It may be supposed, for instance, that helium and neon, of which the molecular mass is very slight, were formerly more abundant on our planet; but at an epoch when the temperature of the globe was higher, the very speed of their molecules may have reached a considerable value, exceeding, for instance, eleven kilometres per second, which suffices to explain why they should have left our atmosphere.

Crypton and neon, which have a density four times greater than oxygen, may, on the contrary, have partly disappeared by solution at the bottom of the sea, where it is not absurd to suppose that considerable quantities would be found liquefied at great depths.[10] [Footnote 10: Another view, viz.

that these inert gases are a kind of waste product of radioactive changes, is also gaining ground.

The discovery of the radioactive mineral malacone, which gives off both helium and argon, goes to support this.

See Messrs Ketchin and Winterson's paper on the subject at the Chemical Society, 18th October 1906 .-- ED.] It is probable, moreover, that the higher regions of the atmosphere are not composed of the same air as that around us.