HELIUM PRODUCED BY RADIUM 99 



remarkable character favoured helium. Thus helium, 

 though it forms no compounds, is found in minerals 

 containing- uranium and thorium, only in the minerals 

 containing uranium and thorium, and always in them. 

 Might not this helium be the a-particles fired off from 

 the uranium and thorium in the mineral, and, unable 

 to escape from the glassy minerals, accumulating in 

 the material over long periods of geological time, 

 until its presence was obvious and striking even to 

 the relatively rough tests of chemistry and the 

 spectroscope? Naturally, if one could only get 

 enough radium the point might be tested directly, for 

 the spectroscopic test for helium is very sensitive, a 

 bubble of the gas, y^Virth of a cubic millimetre in 

 volume, that is, yoVrrth part of a large pin's head, 

 being sufficient to give the characteristic spectrum. 

 This was in 1903, at the time when pure radium 

 compounds were being put on the market for the first 

 time by the enterprise of the German technical 

 chemist, Dr Giesel. The first thing done with it in 

 the late Sir William Ramsay's laboratory in London 

 was to see whether helium was being generated by it 

 continuously, as should be the case if the a-particles 

 were really positively charged atoms of helium. A 

 few milligrams of radium only was available, but it 

 proved sufficient, and the growth of helium from 

 radium was established by the spectroscope by the 

 aid of the beautiful methods of manipulation of gases, 

 devised by Sir William in the course of his investiga- 

 tions on the rare gases of the atmosphere. Later, 

 the writer established the continuous production of 

 helium from uranium and thorium, though here, from 

 a ton of either element in a year, the quantity of 

 helium produced is only -swsih of a milligram by 

 weight a quantity unweighable on the most sensi- 

 tive chemical balance or n cubic millimetres by 

 volume. Helium has also been detected as a 



