and the worlds which revolve in the starry heavens. A universal law rules over the movements and the positions of both. We have not yet discovered this law ; but it is a great deal to have already suspected its existence, to have included in the same sentiment of admiration these two harmonies, differing only in their proportions : the one presiding over masses infinitely small, which we touch • the other over masses which we may call by comparison infinitely great, and which are revealed to us by instruments which enable us to explore the immeasurable profundities of space.

7. Fusion and solidification.

Here is a piece of ice, which we place in a glass vessel near the fire. It melts, and in the place of the solid ice we have soon an equal weight of liquid water. If now we expose this same water to the sharp cold which reigns out of doors, it will freeze again. This transformation is the effect of heat. To melt ice it must be submitted to the action of bodies warmer than itself; to freeze water it must be submitted to the action of colder bodies. The two operations are of the same kind, but both are inverse the one to the other. The sun will gradually bring about the same phenomena in a field covered with snow. By its heating action the white needles which cover the trees, and whose marvellous ramifications are so much admired by all observers of nature, will be reduced into drops of water, which will fall to the earth ; the carpet of snow which preserves the soil from the too sudden variations in the temperature of the air will diminish in thickness ; and the

EVAPORATION AND EBULLITION. 9

water that results will impregnate the earth, and, reanimating the germs contained in it, will restore to it its fertility.

At nightfall, under the clear starlight, the fusing process ceases. The force which produced the result has disappeared \ but more than this — the celestial space is much colder than ice. Thus a part of the water melted during the day will freeze again ; and several sunny days must succeed each other before the snow will completely disappear. Life will thus be revived little by little, and the danger that would attend a too sudden change will be avoided.

8. Evaporation, ebullition, and condensation of vapours.

Fusion, as we have seen, is one of the effects of the action of heat upon solid bodies. There is another effect which is manifested on some solids, but more especially on liquids. During a very sharp cold, a little snow left in the open air on a plate may disappear completely without having suffered fusion. The same thing happens to camphor if put in a large glass bottle. Little crystals are deposited here and there on the interior, changing place slowly if the position of the bottle be changed. Let us inquire what has happened in this case.

We shall find on investigation that vapours have been formed on the surface of the camphor ; their molecules have receded from one another, and constitute a true gas, which seeks to diffuse itself throughout all parts of the bottle. Some of these parts being colder than others, the vapours condense at those particular places, and again take the solid state by an action inverse to that of vaporization. Thus heat has transformed the camphor into vapour, and cold has made it once more assume the solid state. It is to this property that we owe the odour of camphor. Its vapours enter our nostrils and act on the organ of smell ; the same thing happens in the case of all odorous solids.

Liquid substances, however, evaporate most easily. After rain the stones and pavements dry very quickly in the sun, a phenomenon which evidently cannot be referred to absorption. Take a sheet of paper, weigh it, and then moisten

TO THE PHENOMENA AND LAWS OF HEAT.

it with water ; the moisture quickly disappears. Weigh it again, and it will be found to possess exactly the same weight as before. What has become of the water? It exists in a state of vapour, mixed with the air by which we are surrounded, and is therefore not visible. To see the vapour thus disengaged resume the liquid condition, get a decanter of cold water fresh from the well or cellar, and well corked, so that none may come out Much time will not elapse before dew will be seen deposited on the surface of the glass. This happens because the bottle of water has cooled the air of your chamber, and the vapour of water naturally contained in it has been condensed. The atmosphere which surrounds the terrestrial globe, and w r hich forms a gaseous ocean forty miles in depth, evidently contains vapour of water, arising from the evaporation of seas, rivers, and streams. When the sky is very clear, this water remains as an invisible transparent gas ; but various causes here and there determine its condensation, and from this arise clouds, fogs, rain, hail, and snow, circumstances determining which. We shall have to observe all these phenomena, and we shall try to discover the harmony of the laws by which they are all regulated.