18 president's address. 



Without going much into detail, it will suffice to state that 

 difterent observers have determined with some degree of accuracy 

 the proportion of ra(iium contained in various representative 

 rocks and minerals. The Hon. R. J. Strutt in particular has 

 devoted much attention to this investigation and has found that 

 the proportion varies greatly in different rocks. Acidic rocks 

 such as granite are on the whole richest in radium, while basic 

 ones such as basalt contain least. Mineials rich in uranium and 

 certain other rare elements contain relatively large amounts of 

 radium, the latter bearing in all cases a definite relationship to 

 the uranium present; but these minerals are sparsely distributed 

 and exist in insufficient quantities to materiall}^ attect the average 

 composition of the earth's crust as regards radium content. Full 

 details of Mr. Strutt's work on this subject were brought before 

 the Hoj^al Societ}' of London in April, 190G.* The following 

 figures express the proportion of radium existing in a few of the 

 representative rucks examined by Mr. Strutt: — 



It is not easy to realise from figures such as the above how 

 minute are the quantities of radium involved; perhaps this may 

 be better done if we consider the largest of these amounts in 

 another way. 9 56 x 10"^' gram radium per gram rock is equi- 

 valent to 9-56 parts in one billion, or about 1 grain in 6000 tons. 

 It is one of the most remarkable features of modern physical 

 methods that it should be possible to estimate such excessively 



* See Chemical News xciii., 235 & 247, 25 May and 1 June, 1906. 



