﻿Thorium Content of the Earth's Crust. 487 



proportion existing between the two, as will be seen from the 

 figures. There is nevertheless a certain amount of evidence 

 that the ratio of radium to thorium also increases as we pass 

 from the acid to the basic rocks, and for certain rocks this 

 ratio seems to remain fairly constant. It is also remarkable 

 that the value of the ratio always lies within such narrow 

 limits. In only three or four cases does it lie outside the 

 range 1*3 to 2*7 X 10 -7 . This range is perhaps not narrow 

 enough to justify us in assuming that there might be any 

 genetic connexion between uranium and thorium, but that 

 the ratio is as constant as it is, is certainly a remarkable fact. 

 The mixture of the twelve syenites gave rather confusing 

 and disconcerting results. Dr. Joly first determined its 

 activity using a solution containing 10 grins, of the mixture, 

 and obtained a result of 4*08 X 10~ 5 grm. of thorium per 

 gram of rock. He thought that this result was rather high, 

 and suspected that the solution had been contaminated, so 

 two new solutions, one containing six grams and the other 

 five, were made up. Both these solutions gave a value for 

 the thorium content of approximately 1'7 x 10 ~ 5 . This result 

 would naturally lead us to suppose that the first solution had 

 been contaminated as first thought. Unfortunately for this 

 theory, however, when a solution containing six grams of a 

 mixture of only six of the original syenites was tested by the 

 present writer, it gave a result of 4*2 x 10 ~ 5 . It is hard to 

 explain how the two solutions that gave the high results 

 could both have been contaminated to proportional amounts 

 so that they would give nearly the same result. The dis- 

 cordance between the values might possibly be explained by 

 assuming that the thorium in some of the syenites was not 

 evenly distributed through the rock, so that the powder 

 would contain specks of highly active matter. In this case, 

 we could understand that in one case we might by some 

 chance get a good deal more of these particles than in another, 

 and so obtain a higher result. This theory is however 

 incomplete, as it fails to explain the fact that the activity 

 of the mixture seems to possess two fairly definite values, 

 namely, 1*7 and about 4*1 X.10 -5 . It might be thought that 

 the second high value could be explained by the fact that 

 the mixture only contained six syenites. It seems, however, 

 that this cannot be done, as even if we assume that the six 

 syenites rejected in making up the solution contain no 

 thorium at all, still even in this extreme ease wo ought to 

 obtain a value for the thorium content, of only double the 

 value for the twelve syenites. This value would be about 

 3'4x 10~ 5 , which is appreciably Lower than the actual result 



