110 Mr. A. L. Fletcher on tie 



window w of thin cover glass let in for the better observance 

 of the gold leaf. 



It was not found necessary, when observing the natural 

 collapse of the gold-leaf, to remove the flask, bat merely to 

 discontinue the ebullition. 



The thorium content of this granite is very low. In some 

 cases careful and repeated experiment failed to detect with 

 certainty any trace. Efforts were made, but failed to reveal 

 any concentration of thorium existing in any particular con- 

 stituent, as seems to be the case w ith radium. Thus one gram 

 of the biotite showing nearly 12 x 10 -12 gr. radium per gr., 

 showed no detectable trace of thorium ; while 2*5 grams of 

 muscovite with contained biotite picked from No. 17 in the 

 table, also failed to reveal any thorium. 



In the case of those solutions containing any quantity of 

 precipitate (either original or subsequently developed daring 

 the estimation of the radium), a second experiment was made 

 on the clear solution after the precipitate had been removed 

 by filtration. The effect generally was to slightly increase 

 the collapse of the gold leaf. In the case of No. 1 in the 

 table, an increase in the rate of gain of 2 scale-divisions 

 per hour, and of No. 2 in the table an increase of *5 scale- 

 division per hour was observed. 



The mean quantity of thorium found was 0*70 X 10 -5 gr. 

 per gr. of the rock. Professor Joly (Phil. Mag. July 1909) ob- 

 tained on nineteen various primary and secondary rocks a mean 

 thorium content of 1'07 xlO -5 gr. per gr., and on fifty-one 

 rocks, chiefly gneisses of exceptionally high radioactivity from 

 the St. Gothard Tunnel, a mean of 1*12 x 10 — 5 gr. per gr. 



The approximately constant proportions obtaining between 

 the thorium and the radium contained in the granites is a 

 striking feature. The mean ratio borne by the radium to 

 the thorium present is 2*4 X 10 ~ 7 . In no less than ten separate 

 specimens, the proportion borne by radium to thorium varies 

 between 2*0 x 10~ 7 and 2*6 x 10~ 7 , the three specimens from 

 Ballyellin showing this very closely. 



Three remarkable examples, however, are those of Nos. 12, 

 24, and 28 in the accompanying table, these three specimens 

 showing the highest radium content but no detectable thorium. 

 On comparing these latter results with the others, it would 

 appear that an exceptional rise in the quantity of radium, so 

 far from being attended by any corresponding rise in the 

 quantity of thorium, is actually attended by its absence. It 

 cannot be said, however, that previous work supports so 

 marked an inverse ratio ; although it is apparent from the 



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