Which gases should we expect to find in the atmosphere of these planets? According to the Kant-Laplace hypothesis, a theory generally credited with a sound kernel, the planets were segregated from the Sun’s substance at a time when the latter was expanded so as to include the orbits of these planets and beyond. Naturally, therefore, their atmospheres would originally be composed of the very gases that formed the outmost part of the Sun’s atmosphere, notably hydrogen. Slipher, who has photographed the spectra here reproduced of the outer planets, believes that certain strong absorption bands in the spectra of Neptune and Uranus correspond to the distinctive F and C lines of hydrogen, using Fraunhofer’s denotation (see Fig. 13). But because the bands in question, as shown on the figure, are very broad it is difficult to identify them with certainty. Also other gases of unknown origin enter into the vapour envelopes outside of the clouds and cause, as apparent from their spectra, a strong absorption of the sunlight reflected from the clouds below. The absorption increases with the planet’s distance from the Sun; thus, it is most pronounced on Neptune and least on Jupiter. [Illustration: Fig. 12. The appearance of Saturn September 30, 1909, according to F. le Coultre of Geneva.] At any rate, the gas appendages of the heavenly bodies just considered differ in one essential respect from the atmospheres of the inner planets: Mars, Earth, Venus, and Mercury. On the Sun and on the outer planets the atmosphere gradually merges into the interior gas-masses so that no distinct boundary can be found between the rarer and the denser layers. Widely different conditions obtain on the Earth. Here the range of air is sharply defined below by the Earth’s solid crust or by the oceans. In such case alone may we speak of an atmosphere proper, of the kind that enters into our commonplace conceptions. Similar are the conditions on all stellar bodies with a solid or a liquid surface. [Illustration: Fig. 13. Spectra of the major planets, compared with that of the moon. The latter corresponds to the spectrum of sunlight reflected from a planet which lacks a light-absorbing atmosphere. Photos by V. M. Slipher of the Lowell Observatory. ]