s the densities of argon and helium are very
different (2 and 20), the helium should have been visible in the
first portion which diffused. There was no trace to be detected.
But, further, they submitted this portion of argon to a discharge of
electricity for several hours in a vacuum-tube provided with platinum
electrodes. This process, as they proved, carries out the helium
with the platinum which is splashed on to the sides of the tube. The
argon was then removed by pumping it off in the cold. On heating the
splashed-off platinum, no helium spectrum could be observed. In a
similar experiment, in which a very minute trace of helium had been
added to the argon, there was no difficulty in detecting the helium on
heating the tube. The conclusion therefore follows that no helium is
present in our atmosphere.

It is, besides, exceedingly improbable that helium should be present.
For, as shown by Dr. Johnstone Stoney, the rate of motion of a molecule
of hydrogen is so rapid that, on finding its way to the confines of the
atmosphere, it would escape, and travel through space until it found a
planet of sufficient attractive force to hold it. The same is true of
helium. Were helium present in the atmosphere, it would ultimately
leave us for the sun, or for some planet of much greater mass than the
earth. This conception of Dr. Stoney’s tallies with the observation
that the moon, a planet of small mass, is devoid of an atmosphere, and
that the sun, a body with a mass 300,000 times as great as the earth,
shows in its chromosphere the spectra both of hydrogen and of helium
with great brilliancy.

It is now of interest to inquire what are the properties of argon and
how it is related to other elements.

[25] _Proc. Roy. Soc._ vol. liii. p. 134.

[26] _Proc. Roy. Soc._ vol. lix. p. 68.
     _Phil. Trans._ vol. clxxxvi. p. 227.

[27] _Compt. rend._ vol. cxxi. p. 394.




CHAPTER VI

THE PROPERTIES OF ARGON


The density of a gas is one of its most characteristic and important
properties. Avogadro’s law, which postulates t