﻿Average Thorium Content of the Earth's Crust. 483 



42. A third method of identification will be to take 



(a> x , coy, ok) = (a, /3, y), (f ' 5 Vo', ^')=(f,g,h) and f ', 77', £•) = (/ , # , A ) 

 giving ^ : Mj ^7-0 



as in the electronic theory, A = /o, while all other equations 

 are practically the same as before. 



43. Now electrical experiments lead to the conclusion that//, 

 is very nearly constant in a dielectric, but that k varies, while 

 optical experiments make the constancy of n and variability 

 of p probable, so that the second mode of representation- 

 would appear at first sight to be more in accordance with 

 facts. It seems, however, that except in so far as the identi- 

 fication of the constants is concerned, the question of inter- 

 action of matter and aether is resolvable on any of these 



hypotheses, for we may simply have - = y— —V 2 , where V 



is the velocity of propagation, although it may not be possible 

 to identify separately the various quantities, that enter in 

 these investigations, according to any of the schemes tabu- 

 lated above. On this understanding, it does not seem to be 

 possible to pronounce in favour of any of these in preference 

 to the others. In spite of the uncertainty that exists in this 

 respect, these various modes of representation are useful, as 

 we have so little knowledge of the intimate nature of electric 

 and magnetic quantities. 



LII. The Average Thorium Content of the Earth's ("rust. 

 By J. H. J. Poole'*. 



THIS series of experiments was undertaken with the view 

 of supplementing a previous paper of Dr. Joly's on the 

 " Radioactivity of Terrestrial Surface Materials/' which 

 appeared in the Philosophical Magazine for October 1912. 

 The same composite rock mixtures as were used in the 

 previous series of experiments were also used in these. 



The method of procedure and the design of the apparatus 

 have been fully described by him in the Philosophical 

 Magazine for May 1900. It may be briefly said that the 

 rock has first to be got into solution b}^ fusing it with a 

 mixture of the alkaline carbonates; either a blowpipe or a 

 small electric furnace being used for this purpose, as a 



* Communicated bv Prof. J. 3 oh-, F.R.S. 

 2 1 2 



