698 Specific Ionization produced by Corpuscles of Radium. 



support to the theory that the atoms of different sub- 

 stances are different systems of similar corpuscles, the 

 number of which in the atom is proportional to its atomic 

 weight. 



In conclusion I wish to thank Profs. J. J. Thomson and 

 Wilberforce for their adyice and interest in this work. 



University College, Liverpool. 



LXXVI. Specific Ionization produced by Corpuscles of Radium . 



To the Editors of the Philosophical Magazine. 



Gentlemen, 

 IV/TR- DURACK has given an account (Phil. Mag. May 

 -LfJL 1903) of some experiments on the specific ionization 

 produced by corpuscles given out by radium, from which he 

 concludes that the number which I obtained for the same 

 quantity was too large. The specific ionization produced by 

 the corpuscles is the number of ions that a single corpuscle 

 will generate in going through a centimetre of gas at a 

 millimetre pressure. 



From the experiments which I have made with ultra- 

 violet light I have shown that negative ions, when moving 

 with a sufficiently high velocity, produce others by collisions 

 with molecules of the gas. The number of ions produced by 

 a single ion increases as the force is increased, and reaches a 

 maximum when the force is above a certain value depending 

 on the pressure. The voltage in these experiments was com- 

 paratively low, as it seldom exceeded 300 volts. For air at 

 a millimetre pressure the maximum number of ions which a 

 single ion generates is 15. 



I also made some experiments with radium which led me 

 to conclude that each of the corpuscles which are given out 

 generates at least 13 ions per centimetre in air at a milli- 

 metre pressure. 



Mr. Durack finds from his experiments that this number 

 ought to be *T, and he attributes the difference between his 

 result and the result which I obtained with ultra-violet light 

 to the very high velocity of the corpuscles given out by 

 radium, the velocity of these corpuscles being large compared 

 with the velocity corresponding to a fall of potential of 

 300 volts. 



According to this theory the probability that new ions 

 should be produced by a collision would diminish when the 

 velocity is increased above a certain value. 



Mr. Durack states that the result of my experiments is 

 wrong as it does not agree with his number *4. 



