In one particular especially does this difference of quality show itself; besides being non-luminous, the heat radiated from the earth is more easily intercepted and absorbed by almost all transparent substances. A vast portion of the sun's rays, for example, can pa.s.s instantaneously through a thick sheet of water; gunpowder could easily be fired by the heat of the sun's rays converged by pa.s.sing through a thick water lens; the drops upon leaves in greenhouses often act as lenses, and cause the sun to burn the leaves upon which they rest. But with regard to the rays of heat emanating from an obscure source, they are all absorbed by a layer of water less than the 20th of an inch in thickness: water is opaque to such rays, and cuts them off almost as effectually as a metallic screen. The same is true of other liquids, and also of many transparent solids.

[Sidenote: THE ATMOSPHERE LIKE A RATCHET.]

a.s.suming the same to be true of gaseous bodies, that they also intercept the obscure rays much more readily than the luminous ones, it would follow that while the sun's rays penetrate our atmosphere with freedom, the change which they undergo in warming the earth deprives them in a measure of this penetrating power. They can reach the earth, but _they cannot get back_; thus the atmosphere acts the part of a ratchet-wheel in mechanics; it allows of motion in one direction, but prevents it in the other.

De Saussure, Fourier, M. Pouillet, and Mr. Hopkins have developed this speculation, and drawn from it consequences of the utmost importance; but it nevertheless rested upon a basis of conjecture. Indeed some of the eminent men above-named deemed its truth beyond the possibility of experimental verification. Melloni showed that for a distance of 18 or 20 feet the absorption of obscure rays by the atmosphere was absolutely inappreciable. Hence, the _total_ absorption being so small as to elude even Melloni's delicate tests, it was reasonable to infer that _differences_ of absorption, if such existed at all, must be far beyond the reach of the finest means which we could apply to detect them.

[Sidenote: DIFFERENCES OF ABSORPTION BY GASES.]

This exclusion of one of the three states of material aggregation from the region of experiment was, however, by no means satisfactory; for our right to infer, from the deportment of a solid or a liquid towards radiant heat, the deportment of a gas, is by no means evident. In both liquids and solids we have the molecules closely packed, and more or less chained by the force of cohesion; in gases, on the contrary, they are perfectly free, and widely separated. How do we know that the interception of radiant heat by liquids and solids may not be due to an arrangement and comparative rigidity of their parts, which gases do not at all share? The a.s.sumption which took no note of such a possibility seemed very insecure, and called for verification.

My interest in this question was augmented by the fact, that the a.s.sumption referred to lies, as will be seen, at the root of the glacier question. I therefore endeavoured to fill the gap, and to do for gases and vapours what had been already so ably done for liquids and solids by Melloni. I tried the methods heretofore pursued, and found them unavailing; oxygen, hydrogen, nitrogen, and atmospheric air, examined by such methods, showed no action upon radiant heat. Nature was dumb, but the question occurred, "Had she been addressed in the proper language?"

If the experimentalist is convinced of this, he will rest content even with a negative; but the absence of this conviction is always a source of discomfort, and a stimulus to try again.

The principle of the method finally applied is all that can here be referred to; and it, I hope, will be quite intelligible. Two beams of heat, from two distinct sources, were allowed to fall upon the same instrument,[A] and to contend there for mastery. When both beams were perfectly equal, they completely neutralized each other's action; but when one of them was in any sensible degree stronger than the other, the predominance of the former was shown by the instrument. It was so arranged that one of the conflicting beams pa.s.sed through a tube which could be exhausted of air, or filled with any gas; thus varying at pleasure the medium through which it pa.s.sed. The question then was, supposing the two beams to be equal when the tube was filled with air, will the exhausting of the tube disturb the equality? The answer was affirmative; the instrument at once showed that a greater quant.i.ty of heat pa.s.sed through the vacuum than through the air.

The experiment was so arranged that the effect thus produced was very large as measured by the indications of the instrument. But the action of the simple gases, oxygen, hydrogen, and nitrogen, was incomparably less than that produced by some of the compound gases, while these latter again differed widely from each other. Vapours exhibited differences of equal magnitude. The experiments indeed proved that gaseous bodies varied among themselves, as to their power of transmitting radiant heat, just as much as liquids and solids. It was in the highest degree interesting to observe how a gas or vapour of perfect transparency, as regards light, acted like an opaque screen upon the heat. To the eye, the gas within the tube might be as invisible as the air itself, while to the radiant heat it behaved like a cloud which it was almost impossible to penetrate.

[Sidenote: SELECTED HEAT.]

Applying the same method, I have found that from the sun, from the electric light, or from the lime-light, a large amount of heat can be selected, which is unaffected not only by air, but by the most energetic gases that experiment has revealed to me; while this same heat, when it has its _quality_ changed by being rendered obscure, is powerfully intercepted. Thus the bold and beautiful speculation above referred to has been made an experimental fact; the radiant heat of the sun does certainly pa.s.s through the atmosphere to the earth with greater facility than the radiant heat of the earth can escape into s.p.a.ce.

[Sidenote: POSSIBLE HEAT OF NEPTUNE.]

It is probable that, were the earth unfurnished with this atmospheric swathing, its conditions of temperature would be such as to render it uninhabitable by man; and it is also probable that a suitable atmosphere enveloping the most distant planet might render it, as regards temperature, perfectly habitable. If the planet Neptune, for example, be surrounded by an atmosphere which permits the solar and stellar rays to pa.s.s towards the planet, but cuts off the escape of the warmth which they excite, it is easy to see that such an acc.u.mulation of heat may at length take place as to render the planet a comfortable habitation for beings const.i.tuted like ourselves.[B]

But let us not wander too far from our own concerns. Where radiant heat is allowed to fall upon an absorbing substance, a certain thickness of the latter is always necessary for the absorption. Supposing we place a thin film of gla.s.s before a source of heat, a certain percentage of the heat will pa.s.s through the gla.s.s, and the remainder will be absorbed.

Let the transmitted portion fall upon a second film similar to the first, a smaller percentage than before will be absorbed. A third plate would absorb still less, a fourth still less; and, after having pa.s.sed through a sufficient number of layers, the heat would be so _sifted_ that all the rays capable of being absorbed by gla.s.s would be abstracted from it. Suppose all these films to be placed together so as to form a single thick plate of gla.s.s, it is evident that the plate must act upon the heat which falls upon it, in such a manner that the major portion is absorbed _near the surface at which the heat enters_. This has been completely verified by experiment.

[Sidenote: COLD OF UPPER ATMOSPHERE.]

Applying this to the heat radiated from the earth, it is manifest that the greatest quant.i.ty of this heat will be absorbed by the lowest atmospheric strata. And here we find ourselves brought, by considerations apparently remote, face to face with the fact upon which the existence of all glaciers depends, namely, the comparative coldness of the upper regions of the atmosphere. The sun's rays can pa.s.s in a great measure through these regions without heating them; and the earth's rays, which they might absorb, hardly reach them at all, but are intercepted by the lower portions of the atmosphere.[C]

Another cause of the greater coldness of the higher atmosphere is the expansion of the denser air of the lower strata when it ascends. The dense air makes room for itself by pushing back the lighter and less elastic air which surrounds it: _it does work_, and, to perform this work, a certain amount of heat must be consumed. It is the consumption of this heat--its absolute annihilation as heat--that chills the expanded air, and to this action a share of the coldness of the higher atmosphere must undoubtedly be ascribed. A third cause of the difference of temperature is the large amount of heat communicated, _by way of contact_, to the air of the earth's surface; and a fourth and final cause is the loss endured by the highest strata through radiation into s.p.a.ce.

FOOTNOTES:

[A] The opposite faces of a thermo-electric pile.

[B] See a most interesting paper on this subject by Mr. Hopkins in the Cambridge 'Transactions,' May, 1856.

[C] See M. Pouillet's important Memoir on Solar Radiation. Taylor's Scientific Memoirs, vol. iv. p. 44.

ORIGIN OF GLACIERS.

(4.)

[Sidenote: THE SNOW-LINE.]

Having thus accounted for the greater cold of the higher atmospheric regions, its consequences are next to be considered. One of these is, that clouds formed in the lower portions of the atmosphere, in warm and temperate lat.i.tudes, usually discharge themselves upon the earth as rain; while those formed in the higher regions discharge themselves upon the mountains as snow. The snow of the higher atmosphere is often melted to rain in pa.s.sing through the warmer lower strata: nothing indeed is more common than to pa.s.s, in descending a mountain, from snow to rain; and I have already referred to a case of this kind. The appearance of the gra.s.sy and pine-clad alps, as seen from the valleys after a wet night, is often strikingly beautiful; the level at which the snow turned to rain being distinctly marked upon the slopes. Above this level the mountains are white, while below it they are green. The eye follows this _snow-line_ with ease along the mountains, and when a sufficient extent of country is commanded its regularity is surprising.

The term "snow-line," however, which has been here applied to a local and temporary phenomenon, is commonly understood to mean something else.

In the case just referred to it marked the place where the supply of solid matter from the upper atmospheric regions, during a single fall, was exactly equal to its consumption; but the term is usually understood to mean the line along which the quant.i.ty of snow which falls _annually_ is melted, and no more. Below this line each year's snow is completely cleared away by the summer heat; above it a residual layer abides, which gradually augments in thickness from the snow-line upwards.

[Sidenote: MOUNTAINS UNLOADED BY GLACIERS.]

Here then we have a fresh layer laid on every year; and it is evident that, if this process continued without interruption, every mountain which rises above the snow-line must augment annually in height; the waters of the sea thus piled, in a solid form, upon the summits of the hills, would raise the latter to an indefinite elevation. But, as might be expected, the snow upon steep mountain-sides frequently slips and rolls down in avalanches into warmer regions, where it is reduced to water. A comparatively small quant.i.ty of the snow is, however, thus got rid of, and the great agent which Nature employs to relieve her overladen mountains is the glaciers.

Let us here avoid an error which may readily arise out of the foregoing reflections. The princ.i.p.al region of clouds and rain and snow extends only to a limited distance upwards in the atmosphere; the highest regions contain very little moisture, and were our mountains sufficiently lofty to penetrate those regions, the quant.i.ty of snow falling upon their summits would be too trifling to resist the direct action of the solar rays. These would annually clear the summits to a certain level, and hence, were our mountains high enough, we should have a superior, as well as an inferior, snow-line; the region of perpetual snow would form a belt, below which, in summer, snowless valleys and plains would extend, and above which snowless summits would rise.

(5.)

[Sidenote: WHITE AND BLUE ICE.]

At its origin then a glacier is snow--at its lower extremity it is ice.

The blue blocks that arch the source of the Arveiron were once powdery snow upon the slopes of the Col du Geant. Could our vision penetrate into the body of the glacier, we should find that the change from white to blue essentially consists in the gradual expulsion of the air which was originally entangled in the meshes of the fallen snow. Whiteness always results from the intimate and irregular mixture of air and a transparent solid; a crushed diamond would resemble snow; if we pound the most transparent rock-salt into powder we have a substance as white as the whitest culinary salt; and the colourless gla.s.s vessel which holds the salt would also, if pounded, give a powder as white as the salt itself. It is a law of light that in pa.s.sing from one substance to another possessing a different power of refraction, a portion of it is always reflected. Hence when light falls upon a transparent solid mixed with air, at each pa.s.sage of the light from the air to the solid and from the solid to the air a portion of it is reflected; and, in the case of a powder, this reflection occurs so frequently that the pa.s.sage of the light is practically cut off. Thus, from the mixture of two perfectly transparent substances, we obtain an opaque one; from the intimate mixture of air and water we obtain foam; clouds owe their opacity to the same principle; and the condensed steam of a locomotive casts a shadow upon the fields adjacent to the line, because the sunlight is wasted in echoes at the innumerable limiting surfaces of water and air.

[Sidenote: AIR-BUBBLES IN ICE.]

The snow which falls upon high mountain-eminences has often a temperature far below the freezing point of water. Such snow is _dry_, and if it always continued so the formation of a glacier from it would be impossible. The first action of the summer's sun is to raise the temperature of the superficial snow to 32, and afterwards to melt it.

The water thus formed percolates through the colder ma.s.s underneath, and this I take to be the first active agency in expelling the air entangled in the snow. But as the liquid trickles over the surfaces of granules colder than itself it is partially deposited in a solid form on these surfaces, thus augmenting the size of the granules, and cementing them together. When the ma.s.s thus formed is examined, the air within it is found as _round bubbles_. Now it is manifest that the air caught in the irregular interstices of the snow can have no tendency to a.s.sume this form so long as the snow remains solid; but the process to which I have referred--the saturation of the lower portions of the snow by the water produced by the melting of the superficial portions--enables the air to form itself into globules, and to give the ice of the _neve_ its peculiar character. Thus we see that, though the sun cannot get directly at the deeper portions of the snow, by liquefying the upper layer he charges it with heat, and makes it his messenger to the cold subjacent ma.s.s.

The frost of the succeeding winter may, I think, or may not, according to circ.u.mstances, penetrate through this layer, and solidify the water which it still retains in its interstices. If the winter set in with clear frosty weather, the penetration will probably take place; but if heavy snow occur at the commencement of winter, thus throwing a protective covering over the _neve_, freezing to any great depth may be prevented. Mr. Huxley's idea seems to be quite within the range of possibility, that water-cells may be transmitted from the origin of the glacier to its end, retaining their contents always liquid.

[Sidenote: SNOW PRESSED TO ICE.]

It was formerly supposed, and is perhaps still supposed by many, that the snow of the mountains is converted into the ice of the glacier by the process of saturation and freezing just indicated. But the frozen layer would not yet resemble glacier ice; it is only at the deeper portions of the _neve_ that we find an approximation to the true ice of the glacier. This brings us to the second great agent in the process of glacification, namely, pressure. The ice of the _neve_ at 32 may be squeezed or crushed with extreme facility; and if the force be applied slowly and with caution, the yielding of the ma.s.s may be made to resemble the yielding of a plastic body. In the depths of the _neve_, where each portion of the ice is surrounded by a resistant ma.s.s, rude crushing is of course out of the question. The layers underneath yield with extreme slowness to the pressure of the ma.s.s above them; they are squeezed, but not rudely fractured; and even should rude fracture occur, the ice, as shall subsequently be shown, possesses the power of restoring its own continuity. Thus, then, the lower portions of the _neve_ are removed by pressure more and more from the condition of snow, the air-bubbles which give to the _neve_-ice its whiteness are more and more expelled, and this process, continued throughout the entire glacier, finally brings the ice to that state of magnificent transparency which we find at the termination of the glacier of Rosenlaui and elsewhere. This is all capable of experimental proof. The Messrs. Schlagintweit compressed the snow of the _neve_ to compact ice; and I have myself frequently obtained slabs of ice from snow in London.

COLOUR OF WATER AND ICE.

(6.)

The sun is continually sending forth waves of different lengths, all of which travel with the same velocity through the ether. When these waves enter a prism of gla.s.s they are r.e.t.a.r.ded, but in different degrees. The shorter waves suffer the greatest r.e.t.a.r.dation, and in consequence of this are most deflected from their straight course. It is this property which enables us to separate one from the other in the solar spectrum, and this separation proves that the waves are by no means inextricably entangled with each other, but that they travel independently through s.p.a.ce.

In consequence of this independence, the same body may intercept one system of waves while it allows another to pa.s.s: on this quality, indeed, depend all the phenomena of colour. A red gla.s.s, for example, is red because it is so const.i.tuted that it destroys the shorter waves which produce the other colours, and transmits only the waves which produce red. I may remark, however, that scarcely any gla.s.s is of a pure colour; along with the predominant waves, some of the other waves are permitted to pa.s.s. The colours of flowers are also very impure; in fact, to get pure colours we must resort to a delicate prismatic a.n.a.lysis of white light.