Note 16 — Chapter VIII · THE MOON, CHILD OF THE EARTH AND THE SUN

The discovery of free hydrogen in the earth's atmosphere, by Professor Dewar, 1901, bears upon the theory of the escape of gases from a planet, and may modify the view above expressed. Since hydrogen is theoretically incapable of being permanently retained in the free state by the earth, its presence in the atmosphere indicates either that there is an influx from space or that it emanates from the earth's crust. In a similar way it may be assumed that atmospheric gases can be given off from the crust of the moon, thus, to a greater or less extent, supplying the place of the molecules that escape.