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. [15] Geikie, “Geology,” pages 85, 86, and 131-136. 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. [Illustration: 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.