Page:Proceedings of the Royal Society of London Vol 60.djvu/71

56 27. A mixture of soda-lime and. sulphur consisting of polysulphides causes no change of volume in a current of helium passed over it at a bright red heat.

28. Induction sparks in an ozone apparatus passed through a mixture of helium with benzene vapour in presence of liquid benzene for many hours, gave no change of volume. The benzene was, of course, altered, but the sum of the pressures of the helium and the benzene-vapour remained as at first. Had helium been removed, contraction would have occurred.

This ends the catalogue of negative experiments. Any compound of helium capable of existence will probably be endothermic, and the two methods of producing endothermic compounds, where no simultaneous exothermic reaction is possible, are exposure to a high temperature, at which endothermic compounds show greater stability, and the influence of the silent electric discharge. These methods have been tried, so far in vain. There is, therefore, every reason to believe that the elements, helium and argon, are non-valent, that is, are incapable of forming compounds.

The. presence of helium in the residue after removal of nitrogen from this gas was proved in a former paper,f but there was some doubt as to the relative proportions of argon and helium. A fresh sample, kindly collected by Dr. Richardson, has therefore been examined. Of this 2500 c.c., submitted to electric sparks in presence of oxygen, gave a final residue of 37 c.c., after removal of all gases known until recently. The spectrum of the residue, observed at atmospheric pressure, showed argon, and the D3 line of helium very plainly.

The easy visibility of D3 suggested the presence of helium in some such proportion as 10 per cent., and this conjecture has been confirmed by a determination of the refractivity of the mixture. It may be remembered that while the refractivity of argon approaches closely that of air, the relative number being 0'961, the refractivity of helium (as supplied to me by Professor Ramsay) is very low, being only O'146 on the same scale. If we assume that any sample

the Bath waters, which, according to some antiquaries, were called by the Romans Aquce Solis.
 * I am reminded by Mr. Whitaker that helium is appropriately associated with

t ‘Roy. Soc. Froc.,’ vol. 59, p. 206, 1896.