566 On the Masses of the Ions in Gases at Low Pressures. 



A positively electrified atom is an atom which has lost some 

 of its " free mass," and this free mass is to be found along 

 with the corresponding negative charge. Changes in the 

 electrical charge on an atom are due to corpuscles moving 

 from the atom when the positive charge is increased, or to 

 corpuscles moving up to it when the negative charge is in- 

 creased. Thus when anions and cations are liberated against 

 the electrodes in the electrolysis of solutions, the ion with the 

 positive charge is neutralized by a corpuscle moving from 

 the electrode to the ion, while the ion with the negative 

 charge is neutralized by a corpuscle passing from the ion to 

 the electrode. The corpuscles are the vehicles by which 

 electricity is carried from one atom to another. 



We are thus led to the conclusion that the mass of an 

 atom is not invariable : that, for example, if in the molecule 

 of HC1 the hydrogen atom has the positive and the chlorine 

 atom the negative charge, then the mass of the hydrogen 

 atom is less than half the mass of the hydrogen molecule H 2 ; 

 while, on the other hand, the mass of the chlorine atom in 

 the molecule of HC1 is greater than half the mass of the 

 chlorine molecule Cl 2 . 



The amount by which the mass of an atom may vary is 

 proportional to the charge of electricity it can receive; and 

 as we have no evidence that an atom can receive a greater 

 charge than that of its ion in the electrolysis of solutions, and 

 as this charge is equal to the valency of the ion multiplied by 

 the charge on the hydrogen atom, we conclude that the 

 variability of the mass of an atom which can be produced by 

 known processes is proportional to the valency of the atom, 

 and our determination of the mass of the corpuscle shows 

 that this variability is only a small fraction of the mass of the 

 original atom. 



In the case of the ionization of a gas by Rontgen or ura- 

 nium rays, the evidence seems to be in favour of the view that 

 not more than one corpuscle can be detached from any one 

 atom. For if more than one were detached, the remaining 

 part of the atom would have a positive charge greater than 

 the negative charge carried by each of the detached cor- 

 puscles. Now the ions, in virtue of their charges, act as 

 nuclei around which drops of water condense when moist 

 dust-free gas is suddenly expanded. If the positive charge 

 were greater than the individual negative ones, the positive 

 ions would be more efficient in producing cloudy condensation 

 than the negative- one, and would give a cloud with smaller 

 expansion. As a matter of fact, however, the reverse is the 

 case, as C. T. R. Wilson (Phil. Trans. 1899) ha.v shown that 

 it requires a considerably greater expansion to produce a 



