934 Mr. D. C. IT. Florancc 



on 



thus they corresponded to the condition in which the large 

 blocks of carbon wore used. These readings were compared 

 with those obtained with five similar sheets of lead tied 

 together. For the direct radiation, the rate of leak was 

 2 per cent, greater in the case of the lead fastened closely 

 together. But as the electroscope was turned round the 

 secondary radiation from the radiator occupying the large 

 volume produced a leak as much as 20 per cent, greater than 

 that produced from the lead when tied together. This is 

 what might have been expected, as the secondary radiation 

 instead of getting absorbed has in the one case 1 a chance of 

 escaping, and this is more marked the further the electro- 

 is moved from the direct line of radiation. In the 

 present arrangement, where the radius of the arc is about 

 22 cm., the large volume of a radiator such as carbon may 

 not give results comparable with those obtained for 1 cm. 

 thick of lead. 



(2) The Quality of the Secondary y Radiation. 



The apparatus, as previously explained, was set up so as to 

 reduce to a minimum tin' secondary radiation except that 

 due to the radiator itself. A lead screen 3 mm. thick was 

 placed over the window BO as to avoid any constant radiation 

 that might get through them. The electroscope was in such 

 a position, therefore, that when the radiator was placed in 

 position none of the original radiation entering the electro- 

 scope was stopped ; but there was simply an increased leak 

 in the electroscope due to the secondary radiation produced 

 by the radiator. 



The quality or the penetrating power of the radiation was 

 measured by placing a screen G*2-i mm. of lead against the 

 side of the electroscope, which was G mm. thick. Headings 

 were taken without the radiator, first without the lead screen, 

 secondly with the lead screen ; then similar readings with 

 the radiator. A large number of readings were taken, and 

 the mean value is given. In these experiments radium 

 emanation was generally used as a source of 7-rays and a 

 suitable correction was made for its decay. 



The first table shows clearly that for all radiators the 

 secondary 7 radiation gradually becomes softer as the electro- 

 scope is moved further away from the normal position ; and 

 that an increase in thickness of radiation hardens both the 

 primary and the secondary. The ratio of the absorption 

 coefficients for any two metals or for different thicknesses of 

 one metal keeps constant for each position of the electroscope. 



For example, the quality of the direct radiation passing 



