Yet that elementary substances once existed here we have evidence. We find such in volcanic vents. That the Earth was once as hot on its surface as it now is underneath, we know from the condition of the plutonic rocks where sedimentary strata have not covered them up. Volcanoes and geysers are our only avenues now to that earlier state of things. From these pathways to the past, and only from them, do we find elementary substances produced to-day,—hydrogen, sulphur, chlorine, oxygen, and carbon.[15] We are thus made aware that once the Earth was simple, too, on the surface as well as deeper down. A side-light, this, to what we knew must have been the case.
From its primordial state, the least complex compounds were evolved first. As the heat lessened, higher and higher combinations became possible. And this is why the more complex molecules are so unstable, the organic ones the most. Since they are not possible at all under much stir of their atomic constituents, it shows that the bond between them must be feeble—and, therefore, easily broken by other causes besides heat. To the instability of the organic molecule is due its power; and to cooling, the possibility of its expression.
Lowell Observatory Spectrogram showing
water-vapor in the atmosphere of Mars,
January 1908.—V. M. Slipher.
For the steps in the chemical process from Sun to habitable Earth we must look to the spectroscope; not in its older field, the blue end of the spectrum, but in that which is unfolding to our view in Dr. Slipher’s ingenious hands, the extension of the observable part of it into the red. For at that end lie the bands due to planetary absorption. Here we have already secured surprising results as to the atmospheres of the various planets. We have not only found positive evidence of water-vapor in the atmosphere of Mars, but we have detected strange envelopes in the major planets which show a constitution different from that of the Sun on the one hand, and of the Earth on the other. That size and position are for much in these peculiarities, I have already shown you; but something, too, is to be laid at the door of age. The major planets are not so advanced in their planetary history as is our Earth; and Dr. Slipher’s spectrograms of them disclose what is now going on in that prefatory, childish stage.
These spectrograms are full of possibilities, and it is not too much to say that chemistry may yet be greatly indebted to the stars. Compounds, the strange unknown substances there revealed by their spectral lines, may be cryptic as yet to us. Some of the elements missing in Mendeléeff’s table may be there, too. Helium was first found in the Sun; coronium still awaits detection elsewhere. So with these spectral lines of the outer planets. It looks as if chemistry had been a thought too previous in making free for others with what should have been their names, Zenon and Uranium. For we may yet have to speak of Dion and Varunium.
From the chemical aspect of evolution we pass to its physical side; from the indirectly to the directly visible results. Here again, to learn what happened after the sunlike stage, we must turn to the major planets. For the cooling which induced both physical and chemical change has there progressed less far, inasmuch as a large globe takes longer to cool than a small one. To the largest planets, then, we should look for types of the early planetary stages to-day.
Almost as soon as the telescope was directed to Jupiter, among the details it disclosed were the Jovian belts (in the year 1630), dark streaks ruling the planet’s disk parallel to its equator. They are of the first objects advertised as visible in small glasses to-day, vying with the craters in the Moon as purchasable wonders of the sky. As the belts were better and better seen, features came out in them which proved more and more interesting. Cassini, in 1692, noticed that the markings travelled round Jupiter and those nearest his equator the quickest. Sir William Herschel thought them due to Jovian trade-winds, the planet’s swift rotation making up for deficiency of sun; why, does not appear.
Modern study of the planet shows that the bright longitudinal layers between the dark belts are unquestionably belts of cloud. Their behavior indicates this, and their intrinsic brightness bears it out. For they are of almost exactly that albedo. Whether they are the kind of cloud with which we are familiar, clouds of water-vapor, we are not yet sure. But whatever their constitution, their conduct is quite other than is exhibited by our own.