Relation between Uranium and Radium. 277 



less. But it may be stated that the results obtained by the 

 old methods with just detectable amounts of radium in large 

 quantities of uranium are always several times too low. 



The Electroscope. — In the present work the electroscope is 

 the same as that previously used, although the leaf system 

 has been frequently renewed. Here again attempts are being- 

 made to replace it by an improved form, constructed of glass 

 silvered internally, as being more suitable for this class of 

 work. A large number of new brass electroscopes, exactly 

 similar to the old one, were made at the commencement of 

 the work, but for some reason they all had a high natural 

 leak, and the old one has been mostly employed throughout. 

 It consists of a brass tube 10 cm. diameter and 12*5 cm. long 

 provided with glass windows. The leaf system was held in 

 a brass tube 2*5 cm. diameter in the centre of the upper plate. 

 Both the upper and lower plates were of brass, and the joints 

 were made tight with sealing-wax. It required frequent dis- 

 mantling and cleaning throughout the work, for the natural 

 leak tended to increase beyond what was thought desirable. 



Rubbing the inside surface of the brass with a rag moistened 



. . . 



with dilute sulphuric acid was found more efficacious than 



polishing. The normal natural leak was between 0*5 and 1*0.. 



The leaf system consisted of a 2*5 cm. rubber cork coated 



with paraffin and carrying the charging rod made of steel 



wire joined to a piece of watch-spring at one end and an 



ebonite handle at the other. The leaf-system proper was 



carried centrally in the cork on a second steel wire on the 



lower end of which a rod of sulphur about 2 cm. long was 



melted, by repeatedly dipping the end of the cold wire into 



just molten sulphur, and allowing a stick of sulphur gradually 



to build itself up on the end of the wire. Into the lower end 



of the sulphur was melted a brass strip 3 cm. long and 2 mm. 



broad, about 24 mm. of the length projecting beyond the 



sulphur insulation. The effective length of the leaf was about 



2 cm. and the breadth 2 mm. The microscope was that 



previously employed*. One division of the eyepiece scale 



was equal to '0235 mm. The scale was used in the horizontal 



position throughout. 



It was found that the new leaf-systems constructed from 



time to time, although made by eye without any other 



measurement, varied but little in their sensitiveness. For 



this reason it has been thought of interest to give the full 



* It was made by W. Wilson of London, and had, in addition to the 

 horizontal and vertical movements and scales, a focussing movement, 

 which made it perfect for the purpose of following up and keeping- in 

 focus a rapidly moving leaf. 



