Part 18 (1/2)
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 pus.h.i.+ng 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.