204 Prof. J. S. Townsend on 



progress by Prof. Rutherford and myself, which, as it is 

 hoped, will give an accurate value for the period. 



In the case of thorium emanation, where the existence of 

 a product of short life has already been pointed out in a 

 previous note*, similar effects have been observed, and will 

 be communicated in a later paper. 



Attention may be drawn to the fact that the values of the 

 ranges obtained above throw new light on the relation — 

 suggested by Prof. Rutherford t in 1907 — between the period 

 of the active substances and the ranges of the a particles 

 emitted by them. Such a relation has been found to hold 

 more generally than was at first suspected. This will be 

 discussed in a paper shortly to be published. 



Results. 



It has been shown that the actinium emanation is complex,, 

 consisting of two products each of which emits a rays. The 

 first one — the emanation — with a period of 4 seconds emits 

 a rays of a range of 5*7 cm. The second product emits a rays 

 of range 6*5 cm., and has a period of the order of ^J of a 

 second. The latter product carries a positive charge, and is 

 probably a solid. 



My thanks are due to Mr. J. M. Nuttall for the assistance 

 he has given me in some of the experiments. 



Physical Laboratory, 

 "Victoria University, Manchester. 



XVIII. Charges on Ions in Gases. 



To the Editors of the Philosophical Magazine. 

 Dear Sirs, — 



IN a paper by Professor R. A. Millikan and Mr. Harvey 

 Fletcher on the valency of gaseous ionization, which 

 appeared in the June number of the Philosophical Magazine, 

 some experiments on the determination of the charge on an 

 ion are described, from which it is concluded that no ions 

 with double charges are produced in a gas by primary or 

 secondary Rontgen rays. The previous experiments by 

 Professor Millikan, from which it was deduced that such 

 double charges do exist, are said not to be so conclusive, and 

 also all other investigations which show that positive ions 



* H. Geiger and E. Marsden, /. c. 



t E. Rutherford, Phil. Mag. xiii. p. 110 (1907). 



