28 



just be penetrated by the a particle of radium. If there are 

 errors of experiment other than those discussed above they 

 are common to the experiments with uranium and thorium, 

 and disappear when comparison is made. 



RaC. 



A smal] piece of copper foil was rendered active by ex- 

 posure at a negative potential to the emanation from one or 

 two mmg. of radium bromide. Tinfoil was used as the ab- 

 sorbing layer. There was a special difficulty in the experi- 

 ment due to the decay of the active matter. This was over- 

 come by taking measurements of the current with the RaC 

 uncovered before and after each measurement when foil was 

 placed over the radiating material. The observations were 

 equally spaced in point of time, so that the geometric mean 

 of the two former measurements could be matched against the 

 latter. The results are shown in the following table : — 



Thus the a particles from RaC can just penetrate a sheet of 

 tinfoil whose pd = ^0212. A separate experiment by the 

 method employed by Kleeman and myself ('Thil. Mag.,'' 

 Sept., 1905), showed that this was equivalent to 7'4 cm. of air. 

 The actual range is 7'1 floe, cit.), so that the agreement can 

 be considered satisfactory. 



Thorium. 



The material was used in the form of thorium 

 oxide, which had been freed as far as possible from 

 other radio-active substances by means of the processes 

 described by Rutherford and Soddy. The treatment 

 employed, which included heating to a bright red heat as the 

 final stage, was judged to have been successful for the fol- 

 lowing reasons : — In the first place, the recovery of activity 

 was not marked by an initial drop, so far as could be ob- 

 served ; in the second, the ionisation current rose at a rate 

 which showed that it would be halfway to the final value in 

 four days, the final value being about four times the initial. 

 In the third place, no emanation came off the material when 

 first prepared ; even when no draught was employed the read- 

 ings did not alter in 15 minutes ; and, in the fourth place, 

 the observed results fitted closely to the calculated curves, 

 showing only a slight variation, as in the case of the radium. 



