for the Measurement of Recoil Radiations. 707 



balancing chamber is adjusted till the leaf is almost stationary. 

 A reading of its rise or fall is taken, the « particles are 

 allowed to impinge on the hydrogen, and another reading 

 taken. The difference between the ionizations gives a 

 measure of the radiation. Headings such as these are taken 

 wbh different thicknesses of absorbing material, generally 

 aluminium, between the window and the bottom of the 

 ionization chamber. 



The maximum ionization due to the recoil atoms wa?, 

 in the different arrangements used, from 2 per cent, to 

 10 per cent, of the total. 



Tests made to establish (he nature of the radiation. 



Experiments were made with a gap hetween the box in 

 which the recoil particles were produced and the ionization 

 chamber. This gap was in an intense magnetic field, 

 which when varied produced no corresponding change in the 

 ionization. It cannot therefore be a /5 radiation. The form 

 of the absorption curves shows that it is not of <y or X ray 

 type. 



The following experiments were made to show that it is 

 not a primary radiation from the source. 



£ g TT , eg Ionization 



§ S Hydrogen. $ , , 



S ^ J ° « chamber. 



Screen A is between the source and the hydrogen. 

 Screen B between hydrogen and ionization chamber. If 

 the radiation arose at the source, though a change in A+B 

 would affect the ionization in the chamber, it would be 

 immaterial how the total thickness of absorbers was dis- 

 tributed between A and B. Tt was found that the effect was 

 reduced to less than 1/5 when A was changed from 3 cm. 

 air equivalent to 6 cm. air equivalent, instead of from cm. 

 to 3 cm., although in both cases A-f B had changed from 

 G cm. to 1) cm. 



Results. 



The following are the tabulated results of the absorption 

 experiments, plotted in curves I and IT. 



Table A. This table gives the results from two typical 

 absorption experiments made with an a-ray tube as source. 

 u rays of ranges from about 5 cm. in air to zero were 



