PooLK — On the Distribution of Activity in Radium Therapy. 469 



the electroscope. It is made of brass, with a lead base 4 mm. thick, in the 

 centre of which is a circular window, 2-7 ems. in diameter, covered with a 

 sheet of gelatine, 0-02 mm. thick, off an unexposed but " fixed " photographic 

 plate. The electroscope is lined with leather about 2-5 mm. thick, impreg- 

 nated with gelatine containing a little mercuric chloride to increase its 

 electrical conductivity. This lining ensures that the secondary radiation 

 from the walls of the electroscope resembles as nearly as possible that in 

 animal tissue, so that the rate of motion of the gold leaf is a fair measure 

 of the total ionisation that would occur in the 

 tissue under the same conditions of radiation. The 

 electroscope was always charged to about 250 volts 

 by means of a Tucker hygroscopic battery before 

 each test. The approximate constancy of the initial 

 voltage rendered the motion of the leaf more 

 regular. 



The screens to be tested were attached with a 

 little plasticine to the under side of the lead base, 

 the wooden block carrying the radioactive source 

 being so arranged that it could be removed and 

 replaced in the same position. This source consisted 

 of one or more thin glass tubes about 1'5 cm. long 

 by 0'8 mm. diameter, containing emanation, placed 

 in a shallow groove in the block. The activity of 

 the source used varied with the thickness of the 

 screen so as to maintain the rate of motion of the 

 gold leaf within reasonable limits. The screens 

 were placed directly in contact with the electroscope 

 base in order that as large a percentage as possible 

 of the secondary j3 radiation, emerging at various 

 angles from the upper surface of the screen, 



should enter the electroscope. The source was placed only 1'85 cms. below 

 the base in order that the divergence of the j rays passing through the parts 

 of the base surrounding the window might reduce the error due to these 

 rays to a negligible quantity. An allowance was made for the gelatine film 

 in estimating the thickness of the various screens. Experiments with a 

 second film indicated that the absorption of unscreened radiation by such a 

 film amounted to about 6 per cent., so this allowance was made in estimating 

 the effect of a bare emanation tube. In all cases the thickness of the screens 

 used was found by weight. 



8e2 



Fig. 1. 



