856 



Dr. A. F. Kovarik on the Absorption and 



the atomic weight of the absorbing element. If air is under- 

 neath, the /3-particles reflected by the absorbing sheets do not 

 re-enter the ionization vessel in appreciable amount. As a 

 result, the initial portion of the absorption curve under these 

 conditions will suffer a drop not due to absorption alone but 

 due to reflexion as well. This effect is quite noticeable, as 

 may be seen from Table III. and the curves b and c in fig. 3, 

 where aluminium and tinfoil were used respectively as the 

 absorbing matter, air being underneath the active material 

 in both cases. Evidently, to obtain a more accurate absorp- 

 tion curve, the same thickness of the material used for 

 absorption should be placed under the thin radioactive layer 

 as is placed above it, in which case the loss of the /3-particles 

 by reflexion by the absorbing layers is compensated by the 

 reflexion into the ionization chamber by the layers underneath. 

 Column a, Table III., and curve a, fig. 3 (p. 857), show the 

 results when aluminium is so used for the absorption of the 

 /3-particles from RaE. 



Table III. 

 Absorption of the /3-particles from Ra D-i-E. 



a = Aluminium as absorbing substance ; same number of 



layers underneath as used for absorption. 

 b = Aluminium as absorbing substance; air underneath. 

 c = Tinfoil as absorbing substance; air underneath. 



Aluminium 

 in mms. 



a. 



b. 



Tinfoil in 



equivalent value 



of mms. Al. 



c. 



0-059 



1009 



100-0 



0059 



1000 



•118 



659 



60-7 



•099 



65-5 



•177 



463 



44-4 



•139 



49-4 



•236 



34-6 



33-9 



•179 



400 



•295 



25-7 



25-4 



•219 



32-5 



•354 



19-4 



19-3 



•299 



22-8 



•472 



11-6 



11-55 



•339 



19-4 



•590 



670 



697 



•459 



11-55 



•708 



4-10 



4-51 



•599 



7*24 



•826 



2-41 



2-41 



•744 



311 



•944 



152 



1-60 



•870 



1-87 



1-062 



080 



0-84 



•939 



1-38 



The coefficients of absorption of the /3-particles from Ra D, 

 ThA, RaB, RaE, Act C, ThD, and Ra C, obtained in this 

 manner, are given in Table IV. 



