The Question of the Homogeneity ofy-Rays. 725 



the sunshine caused a diminution of the conductivity of the 

 air, but that the effect lagged about two hours behind the 

 cause. It is very probable that the dust is the chief agent 

 by which the sunshine causes the diminution of the conduc- 

 tivity ; for the intense solar radiation of the Indian dry 

 weather raises large quantities of dust into the lower atmo- 

 sphere. This result is important, for it shows that sunlight 

 plays only a small, if any, part in causing the natural ioniza- 

 tion of the lower atmosphere. The earth-air current is 

 determined by the two independent variables potential gradient 

 and conductivity ; it is therefore interesting to note that the 

 variations in these two factors resulted in the current from 

 the earth being least in the early hours of the morning and 

 greatest at 20 hrs. in the evening, while a marked secondary 

 maximum occurred at midday. 



It is hoped that it will be possible to continue this work in 

 Simla ; but it is very desirable that similar measurements 

 should be made in some less abnormal region, and it is with 

 the idea of interesting others in the work that this short note 

 is being published. 



LXXVII. The Question of the Homogeneity ofy-Rays. By 

 F. and W. M. SoDDY, and A. S. Kissjell, 1851 Exhibition 

 Scholar and late Vamegie Scholar, Univeisity of (rlasgow*. 



Part I., by F. and W. M. Sonny and A. S. Russell. 

 Part 11., by F. and W. M. Soddy. 

 Part I1J., by A. S. Russkll. 



[Plate XII.] 



Introduction. 



IN the first systematic work on the absorption of the 

 7-rays of radium (McClelland, Phil. Mag. 1904 [6] 

 viii. p. 67), it was found that the absorption coefficient Xof the 

 exponential equation I T = I e~* T tended to diminish as thick- 

 ness increased, especially for the denser substances, as though 

 the rays were somewhat heterogeneous. The following values 

 were given for lead : — From 0*8 to 1*05 cm. 0*64:1, from 1*05 

 to 1*3 cm. 0*563, from 1*3 to 1*8 cm. 0'480, from 1*8 to 2*3 

 cm. 0-440. Wigger (Jahr. Radioakt. 1 905, ii. p. 430) showed 

 that absorption proceeded exponentially, the absorption 

 coefficient being proportional to the density of the substance, 

 if the ray- were first passed through 2"8 cm. of lead, but an 



* Communicated by the Authors. 



