32 



1905), the loss of range of the a particle of RaC in going 

 through a given sheet of material appears to be slightly 

 greater than the loss of range of an a particle of RaA, and 

 it is not yet quite clear whether this difference is real or appa- 

 rent. 



The difficulties which have just been mentioned occur only 

 in the absolute determination of air ranges, and do not affect 

 the accuracy of the comparison of the ranges of radium, 

 uranium, and thorium. 



I owe my grateful thanks to Dr. W. T. Cooke for his 

 have thought it better to allow it to stand without alteration. 



Note. — Since the above was written I have received the 

 February number of "The Philosophical Magazine," cont-ain- 

 ing an article by Mr. N. F. Campbell on "The Radiation from 

 Ordinary Materials." In finding the formulae necessary to 

 his investigation, Mr. Campbell has covered part of the 

 ground gone over in Part I. of this paper. As the fuller 

 treatment which I have given is required in my own work I 

 have thought it better to allow it to stand without alteration. 



In a footnote Mr. Campbell expresses his inability to see 

 why I introduced an obliquity factor cos into the prelimi- 

 nary calculations of my first paper on the a rays ("Phil. 

 Mag," Dec, 1904), The mistake is mine. I did not discover 

 it until I had occasion to consider the matter again in con- 

 nection with this present investigation. By omitting the fac- 

 tor, Mr. Campbell has obtained the correct formula for the 

 case which he has investigated. 



