The most remarkable part of Dr. Wollaston’s paper is the reductio ad absurdum by which he seeks to finally demonstrate the finite extent of our atmosphere. He maintains, as I do, that if the elasticity of our atmosphere is unlimited, its extension must be commensurate with the universe, that every orb in space will, by gravitation, gather around itself an atmosphere proportionate to its gravitating power, and that, by taking the known quantity of the earth’s atmosphere as our unit, we may calculate the amount of atmosphere possessed by any heavenly body of which the mass is known. On this basis Dr. Wollaston calculates the atmosphere of the sun, and concludes that its extent will be so great as to visibly affect the apparent motions of Mercury and Venus, when their declination makes its nearest approach to that of the sun. No such disturbance being actually observable, he concludes that such an atmosphere as he has calculated cannot exist. In like manner he calculates the atmosphere of Jupiter, and finds it to be so great, that its refraction would be sufficient “to render the fourth satellite visible to us when behind the centre of the planet, and consequently to make it appear on both (or all) sides at the same time.”
On examining these calculations, I have discovered the very curious error above referred to. As this is a matter of figures that cannot be abridged, I must refer the reader to the original calculations. I will here merely state that Wollaston’s method of calculating the solar gravitation atmosphere and that of Jupiter and the moon leads to the monstrous conclusion that, in ascending from the surface of the given orb, we always have the same limited amount of atmospheric matter above as that with which we started, although we are continually leaving a portion of it below.
Wollaston’s mistake is based on the assumption that, under the circumstances supposed, the atmospheric pressure and density, at any given distance from the centre of the given orb, will vary inversely with the square of that distance. As the area of the base upon which such pressure is exerted varies directly with the square of the distance, the total atmosphere above every imaginable starting-distance would thus be ever the same. That this assumption, so utterly at variance with the known laws of atmospheric distribution, should have remained unchallenged for half a century, and that the conclusions based upon it should be accepted by the whole scientific world, and repeated in standard treatises, such as those of the “Encyclopedia Britannica,” etc., etc., is, I think, one of the most remarkable curiosities presented by the history of science. If it were merely a little cobweb in some obscure corner of philosophy, there would be nothing surprising in its escape from the besom of scientific criticism; but this is so far from being the case, that it has hung, since 1822, like a dark veil obscuring another, a wider, and more interesting view of the universe which the idea of an universal atmosphere opens out. But I must now proceed to the next stage of the argument.
Starting from the conclusion reached in the previous chapters, that the atmosphere of our earth is but a portion of an universal elastic medium which it has attached to itself by its gravitation, and that all the other orbs of space must, in like manner, have obtained their proportion, I take the earth’s mass, and its known quantity of atmospheric envelope as units, and calculating by the simple rule I have laid down in opposition to Wollaston’s, I find that the total weight of the sun’s atmosphere should be at least 117,681,623 times that of the earth’s, and the pressure at its base equal, at least, to 15,233 atmospheres. What must be the results of such an atmospheric accumulation?
The experiment of compressing air in the condensing syringe, and thereby lighting a piece of German tinder, is familiar to all who have studied even the rudiments of physical science. Taking the formulæ of Leslie and Dalton, and applying them to the solar pressure of 15,233 atmospheres, we arrive according to Leslie, at the inconceivable temperature of 380,832° C., or 685,529° F., as that due to this amount of compression, or, according to Dalton, at 761,665° F. What will be the effects of such a degree of heat upon materials similar to those of which our earth is composed?
Let us first take the case of water, which, for reasons I have stated, should be regarded as atmospheric, or universally diffused matter.
This brings us to a subject of the highest and widest philosophical and practical importance. I refer to the antagonism between the force of heat and that of chemical combination, to which the French chemists have given the name “dissociation.” Having myself been unable to find any satisfactory English account of this subject at a time when it had already been well treated by French and German authors, in the form of published lectures and cyclopædia articles, I assume that others may have encountered a similar difficulty, and therefore dwell rather more fully upon this part of my present summary.
It appears that all chemical compounds may be decomposed by heat, and that, at a given pressure, there is a definite and special temperature at which the decomposition of each compound is effected. For the absolute and final establishment of the universality of this law further investigations are necessary, actual investigations having established it as far as they have gone, but these have not been exhaustive.
There appears to be a remarkable analogy between dissociation and evaporation. When a liquid is vaporized, a certain amount of heat is “rendered latent,” and this quantity varies with the liquid and with the pressure, but is definite and invariable for each liquid at a given pressure. In like manner, when a compound is dissociated, a certain amount of heat is “rendered latent,” or converted into dissociating force, and this varies with each compound and with the pressure, but is definite and invariable for each compound at a given pressure. Further, when condensation occurs, an amount of heat is evolved, as temperature, exactly equal to that which was rendered latent in the evaporation of the same substance under the same pressure; and, in like manner, when chemical re-combination of dissociated elements occurs, an amount of heat is evolved, as temperature, exactly equal to that which disappeared when the compound was dissociated by heat alone under the same pressure.
According to the recently adopted figures of M. Deville, the temperature at which the vapor of water becomes dissociated under ordinary atmospheric pressure is 2800° C., and the, quantity of heat which disappears, as temperature, in the course of dissociation is 2153 calorics, i.e., sufficient to raise 2153 times its own weight of liquid water 1° C.; but, as the specific heat of aqueous vapor is to that of liquid water as 0·475 to 1, that latent heat expressed in the temperature it would have given to aqueous vapor is = 4532° C., or 8158° F.
In order to render the analogy between the ebullition and dissociation of water more evident and intelligible, I will state it as follows:—
| To commence the ebullition of water under ordinary pressure, a temperature of 100° C., or 212° F., must be attained. | To commence the dissociation of aqueous vapor under ordinary pressures, a temperature of 2800° C., or 5072° F., must be attained. |
| To complete the ebullition of a given quantity of water, an amount of heat must be applied, sufficient to have raised the water 537° C., or 968° F., above its boiling-point, had it not evaporated. | To complete the dissociation of a given quantity of aqueous vapor, an amount of heat must be applied sufficient to have raised the vapor 4532° C., or 8158° F., above its dissociation-point had it not decomposed. |
| In order that a given quantity of vapor of water shall condense, it must give off sufficient heat to raise its own weight of water 537° C., or 968° F. | In order that a given quantity of the elements of water may combine, they must give off sufficient heat to raise their own weight of aqueous vapor 4532° C., or 8158° F. |
I have expressed these generalizations and analogies rather more definitely than they have been hitherto stated, but those who are acquainted with the researches of Deville, Cailletet, Bunsen, etc., will perceive that I am justified in doing so.2
With the general laws of the dissociation of water thus before us, we may follow out the necessary action of the