152 



The Hon. Robert John Strutt 



[March 27, 



for helium in the naturally occurring ores of all the elements, 

 common and rare. This had indeed been done, to some extent, from 

 quite a different point of view, by Sir William Ramsay and his co- 

 adjutors, in their first investigations on hehum ; but their observa- 

 tions were directed to finding a practical source of the gas, and were 

 not carried out with anything approaching the minuteness required 

 for the present purpose. 



The upshot has been to prove the presence of helium in almost 

 every mineral examined, and even in such unpromising materials as 

 rock crystal, or common quartz sand. The quantity found in the 

 various cases has varied very widely. In fact, minerals may be found 

 having any hehum content, from thorianite, which contains 10 cubic 

 centimetres per gram, down to rock crystal, which contains about a 

 ten-millionth part of that quantity. 



I have here a small tube of helium obtained from clear, colourless 

 rock crystal, and you will have no difficulty in seeing the characteristic 

 yellow glow as before. 



Are we to regard the helium in common minerals as due to a 

 feeble radio-activity of the common elements ? No doubt such an 

 hypothesis is tempting, but it must be rejected. Radium is present 

 everywhere in traces, and these traces are in general sufficient to 

 account for the minute quantities of helium. This is illustrated in 

 the table below, which gives in round numbers the actual amount of 

 helium extracted from various minerals by heat, and the amount of 

 helium reckoned relatively to the radium. 



Mineral. 

 -^ [Samarskite 

 S 1 Haematite 

 o 1 Galena 

 ^ i, Quartz 



g 



Beryl 



There is reason to think, as already mentioned, that the presence 

 of thorium would constitute another source of helium. But it is 

 believed that this complication does not produce any appreciable 

 effect in these cases. You will see that minerals like quartz, though 

 they contain actually only an infinitesimal quantity of either substance, 

 still show about the same proportion of helium to radium as the 

 minerals which are rich in both. We may conclude that helium is 

 connected with radium in the poor minerals as in the rich ones. 



I have, however, encountered an interesting exception to this 

 rule in the mineral beryl. Beryl is, in all essentials, the same as 

 emerald : the latter name is kept for stones which are of a clear 



