[m'lennan et KENNEDY] RADIOACTIVITY OF POTASSIUM 23 



tinfoil used for a screen was increased. The values obtained by 

 Rutherford ^ for the constant X when studying the absorption 

 of the f^ rays from Uranium with screens of tinfoil was 96, and 

 later the value found by Crowther' for the same constant under 

 similar circumstances was 69.3. A comparison of these values and 

 those given in Table IX makes it evident that while the rays emitted 

 by the potassium salts were very heterogeneous in character they did 

 not contain any types possessing greater penetrating power than the 

 /3 rays emitted by Uranium salts. This result too is in keeping with 

 what Campbell and Wood found in their investigations on the absorp- 

 tion of these potassium rays. 



VI. Activity of Potassium Salts and Secondary Radiations, 



A set of measurements was made with the object of ascertaining 

 whether the radiation emitted by the potassium salts, which from its 

 characteristics evidentl}^ consisted of yS rays, was due to some pro- 

 perty inherent in the salts or whether it was due to a secondary 

 radiation, of exceptionally high intensity, excited in the salt by the 

 penetrating radiation which is known to exist at the surface of the 

 earth. 



A cylinder of lead some 60 cm. high and 25 cm. in diameter was 

 prepared and the natural conductivity of the air in this cylinder 

 determined. A layer of potassium sulphate was then placed in a 

 tray on the bottom of the cylinder and when the conductivity of the 

 air was again measured it was found to be approximately twice as high 

 as it was before the salt was introduced. 



From this it was evident that the radiation from the salt con- 

 tributed to the conductivity practically as much as the combined 

 action of the earth's penetrating radiation and that emitted by the 

 walls of the cylinder. If then the activity of the salt consisted solely 

 of a secondary radiation one should expect with a penetrating radia- 

 tion gradually increasing in intensity to obtain proportionately greater 

 effects when the salt was in the cylinder than when it was removed 

 from it. 



In order to realise these conditions a few milligrams of radium 

 bromide sealed in a glass tube were enclosed in a heavy block of lead 

 and this was placed at a number of different distances from the 

 cylinder and the conductivity of the air in the chamber ascertained, 

 with the radium in each position. The observations were made first 

 with the salt in the cylinder and then repeated for each position of 

 the radium after the salt had been removed. 



^ Rutherford " Radioactivity." Edition II., page 137. 

 2 Phil. Mag., Oct. 1906, page 379. 



