40 Mr. H. Mitel i ell on Ratios which Amounts of 



Note added Oct. 26, 1910. 



Since this paper was written there have appeared two 

 communications by Wulf (f.e Radium, June 1910 ; Rhys. 

 Zeit. 15 Sept. 1910). These throw much light on the subject 

 of penetrating radiation and confirm some of the conclusions 

 set forth above. 



Wulf, with the vessels he used, finds for the penetrating- 

 radiation in Holland ^ = 10, in Paris q=6, and at a height 

 of 300 m. on the Eiffel Tower q = 35. 



The reduction in q over a lake was about 4*9, and a similar 

 reduction was determined at a depth of 12 m. beneath the 

 surface of the water. His interesting experiments seem to 

 establish the terrestrial origin of the penetrating radiation 

 and the diminution of intensity with altitude. 



The figures appear to require reduction for values in free 

 air, perhaps on the basis of lead 100, zinc 60, aluminium 52, 

 free air 42. 



It will be seen that Wulf s value for q, even at Paris, is 

 larger than that calculated in this paper, and that a loss of 

 40 per cent, for an altitude of 300 m. is less than the loss 

 of 64 per cent, which I caleulate for an altitude of 100 m. 

 It will no doubt take time to reconcile these points. 



At present it would seem that the penetrating radiation 

 both passes through the air more readily, and also produces 

 more ions than laboratory experiments, with Ra and metal 

 screens, lead us to infer. It will, however, be remembered 

 that no direct measurements of the coefficient of absorption 

 of the 7 rays by gases have yet been made. A. S. E. 



Y. Note on the Ratios which the Amounts of Substances in 

 Radioactive Equilibrium bear to one another. By Hugh 

 Mitchell, M.A.* 



(1) fT^HE law of radioactive change is that the rate of 

 JL disintegration of an active substance is proportional 

 to the amount of it present. A series of radioactive sub- 

 stances is said to be in equilibrium when the ratio of the 

 amount of one substance to that of any other substance in 

 the series remains unchanged as time goes on. It is known 

 that assuming the disintegration rate of the parent substance 

 to be small in comparison with that of any other in the 

 series, the ratio of the amounts of any two substances in the 

 series is approximately equal to the ratio of their average 

 lives. The present note is intended to show that without 



* Communicated by F. Soddy, M.A., F.R.S. 



