46 Dr. H. E. Watson and Mr. Gostabehari Pal on 



anhydride, by opening the tap F connected to a tube leading 

 nearly to the bottom of the flask. The electroscope itself 

 was similar to those used by Joly, but about twice as 

 sensitive. The moving leaf was of aluminium about 

 30 x 2 mm., and a small piece of quartz fibre attached to 

 the end and illuminated from the side, made it possible 

 to take readings to the tenth part of a scale-division with 

 ease. The insulation was a quartz tube with a sulphur rod 

 shaded by black paper on the end. Charging was effected 

 by means of a wire movable through an air-tight ground- 

 glass joint H. A potential of 300 volts from a battery was 

 used, and the wire always replaced in the same position and 

 earthed after charging. The whole was contained in a 

 500 c.c. glass flask silvered on the inside except for two small 

 holes, and earthed. 



The quartz fibre was observed through a small microscope 

 with a scale in the eyepiece. The constant was determined 

 roughly at first by weighing out 5 milligrams of Joachimsthal 

 pitchblende which was mixed with a little rock and fused up 

 in the usual way. This quantity was found to be much too 

 large, and so 10 milligrams were accurately weighed, well 

 digested with nitric acid, and the solution made up to 100 c.c. 

 1 c.c. of this was taken and evaporated to dryness in a small 

 glass capsule. After three weeks this was fused up with 

 25 grams of potassium hydroxide, and the resulting leak 

 determined. Two such determinations were made at intervals 

 of eight months with different portions of pitchblende, and 

 in the second case twice the quantity was taken. 



These two experiments gave the quantity of radium 

 which produced a leak of 1 scale-division per hour as 

 0-310 xlO -12 gram and 0'315xl0~ 12 gram respectively. 

 Another preliminary experiment with a different leaf gave 

 0*275 X 10" 12 gram. The pitchblende was found to contain 

 60*0 per cent, of uranium ; and it was assumed that the 

 amount of radium in equilibrium with 1 gram of uranium 

 was 3*15 x 10~ 7 gram (Pirret & Soddy, Phil. Mag. [6] xxi. 

 1911, p. 652). 



The natural leak was fairly constant. It varied from 5*5 

 to 6'2 scale-divisions per hour, and was determined at frequent 

 intervals. 



The materials used for fusion appeared to be fairly free 

 from radioactive matter. Blank experiments showed that a 

 correction of about 1 scale-division per hour for 50 grams was 

 necessary. 



It was found at once that the quantity of radium in the 

 rocks under examination was very minute ; and in order to 



