360 Prof. J. S. Townsend on Ionization produced 



results from which the ionizing action of the positive ions 

 may be determined. With regard to these experiments 

 Dr. Stark states :— " Further, J. A. McClelland has already 

 made an observation which is similar to the phenomenon 

 studied by Townsend, but more simple."" Professor 

 McClelland's experiments *, to which this statement refers, 

 were made on the conductivity that takes place between a 

 cylinder and a concentric wire heated to incandescence. The 

 phenomena connected with the passage of a current through 

 a gas surrounding a hot wire are so complicated that 

 Professor McClelland was unable to come to a definite con- 

 clusion as to the action of the positive ions. Professor 

 McClelland states that the particular experiments which seem 

 to indicate that positive ions produce others by collision may 

 be explained by the action of the negative ions. Even if we 

 accept the first explanation and attribute the results of his 

 experiments to positive ions, we cannot be sure that they are 

 the same as the positive ions generated in a gas at ordinary 

 temperature as in my experiments. On this point Professor 

 McClelland says : — '^ Possibly the positive ions which are 

 active in producing secondary ionization also come from the 

 wire, which may account for the apparent difference between 

 them and the positive ions investigated in other cases of 

 ionization.'^ 



Dr. Stark is therefore under a misapprehension as to the 

 Avork in which I have been engaged. 



The first paper which Dr. Stark has published on the 

 genesis of ions by collision seems to be a communication to 

 the PhysiJcalisclie ZeitscJirift, which bears the date 13th 

 October, 1900. Since then he has published numerous 

 articles on the subject^ in which he has added to the theory 

 the '^ catalytic action " of the metal on the gas. It appears 

 that I have not understood the full import of the catalytic 

 action. My experiments with Rontgen rays were made 

 between two parallel plates, one of aluminium, the other of 

 brass. One set of experiments were made with the plates 

 one centimetre apart, and the other set with the plates 

 two centimetres apart. The results showed that the conditions 

 under which ionization by collision is brought about are pre- 

 cisely the same in the two cases. Dr. Stark interprets these 

 experiments as illustrating the catalytic action of brass on 

 air. Presumably therefore the catalytic action is the same 

 at tw^o centimetres from the metal as it is at one centi- 

 metre. As later experiments with shorter distances between 



* J. A. McClelland, Proc. Camb. PhH. Soc. xi. p. 296 (1901). 



