The only conceivable motion for thus heating the Earth as a whole was the falling together of its parts. The present heat of the Earth, then, accuses the concourse of particles in the past to its formation, or in other words proves that the Earth was evolved out of material originally more sparcely strewn. It does so not only in a generic but in a most particular manner, for the heat is distributed just where it would be by such a process. It is greater to-day within, increasingly, because when the globe began to cool, the surface necessarily cooled first and established a regular gradient of heat from core to cuticle.
It is possible to test this qualitative inference quantitatively and see if the falling together of the meteorites was equal to the task. Knowing the mechanical equivalent of heat, what we do is to calculate the quantity of motion involved and then evaluate it in heat. As we are unaware of the exact law of density of the Earth, and are ignorant of how much was radiated away in the process, the problem is a little like estimating the fortune of a man when we do not know the stocks in which he has invested, and ignore how much he has spent the while. We only know what he would have been worth had he followed our advice in the matter of investments and lived as frugally as we recommended. For here, too, we are obliged to make certain assumptions. Nevertheless the figure obtained in the case of the planets’ stores of heat is so enormous as to leave a most ample margin for dissipation. Had the Earth contracted from a fairly generous expansion to its present state under the probable law of density suggested by Laplace in another connection, the heat developed would have been enough to raise the whole globe to 160,000° F. if of iron, 90,000° F. if of stone. As 10,000° F. would have sufficed for the Earth to have kept up its past, to say nothing of its present, state, we are justified of our deduction.
Nor is the Earth the only body in the system which thus argues itself evolved by the falling together of its present constituents. In the larger planets Jupiter and Saturn we seem to see the heat, far as we are away. For the cherry hue they disclose between their brighter belts proves to come from greater absorption there of the green and blue rays of the spectrum, indicating a greater depth of atmosphere traversed. Thus these parts lie at a lower level, and their ruddy hue is just what they should show were they still glowing with a dull red heat.
Spectrogram of Jupiter, Moon Comparison.
Lowell Observatory. V. M. Slipher.
Heat is not only the end of the beginning, it is the beginning of the end as well. It is both the result of the evolving of definite bodies out of the agglomeration of matter-strewn space, and the cause of the higher evolution of those globes themselves. For the acquisition of heat is the necessary preface to all that follows. Heat is a body’s evolutionary capital whose wise expenditure through cooling down makes all further advance to higher products possible. A body too small to have acquired it must remain forever lifeless, as dead as the meteorites themselves that enter our air as mere inert bits of stone or iron.
Curiously enough, heat both must have been and then must have been lost. Like the loss of fortune or of friends sometimes in the ennobling of character, it is through its passing away that its effects are realized. For in cooling down from a once heated condition, that train of events occurs which we most commonly particularize as evolution. So far in our survey the march of advance has been through masses of matter, a molar evolution; from this point on it passes into its minute constituents and becomes a molecular one. The one is the necessary prelude to the other. Up to this great turning-point in the history of each member of a solar system we have been busied with the acquisition of heat, though we may not have been aware of it the while. All the motions we have studied tended to that end. During these three chapters, I, II, V, we have been gradually rising in our point of view until we stand at the temperature pinnacle of the whole process. In the next three we are to descend upon the other side. The slope we have come up was of necessity barren; the one we are to go down brings us to verdure and the haunts of men. Coming from the causes above, we reach at each step effects more and more related to ourselves which those causes will help us to explain.