378 



Mr. R. W. Boyle on the Absorption of 



slight rise of activity due to the active deposit, but not nearly 

 in the same degree as if the charcoal had been absent. 



The above experiments were all performed in a constant- 

 temperature room, so that all effects on the diffusion by 

 changes of temperature and draughts o£ air were avoided. 



More experiments, along the same lines as the above, are 



being carried on. 



Thorium Emanation. 



In the case of thorium emanation, owing to its short period 

 of decay — to half-value in 54 seconds,— it is necessary to use 

 a flow method of experiment. In other words, the emanation 

 must be mixed with some gas, and conducted through tubes 

 containing absorbing, or non-absorbing, material into a testing- 

 vessel, where it ionizes the air, and the ionization currents 

 can be measured by means of an electrometer. The apparatus 

 used for the experiments is represented by the accompanying 

 diagram (fig. 2). 



Fig-. 2 



An air-pump supplies a current of air through a tube of 

 thorium hydroxide, where the emanation mixes with the air 

 and thence passes through tubes containing absorbing or 

 non-absorbing material, as the case may be, into a testing- 

 vessel of the ordinary cylindrical type. An accurately 

 calibrated manometer iu the circuit measures the velocity of 

 the air-current. The testing vessel is connected to one pole 

 of a battery of E.M.F. sufficient for complete saturation, the 

 other pole of the battery being earthed. The central rod of 

 the testing vessel is connected with a suitable electrometer, 

 and a condenser of adjustable capacity is placed in parallel 

 with it. It should be noticed that, with this arrangement 

 absorption of the emanation takes place when the charcoal 

 has absorbed its full amount of the atmospheric gases. 



The weight of thorium hydroxide used was 37 grms. 



