﻿204 Prof. J. Joly on the Radioactivity of 



rocks of the central and south divisions. There was a 

 similarity in the proportionate distribution of the two radio- 

 active elements wliich may perhaps be regarded as an instance 

 of the curious and as yet unexplained relation between the 

 uranium and thorium content of many rocks. 



The rock specimens originally de;ilt with were kindly 

 given to me by the Trustees of the South Kensington 

 Museum of Natural History, and consisted of chips removed 

 from the series collected by Stapff during the construction 

 of the tunnel. At a later date I ventured to apply for some 

 further specimens. I have to thank the Trustees for again 

 acceding to my request. The number of specimens applied 

 for was 33; they were selected by Mr. Prior, whom I desire 

 also to thank for the care with which the chips were selected 

 and labelled. In this second selection I gave no guidance 

 save the indication of the approximate position in the tunnel 

 from which it was desirable the chips should come. These 

 positions were selected so as to fill up the wider gaps left in 

 the first selection. A few additional specimens in the form 

 of a small collection purchased from MM. Grebel, Wendler 

 et C'% enabled me to add yet three more determinations 

 among the granitic rocks at the north end. The purchased 

 specimens were part of a Stapff collection and undoubtedly 

 auihentic. The total number of additional rocks obtained 

 was 36. The original specimens numbered 51. 



The examination of the additional specimens I postponed 

 pending the development of a new method of determining 

 the radioactivity of rocks, which would save much of the 

 exacting labour attending the method by solution *. During 

 the month of August last I accordingly examined the 36 

 rock samples in the electric tube-furnace. 



The rock specimen to the amount of about 7 grams was 

 reduced in the agate mortar to a degree of fineness sufficient 

 to pass it through a sieve of 60 mesh to the inch, but no 

 finer, as far as finer subdivision could be avoided. The 

 powder after weighing was mixed with 28 grams of the 

 mixed carbonates of sodium and potassium. The mixture 

 was placed in a platinum boat and introduced into the 

 furnace, which in some cases was raised somewhat in tem- 

 perature before the introduction of the charge, in order that 

 the volume of gas expelled into the gas-collector during the 

 heating should not be too great for the electroscope. The 

 temperature was raised for the last ten minutes to about 

 1100° C, the total duration of heating being about an hour. 

 * Phil. Mag. July 1911. 



