f supplying), and they even seem to have no
chemical energy. Everything leads to the belief that they show the
existence on the earth of an earlier state of things now vanished. 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. Sir James Dewar
points out that Dalton's law demands that every gas composing the
atmosphere should have, at all heights and temperatures, the same
pressure as if it were alone, the pressure decreasing the less
quickly, all things being equal, as its density becomes less. It
results from this that the temperature becoming gradually lower as we
rise in the atmosphere, at a certain altitude there can no longer
remain any traces of oxygen or nitrogen, which no doubt liquefy, and
the atmosphere must be almost exclusively composed of the most
volatile gases, including hydrogen, which M.A. Gautier has, like Lord
Rayleigh and Sir William Ramsay, proved to exist in the air. The
spectrum of the _Aurora borealis_, in which are found the lines of
those parts of the atmosphere which cannot be liquefied in liquid
hydrogen, together with the lines of argon, crypton, and xenon, is
quite in conformity with this point of view. It is, however, singular
that it should be the spectrum of crypton, that is to say, of the
heaviest gas of the group, which appears most clearly in the upper
regions of the atmosphere.

Among the gases most difficult to