A LITTLE more than ten years ago this remarkable element was only known to astronomers through the medium of the spectroscope. Now it is not only to be found in all laboratories, but appears to occur in almost all constituents of the earth's crust and in amounts proportional to their radio-activity, except in beryl (Strutt). In some cases it occurs in quantities far from minute, as in certain minerals, particularly cleveite and monazite, where the number of litres of gas obtained is comparable with the number of kilograms of mineral treated. Again, it constitutes more than 5 per cent, of the gases evolved from some mineral springs, as at Maizieres, and 1.4 per cent, of the vast supplies of natural gas at Dexter, Kansas, while it occurs everywhere as four parts in a million of the atmosphere (Ramsay). Its mode of occurrence and origin are too complex and still too uncertain to be treated adequately here, but it is apparently not liberated from minerals by grinding alone to an impalpable powder (Moss), and it certainly permeates quartz at temperatures above 220° C., and with a velocity rising with the temperature (Jaquerod and Perrot). Moreover, it appears to be frequently produced in the gradual breaking down of the uranium molecule and the various radio-active transformations of this into radium and other substances. Helium was first known from its yellow line D3, and was first detected on the earth by the same characteristic (Ramsay). In nitrogen or hydrogen it appears that a proportion of 10 per cent, can just be detected by its spectrum (Collie and Ramsay). Very shortly it was shown to be a very light, unreactive gas with monatomic molecules. Hence it was taken to be the lightest known member of the argon group. Later determinations have shown that its density cannot differ much from 2.0 (Onnes), and that the value of the ratio of its specific heats is 1.63 (Geiger), which confirms the earlier results.