140 



Messrs. A. S. Russell and F. Soddv on the 



the values of A for zinc were (1) 0*23, (2) 0*21 when 1*24 cm. of 

 lead were placed (1) between the zinc and the electroscope, 

 (2) between the zinc and the radium. Similarly the values 

 for iron were (1) 0'304, (2) 0'280, 1 cm, thickness of lead 

 being used. These results indicate at once that the values 

 found for X are to some extent influenced by the nature and 

 thickness of the base employed. Thus the results given, for 

 example, in Table II. (I. p. 644), in which the mean value of 

 X/d for Class II. bodies (7-rays of radium) was 0*040, refer 

 to a lead base (0*975 cm.). The mean values for other bases 

 were as follows : — - 



Base 



Lead 

 0-98 cm. 



Lead 

 2'86 cm. 



Brass 

 00 cm. 



Brass 

 1-73 cm. 





Mean A/dxl00 



399 



3-81 



3-92 



3'82 





An experiment was conducted with zinc to see to what 

 degree the process of hardening could be pushed. The dis- 

 position used in this case was different to that of the last, the 

 radium being placed 25 cm, below the usual lead electroscope 

 and the absorbing zinc clamped up to form the base. Seven 

 thicknesses of lead were placed in turn over the preparation 

 and the absorption by the zinc for each thickness of lead 

 measured. The results are given in the following table*. 



Thickness of Lead . . . 



0-124 



0-249 



0373 



064 



1-24 



1 

 2-50 3-75 



A fern ) -1 



0-325 



0-322 



0-309 



0'300 



0-291 



0-272 



0-265 0258 





Thus the final value is 0'79 of the initial, and there does 

 not seem to be a limit to the degree to which the hardening 

 process may be carried. The remarkable point is that the 

 curves are in every case, as nearly as can be seen, exponential 

 with the new value of X, after from 1 to 035 cm. equivalent 

 thickness of zinc has been penetrated, according to the amount 

 of lead covering the preparation. For lead the value of X is, 

 as we have shown, independent of thickness up to 22 cm. 

 (II. p. 752). Provided there is a base of lead 1 cm. thick 



* Incidentally it may be noticed that the value of X for the base prepa- 

 ration (0-325) is 7 per cent, greater than the value found in the standard 

 series of measurements given later (Table B, p. 148). The cause of this 

 difference is not yet clear, but may be due to the fact that in this experi- 

 ment the preparation rested in a groove in a lead disk, which somewhat 

 confined the beam of y-rays. 



