131 Messrs. A. S. Russell and F. Soddy on the 



thickness of aluminium as the radium C disk. Fifteen 

 minutes alter the disk had been removed from the emanation 

 the first measurements were taken. Experiments in each of 

 the four different dispositions were made with thorium D, 

 just as they had been made with radium C, and the relative 

 intensities of the rays at each disposition accurately determined. 

 The mesothorium 2 used in the experiments was separated 

 chemically from a preparation of mesothorium 1 previously 

 prepared from several kilograms of thorianite. The preci- 

 pitate, which weighed only a few milligrams, was evaporated 

 down on a small watch-glass of diameter exactly the same 

 as that of the disks used for the other two active bodies. The 

 quantity of matter was so small that self-absorption can be 

 neglected. The watch-glass was covered with a piece of 

 aluminium foil, 0*095 mm. thick, and several measurements 

 were made for each disposition over a period of 3J hours. 

 The four decay curves were exponential (\=0"12 (hour) -1 ). 

 The residual activity remaining two days later was negligible. 

 In the following table the actual results found for each 

 disposition for each source are given : — 





7i//3- 



rJP- 



y 3 //3- 



Radium C 



1-081 



1-849 



0711 



Mesothorium 2... 



1-222 



2-099 0-813 



Thorium D 795 



1-420 0525 



Or, taking the radium values as the standard :- 





ri//3- 



rJP- 



y 3 //3- 



Radium C 



100 



1-00 



1-00 



Mesothorium 2... 



113 



1-13 



114 



Thorium D 



0-736 



768 



0-738 



For these dispositions therefore mesothorium 2 gave 

 13 per cent, more 7-rays per yS-ray than radium C, and 

 thorium D about 25 per cent. less. Also, the variation of 

 the ratios thorium D to radium C, or mesothorium 2 to 

 radium C, with the thickness of the lead base is slight, if 

 any. This is to be expected because, as will be shown in 



