232 



Mr. D. C, H. Florance 



was airtight, so that methyl iodide could be introduced when 

 necessary. 



The general conclusions drawn from a large number of 

 experiments are these : When elements such as aluminium, 

 carbon, and iron are used as radiators, there is no evidence 

 or! the presence of a very easily absorbed radiation. The 

 "returned" radiation has an absorption coefficient of '20 cm. -1 

 to '30 cm. -1 in aluminium compared with '115 for the direct 

 radiation. Aluminium is not so effective in absorbing the 

 secondary radiation as lead, and in consequence is not so 

 suitable for measuring the quality of the radiation. 



On the other hand, in the case of the elements lead, 

 platinum, and mercury, there is always an indication of the 

 presence of a soft type of radiation, especially noticeable in 

 the " returned " radiation. This appeared most marked in 

 the case of platinum, but it was difficult to prove conclusively 

 since nothing better than a platinum dish was available for a 

 radiator. The absorption coefficients of the returned radia- 

 tion in aluminium varied between '50 cm. -1 and '80 cm. -1 

 for the first 3 or 4 mm. of absorbing material. 



Additional experiments have been made using a thin sheet 

 of lead about O'Ol cm. thick and 15 cm. square as a radiator. 

 The returned radiation could be detected with certainty 

 through lead 0'084 cm. thick. With an absorbing screen of 

 lead 0"01 cm. thick the absorption coefficient was found to be 

 about 40 cm. -1 . Accurate readings were impossible on 

 account of the smallness of the radiation observed. The 

 absorption coefficient in O202 cm. of aluminium was found 

 to be about 1*5 cm. -1 . In these experiments a mixture of 

 hydrogen and methyl iodide had been introduced into the 

 electroscope. The absorption coefficients in successive thick- 

 nesses of lead absorbing-screens are included in the following 

 table : — 



Absorbing-screens 

 of lead 



0-01 cm. 0-02 cm. 



0-03 cm. 



0-04 cm. 



0-088 cm. 





1 



Absorption coeffi- 



j 

 40 32 



24 



20 



13 





While there is undoubtedly a very soft radiation present in 

 the 7 rays returned from lead, it is difficult to be certain 

 whether this is to be ascribed to the excitation of one of the 

 characteristic types of radiation from lead. At the same 

 time there is no evidence to contradict such an assumption. 



