Fe ee 
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142 NATURE OF THE RADIATIONS [CH. 
97. In an examination of the active substances by the elec- 
trical method the writer’ found that both uranium and thorium 
gave out y rays in amount roughly proportional to their activity. 
An electroscope of the type shown in Fig. 11 was employed. This 
was placed on a large lead plate ‘65 cm. thick, the active substance 
being placed in a closed vessel beneath. 
The discharge due to the natural ionization of the air in the 
electroscope was first observed. The additional ionization due to 
the active substance must be that produced by rays which have 
passed through the lead plate and the walls of the electroscope. 
The following table shows that the discharge due to these rays 
decreases according to an exponential law with the thickness of 
lead traversed. 
Thickness of lead Rate of discharge 
“62 cms. 100 
» + 64cms. 67 
AO OMe 23 
OS). op 8 
Using 100 gr. of uranium and thorium, the discharge due to 
the rays through 1 cm. of lead was quite appreciable and readily 
measured, The results showed that the amount of y rays was 
about the same for equal weights of thorium and uranium oxides. 
The penetrating power was also about the same as for the radium 
rays. 
Results originally obtained with an electrometer in the place of 
an electroscope gave results indicating about 20 per cent. less pene- 
trating power. The electroscopic results are probably the more 
accurate, but those obtained with the electrometer, as given below, 
serve for the purpose of comparison. 
Thickness of metal to 
sue absorb half of the rays 
Mercury ... ‘75 cms. 
Lead 8) 5 
Tin 1:3 
Copper DD i. 
Zinc Day |, 
Iron DAES Ness 
1 Phys. Zeit. p. 517, No. 22, 1902. 
