Rays of Positive Electricity. 295 



(4) The residual activity of actinium is not due to radium 

 and no appreciable growth o£ radium has yet occurred from it, 

 although the activity has decayed to a small fraction of its 

 initial value. 



(5) An experiment to detect a possible gaseous intermediate 

 prod act between uranium and radium has so far given a 

 negative result. 



(6) The residues from the purification of the uranium by 

 ether have been very perfectly freed from radium by barium 

 sulphate, and sealed up for future examination. 



XXIII. Rays of Positive Electricity. 



To the Editors of the Philosophical Magazine. 



Cavendish Laboratory, Cambridge. 



Gentlemen,— July 12, i907. 



IN his letter published in the June number of the Philo- 

 sophical Magazine, Professor W. Wien ascribes the 

 constancy of the value of e/m observed for Ganalstrahlen in 

 different gases to the presence of hydrogen as an impurity 

 in the discharge-tube. The reasons which have led me to 

 the conclusion that this explanation is not adequate are as 

 follows. Although it is impossible to be sure that all the 

 hydrogen has been eliminated from a discharge-tube, the 

 amount present can be varied within very wide limits ; and 

 when this is done there is no corresponding variation in the 

 brightness of the parts of the Ganalstrahlen which are ascribed 

 by Professor Wien to the presence of hydrogen. In all my 

 experiments at low pressures, by far the greater part of the 

 luminosity on the phosphorescent screen was produced by 

 rays corresponding to values of e/m equal to 10 4 and 5 x 10 3 , 

 and this was equally the case whether the most elaborate pre- 

 cautions were taken to exclude hydrogen, or whether hydrogen 

 was purposely introduced into the tube. I described in my 

 paper an experiment where the tube was exhausted until the 

 discharge would not pass ; at this stage small quantities of 

 one or other of the different gases, air, oxygen, carbonic oxide, 

 helium, argon, neon, and hydrogen itself, were let into the 

 tube through drying-tubes, so as to increase the pressure 

 sufficiently to allow the discharge to pass, although it was 

 still very small ; the appearance of the Ganalstrahlen with 

 equal currents through the tube was the same in all cases 

 although the amount of hydrogen must have varied enormously. 

 In another experiment, made since the publication of my 



