218 Prof. Town send on the Conductivity produced in 



We can find S from the present experiments since it is 

 practically the same as d. 



The number of collisions made by an ion in going through 



a centimetre of air at one millimetre pressure is : — ■ = 21 , 



n being the number of molecules in a c.c. at atmospheric 

 pressure. Taking for n the value 2 X 10 19 , we obtain 

 S = l'6xl0- 8 *. " 



I may mention that the mean free path of an ion in air is 

 shorter than its mean free path in hydrogen, but as the 

 experiments with this gas are not yet completed, I must 

 reserve the account of them for a future paper. 



The number of ions produced in a gas by a single ion 

 moving rapidly through it could be easily found by another 

 method. It has been shown that the charge on the corpuscles 

 emanating from radium can be measured f, when a highly 

 active specimen of the substance is used. The ratio of the 

 ionization produced per centimetre of a gas to the charge 

 carried by the corpuscles would give the number of ions pro- 

 duced by each corpuscle in travelling through a centimetre. 



1 have made experiments with a view to finding the above 

 ratio, but the specimen of the radio-active substance at my 

 disposal was so weak that it was impossible to detect its charge 

 although a very sensitive method was employed. The ex- 

 periments, however, showed distinctly that each corpuscle in 

 travelling through a centimetre of air at atmospheric pressure 

 produces at least 10,000 ions. According to the theory 

 which I have described, the number should be 7(50x21, it 

 the absorption of the radiation by the gas be neglected. I 

 hope to be able to repeat this investigation with some highly 

 active radium and obtain an accurate result. 



9. I have already mentioned that the conductivity be- 

 tween the two parallel plates is altered when the electro- 

 motive force is reversed. The difference between the 

 currents in the two directions is a direct consequence of 

 an effect discovered by Curie and Sagnac J. They found 

 that the secondary rays, which are given out when 

 Bontgen rays fall on a metal, carry a small negative charge. 

 ]t would therefore appear that the ionization produced by 

 secondary rays is due to charged corpuscles emitted by the 

 metal. The bodies which produce the greatest secondary 

 ionization also emit the largest charges. Thus the bra&s 



* John S. Townsend, Phil. Trans, vol. cxciii. (1899). 

 t M. P. Curie et Mme. Curie, Comptes liendus, vol. cxxx., 5th March, 

 1900. 



j P. Curie et G. Sagnac, Comptes Bendus, vol. cxxx., 9th April, 1900, 



