108 



Mr. A. L. Fletcher on the 



evolved gases to the soda-lime tubes (see fig. 2). This 

 method gave 2*3 x 10~ 12 gr. per gr. for the contained radium. 

 My own result on a Ballyknockan granite by the solution 

 method used by Professor Strutt yielded a result of 1*6 X 10~ 12 

 gr. per gr. 



The variations in these results, together with other differ- 

 ences in the quantities of contained ladium, seem to indicate 

 that a moderately large divergence may exist between closely 

 adjoining specimens, but that the general average fairly 

 approximates to the mean result in any particular district. 

 In no case was the comparatively high result of 4 x 10~ 12 

 gr. per gr. maintained in a series of specimens from the 

 same locality. These results have been included therefore 

 in estimating the mean radium content of the whole chain. 

 The thorium was estimated from the same solutions, by the 

 method introduced and described by Prof, Joly (Phil. Mag. 

 July 1909), using his apparatus, and the constants determined 

 by him. I add a description of this apparatus in its latest 

 form. 



Apparatus for Detection and Measurement of Thorium. 



The electroscope in which the thorium was estimated was 

 made and calibrated by Prof. Joly (Phil. Mag. July 1909) 

 from solutions containing a known quantity of thorite. The 

 vacuum-pump P, which is fed from an overhead tank main- 

 tained at a constant level, is first started, the partial vacuum 

 caused in the apparatus being observed by the rise of the 

 oil in the tube T, reading on to an arbitrary sc;de S behind, 

 which thus serves as an indication of the velocity of the air- 

 current. The vessels B, B serve to steady the influx of air, 

 and thus keep the indicator steady. 



The actual rate of flow of the air is about 4 c.c. per sec, and 

 is measured by means of the 100 c\c. flask F. The stopcock 



