Clouds, Rains, and Rivers
( Originally Published Early 1900's )
1. Every occurrence in Nature is preceded by other occurrences which are its causes, and succeeded by others which are its effects. The human mind is not satisfied with observing and studying any natural occurrence alone, but takes pleasure in connecting every natural fact with what has gone before it, and with what is to come after it.
2. Thus, when we enter upon the study of rivers and glaciers, our interest will be greatly augmented by taking into account not only their actual appearances, but also their causes and effects.
3. Let us trace a river to its source. Beginning where it empties itself into the sea, and following it backwards, we find it from time to time joined by tributaries which swell its waters. The river of course becomes smaller as these tributaries are passed. It shrinks first to a brook, then to a stream; this again divides itself into a number of smaller streamlets, ending in mere threads of water. These constitute the source of the river, and are usually found among hills.
4. Thus the Severn has its source in the Welsh Mountains ; the Thames in the Cotswold Hills ; the Danube in the hills of the Black Forest; the Rhine and the Rhone in the Alps ; the Ganges in the Himalaya Mountains ; the Euphrates near Mount Ararat; the Garonne in the Pyrenees ; the Elbe in the Giant Mountains of Bohemia ; the Missouri in the Rocky Mountains, and the Amazon in the Andes of Peru.
5. But it is quite plain that we have not yet reached the real beginning of the rivers. Whence do the earliest streams derive their water l A brief residence among the mountains would prove to you that they are fed by rains. In dry weather you would find the streams feeble, sometimes indeed quite dried up. In wet weather you would see them foaming torrents. In general these streams lose themselves as little threads of water upon the hillsides; but sometimes you may trace a river to a definite spring. The river Albula in Switzerland, for instance, rushes at its origin in considerable volume from a mountain side. But you very soon assure yourself that such springs are also fed by rain, which has percolated through the rocks or soil, and which, through some orifice that it has found or formed, comes to the light of day.
6. But we cannot end here. Whence comes the rain which forms the mountain streams? Observation enables you to answer the question. Rain does not come from a clear sky. It comes from clouds. But what are clouds? Is there nothing you are acquainted with which they resemble? You discover at once a likeness between them and the condensed steam of a locomotive. At every puff of the engine a cloud is projected into the air. Watch the cloud sharply: you notice that it first forms at a little distance from the top of the funnel. Give close attention and you will sometimes see a perfectly clear space between the funnel and the cloud. Through that clear space the thing which makes the cloud must pass. What, then, is this thing which at one moment is transparent and in-visible, and at the next moment visible as a dense opaque cloud?
7. It is the steam or vapor of water from the boiler. Within the boiler this steam is transparent and invisible; but to keep it in this invisible state a heat would be required as great as that within the boiler. When the vapor mingles with the cold air above the hot funnel it ceases to be vapor. Every bit of steam shrinks, when chilled, to a much more minute particle of water. The liquid particles thus produced form a kind of water-dust of exceeding fineness, which floats in the air; and is called a cloud.
8. Watch the cloud-banner from the funnel of a running locomotive ; you see it growing gradually less dense. It finally melts away altogether, and if you continue your observations you will not fail to notice that the speed of its disappearance depends upon the character of the day. In humid weather the cloud hangs long and lazily in the air; in dry weather it is rapidly licked up. What has become of it? It has been reconverted into true invisible vapor.
9. The drier the air, and the hotter the air, the greater is the amount of cloud which can be thus dissolved in it. When the cloud first forms, its quantity is far greater than the air is able to maintain in an invisible state. But as the cloud mixes gradually with a larger mass of air it is more and more dissolved, and finally passes altogether from the condition of a finely divided liquid into that of transparent vapor or gas.
10. Make the lid of a kettle air-tight, and permit the steam to issue from the pipe; a cloud is precipitated in all respects similar to that issuing from the funnel of the locomotive.
11. Permit the steam as it issues from the pipe to pass through the flame of a spirit-lamp, the cloud is instantly dissolved by the heat, and is not again precipitated. With a special boiler and a special nozzle the experiment may be made more striking, but not more instructive, than with the kettle.
12. Look to your bedroom windows when the weather is very cold outside; they sometimes stream with water derived from the condensation of the aqueous vapor from your own lungs. The windows of railway carriages in winter show this condensation in a striking manner. Pour cold water into a dry drinking-glass on a summer's day: the outside surface of the glass becomes instantly dimmed by the precipitation of moisture. On a warm day you notice no vapor in front of your mouth, but on a cold day you form there a little cloud derived from the condensation of the aqueous vapor from the lungs.
13. You may notice in a ballroom that as long as the doors and windows are kept closed, and the room remains hot, the air remains clear, but when the doors or windows are opened a dimness is visible, caused by the precipitation to fog of the aqueous vapor of the ballroom. If the weather be intensely cold the entrance of fresh air may even cause snow to fall. This has been observed in Russian ballrooms; and also in the subterranean stables at Erzeroom, when the doors are opened and the cold morning air is permitted to enter.
14. Even on the driest day this vapor is never absent from our atmosphere. The vapor diffused through the air of this room may be congealed to hoarfrost in your presence. This is done by filling a vessel with a mixture of pounded ice and salt, which is colder than the ice itself, and which, therefore, condenses and freezes the aqueous vapor. The surface of the vessel is finally coated with a frozen fur, so thick that it may be scraped away and formed into a snowball.
15. To produce the cloud, in the case of the locomotive and the kettle, heat is necessary. By heating the water we first convert it into steam, and then by chilling the steam we convert it into cloud. Is there any fire in nature which produces the clouds of our atmosphere? There is : the fire of the sun.
16. Thus, by tracing backward, without any break in the chain of occurrences, our river from its end to its real beginning, we come at length to the sun.
17. There are, however, rivers which have sources somewhat different from those just mentioned. They do not begin by driblets on a hillside, nor can they be traced to a spring. Go, for example, to the mouth of the river Rhone, and trace it backwards to Lyons, where it turns to the east. Bending round by Chambéry, you come at length to the Lake of Geneva, from which the river rushes, and which you might be disposed to regard as the source of the Rhone. But go to the head of the lake, and you find that the Rhone there enters it, that the lake is in fact a kind of expansion of the river. Follow this upwards; you find it joined by smaller rivers from the mountains right and left. Pass these, and push your journey higher still. You come at length to a huge mass of ice—the end of a glacier—which fills the Rhone valley, and from the bottom of the glacier the river rushes. In the glacier of the Rhone you thus find the source of the river Rhone.
18. But again we have not reached the real beginning of the river. You soon convince yourself that this, earliest water of the Rhone is produced by the melting of the ice. You get upon the glacier and walk upwards along it. After a time the ice disappears and you come upon snow. If you are a competent mountaineer you may go to the very top of this great snow-field, and if you cross the top and descend at the other side you finally quit the snow, and get upon another glacier called the Trift, from the end of which rushes a river smaller than the Rhone.
19. You soon learn that the mountain snow feeds the glacier. By some means or other the snow is converted into ice. But whence comes the snow? Like the rain, it comes from the clouds, which, as before, can be traced to vapor raised by the sun. Without solar fire we could have no atmospheric vapor, without vapor no clouds, without clouds no snow, and without snow no glaciers. Curious then as the conclusion may be, the cold ice of the Alps has its origin in the heat of the sun.
20. But what is the sun? We know its size and its weight. We also know that it is a globe of fire far hotter than any fire upon earth. But we now enter upon another inquiry. We have to learn definitely what is the meaning of solar light and solar heat; in what way they make themselves known to our _senses ; by what means they get from the sun to the earth, and how, when there, they pro-duce the clouds of our atmosphere, and thus originate our rivers and our glaciers.
21. If in a dark room you close your eyes and press the eyelid with your finger nail, a circle of light will be seen opposite to the point pressed, while a sharp blow upon the eye produces the impression of a flash of light. There is a nerve specially devoted to the purposes of vision which comes from the brain to the back of the eye, and there divides into fine filaments, which are woven together to a kind of screen called the retina. The retina can be ex-cited in various ways so as to produce the consciousness of light ; it may, as we have seen, be excited by the rude mechanical action of a blow imparted to the eye.
22. There is no spontaneous creation of light by the healthy eye. To excite vision the retina must be affected by something coming from without. What is that some-thing? In some way or other luminous bodies have the power of affecting the retina—but how?
23. It was long supposed that from such bodies issued, with inconceivable rapidity, an inconceivably fine matter which flew through space, passed through the pores sup-posed to exist in the humors of the eye, reached the retina behind, and by their shock against the retina aroused the sensation of light.
24. This theory, which was supported by the greatest men, among others by Sir Isaac Newton, was found competent to explain a great number of the phenomena of light, but it was not found competent to explain all the phenomena. As the skill and knowledge of experimenters increased, large classes of facts were revealed which could only be explained by assuming that light was produced, not by a fine matter flying through space and hitting the retina, but by the shock of minute waves against the retina.
25. Dip your finger into a basin of water, and cause it to quiver rapidly to and fro. From the point of disturbance issue small ripples which are carried forward by the water, and which finally strike the basin. Here, in the vibrating finger, you have a source of agitation; in the water you have a vehicle through which the finger's motion is transmitted, and you have finally the side of the basin which receives the shock of the little waves.
26. In like manner, according to the wave theory of light, you have a source of agitation in the vibrating atoms, or smallest particles, of the luminous body; you have a vehicle of transmission in the substance which is supposed to fill all space, and to be diffused through the humors of the eye; and finally, you have the retina, which receives the successive shocks of the waves. These shocks are supposed to produce the sensation of light.
27. We are here dealing, for the most part, with suppositions and assumptions merely. We have never seen the atoms of a luminous body, nor their motions. We have never seen the medium which transmits their motions, nor the waves of that medium. How, then, do we come to assume their existence?
28. Before such an idea could have taken any real root in the human mind, it must have been well disciplined and prepared by observations and calculations of ordinary wave-motion. It was necessary to know how both water-waves and sound-waves are formed and propagated. It was above all things necessary to know how waves, passing through the same medium, act upon each other. Thus disciplined, the mind was prepared to detect any resemblance presenting itself between the action of light and that of waves. Great classes of optical phenomena accordingly appeared which could be ac-counted for in the most complete and satisfactory manner by assuming them to be produced by waves, and which could not be otherwise accounted for. It is be-cause of its competence to explain all the phenomena of light that the wave theory now receives universal acceptance on the part of scientific men.
Let me use an illustration. We infer from the flint implements recently found in such profusion all over England and in other countries, that they were produced by men, and also that the Pyramids of Egypt were built by men, because, as far as our experience goes, nothing but men could form such implements or build such Pyramids. In like manner, we infer from the phenomena of light the agency of waves, because, as far as our experience goes, no other agency could produce the phenomena.
THE WAVES OF HEAT WHICH PRODUCE THE VAPOR OF OUR ATMOSPHERE AND MELT OUR GLACIERS.
29. Thus, in a general way, I have given you the conception and the grounds of the conception, which regards light as the product of wave-motion : but we must go farther than this, and follow the conception into some of its details. We have all seen the waves of water, and we know they are of different sizes—different in length and different in height. When, therefore, you are told that the atoms of the sun, and of almost all other luminous bodies, vibrate at different rates, and produce waves of different sizes, your experience of water-waves will enable you to form a tolerably clear notion of what is meant.
30. As observed above, we have never seen the light-waves, but we judge of their presence, their position, and their magnitude, by their effects. Their lengths have been thus determined, and found to vary from about 1-30,000 to 1-60,000 of an inch.
31. But besides those which produce light, the sun sends forth incessantly a multitude of waves which pro-duce no light. The largest waves which the sun sends forth are of this non-luminous character, though they possess the highest heating power.
32. A common sunbeam contains waves of all kinds, but it is possible to sift or filter the beam so as to intercept all its light, and to allow its obscure heat to pass unimpeded. For substances have been discovered, which, while intensely opaque to the light-waves, are almost perfectly transparent to the others. On the other hand, it is possible, by the choice of proper substances, to intercept in a great degree the pure heat-waves, and to allow the pure light-waves free transmission. This last separation is, however, not so perfect as the first.
33. We shall learn presently how to detach the one class of waves from the other class, and to prove that waves competent to light a fire, fuse metal, or burn the hand like a hot solid, may exist in a perfectly dark place.
34. Supposing, then, that we withdraw, in the first instance, the large heat-waves, and allow the light-waves alone to pass. These may be concentrated by suitable lenses and sent into water without sensibly warming it. Let the light-waves now be withdrawn, and the larger heat-waves concentrated in the same manner; they may be caused to boil the water almost instantaneously.
35. This is the point to which I wished to lead you, and which without due preparation could not be understood. You now perceive the important part played by these large darkness-waves, if I may use the term, in the work of evaporation. When they plunge into seas, lakes, and rivers, they are intercepted close to the surface, and they heat the water at the surface, thus causing it to evaporate; the light-waves at the same time entering to great depths without sensibly heating the water through which they pass. Not only, therefore, is it the sun's fire which produces evaporation, but a particular constituent of that fire, the existence of which you probably were not aware of.
36. Further, it is these self-same lightless waves which, falling upon the glaciers of the Alps, melt the ice and produce all the rivers flowing from the glaciers; for I shall prove to you presently that the light-waves, even when concentrated to the uttermost, are unable to melt the most delicate hoarfrost; much less would they be able to produce the copious liquefaction observed upon the glaciers.
37. These large lightless waves of the sun, as well as the heat-waves issuing from non-luminous hot bodies, are frequently called obscure or invisible heat. We have here an example of the manner in which phenomena, apparently remote, are connected together in this wonderful system of things which we call Nature. You cannot study a snowflake profoundly without being led back by it step by step to the constitution of the sun. It is thus through-out Nature. All its parts are interdependent, and the study of any one part completely would really involve the study of all.
EXPERIMENTS TO PROVE THE FOREGOING STATEMENTS
38. Heat issuing from any source not visibly red cannot be concentrated so as to produce the intense effects just referred to. To produce these it is necessary to employ the obscure heat of a body raised to the highest possible state of incandescence. The sun is such a body, and its dark heat is therefore suitable for experiments of this nature.
39. But in the atmosphere of London, and for experiments such as ours, the heat-waves emitted by coke raised to intense whiteness by a current of electricity are much more manageable than the sun's waves. The electric light has also the advantage that its dark radiation em-braces a larger proportion of the total radiation than the dark heat of the sun. In fact, the force or energy, if I may use the term, of the dark waves of the electric light is fully seven times that of its light-waves. The electric light, therefore, shall be employed in our experimental demonstrations.
40. From this source a powerful beam is sent through the room, revealing its track by the motes floating in the air of the room; for were the motes entirely absent the beam would be unseen. It falls upon a concave mirror (a glass one silvered behind will answer) and is gathered up by the mirror into a cone of reflected rays ; the luminous apex of the cone, which is the focus of the mirror, being about fifteen inches distant from its reflecting surface. Let us mark the focus accurately by a pointer.
41. And now let us place in the path of the beam a sub-stance perfectly opaque to light. This substance is iodine dissolved in a liquid called bisulphide of carbon. The light at the focus instantly vanishes when the dark solution is introduced. But the solution is intensely trans-parent to the dark waves, and the focus of such waves remains in the air of the room after the light has been abolished. You may feel the heat of these waves with your hand; you may let them fall upon a thermometer, and thus prove their presence ; or, best of all, you may cause them to produce a current of electricity, which deflects a large magnetic needle. The magnitude of the deflection is a measure of the heat.
42. Our object now is, by the use of a more powerful lamp, and a better mirror (one silvered in front and with a shorter focal distance) to intensify the action here rendered so sensible. As before, the focus is rendered strikingly visible by the intense illumination of the dust particles. We will first filter the beam so as to intercept its dark-waves, and then permit the purely luminous waves to exert their utmost power on a small bundle of gun-cotton placed at the focus.
43. No effect whatever is produced. The gun-cotton might remain there for a week without ignition. Let us now permit the unfiltered beam to act upon the cotton. It is instantly dissipated in an explosive flash. This experiment proves that the light-waves are incompetent to explode the cotton, while the waves of the full beam are competent to do so; hence we may conclude that the dark-waves are the real agents in the explosion.
44. But this conclusion would be only probable; for it might be urged that the mixture of the dark-waves and the light-waves is necessary to produce the result. Let us then, by means of our opaque solution, isolate our dark-waves and converge them on the cotton. It explodes as before.
45. Hence it is the dark-waves, and they only, that are concerned in the ignition of the cotton.
46. At the same dark focus sheets of platinum are raised to vivid redness; zinc is burnt up; paper instantly blazes; magnesium wire is ignited ; charcoal within a receiver containing oxygen is set burning; a diamond similarly placed is caused to glow like a star, being after-wards gradually dissipated. And all this while the air at the focus remains as cool as in any other part of the room.
47. To obtain the light-waves we employ a clear solution of alum in water ; to obtain the dark-waves we em-ploy the solution of iodine above referred to. But as before stated (32), the alum is not so perfect a filter as the iodine; for it transmits a portion of the obscure heat.
48. Though the light-waves here prove their incompetence to ignite gun-cotton, they are able to burn up black paper; or, indeed, to explode the cotton when it is blackened. The white cotton does not absorb the light, and without absorption we have no heating. The blackened cotton absorbs, is heated, and explodes.
49. Instead of a solution of alum, we will employ for our next experiment a cell of pure water, through which the light passes without sensible absorption. At the focus is placed a test-tube also containing water; the full force of the light being concentrated upon it. The water is not sensibly warmed by the concentrated waves. We now remove the cell of water; no change is visible in the beam, but the water contained in the test-tube now boils.
50. The light-waves being thus proved ineffectual, and the full beam effectual, we may infer that it is the dark-waves that do the work of heating. But we clinch our inference by employing our opaque iodine filter. Placing it on the path of the beam, the light is entirely stopped, but the water boils exactly as it did when the full beam fell upon it.
51. The truth of the statement made in paragraph (34) is thus demonstrated.
52. And now with regard to the melting of ice. On the surface of a flask containing a freezing mixture we obtain a thick fur of hoarfrost (Paragraph 14). Sending the beam through a water-cell, its luminous waves are concentrated upon the surface of the flask. Not a spicula of the frost is dissolved. We now remove the water-cell, and in a moment a patch of the frozen fur as large as half-a-crown is melted. Hence, inasmuch as the full beam produces this effect, and the luminous part of the beam does not produce it, we fix upon the dark portion the melting of the frost.
53. As before, we clinch this inference by concentrating the dark-waves alone upon the flask. The frost is dissipated exactly as it was by the full beam.
54. These effects are rendered strikingly visible by darkening with ink the freezing mixture within the flask.
When the hoarfrost is removed, the blackness of the surface from which it has been melted comes out in strong contrast with the adjacent snowy whiteness. When the flask itself, instead of the freezing mixture, is blackened, the purely luminous waves being absorbed by the glass, warm it; the glass reacts upon the frost, and melts it. Hence the wisdom of darkening, instead of the flask itself, the mixture within the flask.
55. This experiment proves to demonstration the statement in paragraph (36) ; that it is the dark-waves of the sun that melt the mountain snow and ice, and originate all the rivers derived from glaciers.
There are writers who seem to regard science as an aggregate of facts, and hence doubt its efficacy as an exercise of the reasoning powers. But all that I have here taught you is the result of reason, taking its stand, however, upon the sure basis of observation and experiment.