126 Prof. J. Jolj on the Amount of 



residue was, after fusion, divided into an acid and an alkaline 

 solution; the former being alone added to the original acid 

 solution, and the alkaline solution reserved for a separate 

 test. 



It was thought best to determine the constant of the 

 electroscope by an experiment in which a known amount of 

 a standard solution of thorianite was added to one of the 

 acid rock solutions which had been already tested for thorium. 

 Throughout the experiments all the solutions were boiled in 

 the same flask and brought to the same bulk. These con- 

 dilions, as well as those of velocity of air-current &c, were 

 preserved unchanged when finding the constant of the 

 electroscope. It was found that when one cubic centimetre 

 of a standard thorianite solution containing 6*977 x 10~ 4 gram 

 thorium per cc. was added to the rock solution, the rate of 

 discharge of the electroscope increased from a quite steady 

 rate of 1''6 scale-divisions per hour to 50 scale-divisions per 

 hour, a gain of 42*7 scale-divisions. Consequently one scale- 

 division per hour increase corresponds to 1*63 x 10~ 5 gram 

 thorium, which is the required constant. A previous deter- 

 mination of the constant of this electroscope under the same 

 conditions, but effected by adding the thorianite to the solu- 

 tion of a trachyte of the Andes, gave the constant as 

 1*68 x 10~ 5 gram. The first value was used throughout. 



The figures in the brackets, given in the table which 

 follows, refer to the weight in grams of the amount of rock 

 dealt with. 



It will be noticed that in most cases the quantity of 

 thorium present is so small that it could not be certainly 

 determined in the quantity of rock used. The major limit, 

 in such cases, is obtained by dividing the constant of the 

 electroscope by the weight of rock taken; it being assumed 

 that a change in rate of discharge of one scale- division per hour 

 is readable. This assumption is certainly justifiable. 



It would appear from these results that thorium is not 

 abstracted in any considerable degree from waters in which 

 calcareous materials are formed. This may be due to a 

 process of organic selection among the dissolved materials. 

 I have obtained, in the case of a sample of sea-water from 

 the Indian Ocean, 0*9 xlO -8 gram thorium per gram of 

 water (Phil. Mag. July 1909). If this approximates to the 

 average thorium content of sea-water, by far the greater 

 part of the thorium accompanying a given amount of calcium 

 salts in sea-water must be rejected in the organic processes 

 attending the abstraction of the lime. 



