The various theories thus reviewed, while not sufficient in themselves to account for the facts of our own solar system, are fatally defective in another respect. While they aim to account for the sun's light and heat, they all fail to consider the active medium of the solar light and heat in the sun itself. It is not simply a highly-heated central mass glowing in space. It is a vast orb surrounded by different envelopes of incandescent vapors or gases, and by far the most vast in volume, as well as in light and heat-radiating power, are the photosphere and its superincumbent chromosphere, composed almost entirely of free hydrogen gas in a state of intense incandescence. Whence comes this enormous mass of hydrogen? And how explain the entire absence of free hydrogen gas from our own atmosphere and its replacement by oxygen? There is a recent theory propounded by Mr. A. Mott, which is set forth in detail in Professor Ball's "In the High Heavens," and which endeavors to account for the remarkable absence of free hydrogen gas from the earth's atmosphere, for, as the author states, "It is a singular fact that hydrogen in the free state is absent from our atmosphere." The theory, in brief, is that the molecules of hydrogen gas have an average speed of about a mile a second,--which, however, is only one-seventh that required to shoot them off into space,--but that these molecules are continually changing their velocity, and may sometimes attain a speed of seven miles a second; the result is that "every now and then a molecule of hydrogen succeeds in bolting away from the earth altogether and escaping into open space." During past ages the molecules of hydrogen would thus have gradually wiggled up through the air, and finally disappeared into outer darkness for good and all; and thus "the fact that there is at present no free hydrogen in the air over our heads may be accounted for." Since the molecules of oxygen have only a velocity of a quarter mile a second, that unfortunate gas remains behind and is consumed.