COXDLXTIOX THROUGH GASES 131 



the same as the mass of an atom, the mass of 

 the nesfative ion is about the eioi'hteen hundredth 

 part of the mass of the Hghtest atom known 

 to chemistry, that of hydrogen. The decrease 

 of the ionic velocity at low pressures probably 

 indicates an approach to this state of low ionic 

 mass. 



A similar decrease in the size of the negative 

 ion, compared with that of the positive, is produced 

 by raising the temperature. H. A. Wilson found 

 that, at 2000 C, the velocity of the negative 

 ions, produced by salts volatilised in flames, was 

 seventeen times greater than the velocity of the 

 positive ions. - 



The problem of determining the dimensions 

 of the ions at atmospheric pressure has been 

 attacked by measuring their rates of diffusion 

 into non-ionized gases. The rate of diffusion of 

 a gas depends on the mass of its molecule, and 

 experiments show that the mass of an ion at 

 atmospheric pressure is considerably greater than 

 that of the molecule of an ordinary gas. 



All these results may be explained by the 

 theory that the normal process of gaseous ioniza- 

 tion consists in the detachment from an atom of 

 the gas of a minute particle, called by Sir J. J. 

 Thomson a corpuscle. At extremely low pressures 

 the corpuscle constitutes the negative ion, and the 

 atom or molecule from which it has been separated 

 forms the positive ion. As the pressure rises, 

 neutral molecules become attached to the ions, 

 probably by virtue of the electric forces, and 

 collect round the original ion, which constitutes 

 the nucleus. These complex systems form the 

 ions of gases at atmospheric pressures. 



