Experiments on the chemical activity of helium

Although helium is chemically inert under ordinary conditions, evidence obtained in recent years suggests the possibility that under special conditions it may exhibit chemical activity. Particularly under the influence of the electric discharge, helium atoms may be brought into a special state in which they possess the power of uniting with other elements to form compounds either of a fugitive or permanent character. At the suggestion of Prof. Sir E. Rutherford, a search has been made to find whether evidence could be obtained of the formation of such compounds under the action of intense electronic bombardment. It may be stated at this point that strong evidence has been obtained which leads to the belief that under certain conditions helium forms a tungsten helide of the formula WHe2 , a black solid at ordinary temperatures which decomposes at high temperatures. Some evidence has also been obtained that the vapours of mercury, iodine, phosphorus and sulphur unite with helium, forming substances which are stableat liquid-air temperatures and decompose at slightly higher temperatures. Before discussing the actual experiments, it is desirable to give a brief account of previous observations on the problem. Attention has been drawn on many occasions to peculiarities in the behaviour and occurrence of helium. It is known that helium occurs in small amount in widely different substances, and in many cases this is accounted for by the presence of radioactive matter. Strutt (1), however, has drawn attention to a very remarkable case, in which helium is occluded in very considerable amounts in beryl, where no appreciable radioactive matter is to be found. Many years ago Berthelot (2) believed he had obtained helium in the combined state by subjecting mixtures of helium and the vapours of carbon disulphide or benzene to the silent electric discharge. Strutt (3) investigated the point, and on repeating Berthelot’s work failed to confirm his results. Special reference should be made to the absorption of helium in discharge tubes (4) by the electrodes, the sputtered metal and the glass. It is generally agreed that the helium is not chemically retained, but that the phenomena are due to occlusion. In some instances (5) helium has been found to appear in a discharge tube after running it for some time, and its origin is not yet clear.