the Method of its Transmission. 105 



removal decreases. For a long interval of exposure the activity 

 begins to decrease at once after removal. This result is to be 

 expected, for the activity o£ each portion of the radioactive 

 matter deposited increases with time for two or three hours, 

 and then diminishes. Consequently, after the electrode has 

 been exposed for about ten hours or more, the increase of 

 activity of the matter deposited in the last few hours does not 

 compensate for the decrease of activity of the radioactive 

 matter as a whole. This increase of activity with time 

 explains an irregularity in the curve of increase of excited 

 activity from thorium with time of exposure. It was pointed 

 out in a previous paper (Phil. Mag. February 1900, p. 178) 

 that, on the hypothesis of a uniform rate of deposit of radio- 

 active matter, the activity of which decreased in a G. P. with 

 the time, the curve of rise of excited activity with time of 

 exposure is the same as the curve of rise of an electric 

 current in a circuit of constant self-inductance. It was 

 experimentally observed, however, that the rate of increase 

 for the tirst few hours was much smaller than would be 

 expected on this hypothesis. In the light of the present 

 results, the explanation of this effect is simple. The matter 

 deposited during the first few hours does not reach its maximum 

 activity for several hours, and the initial effect is consequently 

 much smaller than would be expected on the simple theory. 



§ 6. Radium Excited Radioactivity. 



Experiments were made to determine the velocity of the 

 carriers responsible for the excited radioactivity of radium in 

 the same way as for thorium. The radium in my possession 

 gave out too little emanation at ordinary temperatures in the 

 solid state to enable me to use it in the apparatus in place of 

 thoria. The amount of emanation from radium can, however, 

 be increased several thousand times by heating the radium 

 compound below a red heat. A more convenient method of 

 obtaining a large amount of emanation is to dissolve a small 

 quantity of radium chloride in water. Radium in solution 

 gives off several hundred times more emanation than in the 

 solid state. If the solution is kept in a closed vessel the 

 emanation continually collects in the upper part of the vessel 

 It can be transferred at any time to another vessel by bubbling 

 a slow current of air through the solution. The procedure 

 adopted in introducing the emanation into the apparatus was 

 as follows : — 



A large amount of emanation was introduced into ;; 

 metal cylinder of about 3 litres capacity. The plates to be 

 tested were then placed in position in the apparatus of tig. L, 



