380 Prof. Thomson, On the theory of the motion, etc. 



the velocity in the mixture instead of being equal to it. We may 

 therefore conclude that if there is no transference of the charge 

 the mass of the ion must be a considerable multiple of that of the 

 molecule. 



We must therefore consider whether the hypothesis of 

 complex positive ions is sufficient to explain the results which 

 have been obtained as to the mobility of the positive ion. The 

 expressions given for the mobility shew that when the mass of 

 the ion is large compared with the mass of a molecule of the gas 

 through which it is moving the mobility of the ion will be 

 independent of the electric charge carried by it, practically 

 independent of the mass of the ion, and at constant pressure will 

 vary directly as the absolute temperature, and will be inversely 

 proportional to the V^n (//, — !), where m is the mass of a molecule 

 of the gas through which the ion is moving and fj, its refractive 

 index. The result that the velocity is independent of the charge 

 and mass of the ion and depends only on the nature of the gas 

 through which the ions are moving is not in accordance with the 

 results obtained by Professor H. A, Wilson for the mobility of 

 ions in salted flames, for he found that the velocity of the 

 positive ions when salts of the alkaline earths were placed in 

 the flame was but little more than half the velocity of the positive 

 ions when salts of the alkali metals were placed in the flame. 

 Again though, as Mr Phillips' results shew, the law that the 

 mobility at constant pressure is directly proportional to the 

 absolute temperature is very approximately obeyed at moderate 

 temperatures, it breaks down altogether at temperatures as high 

 as those which occur in flames. Thus the experiments of H. A. 

 Wilson and Moreau shew that at a temperature of about 2000° C. 

 the velocity of the positive ions in flames impregnated with salts 

 of the alkali metals is about 60 cm./sec. for a potential gradient 

 of 1 volt per centimetre ; the velocity of positive ions through 

 air at 0° C. is about 1*4 cm./sec, so that the velocity at 2000° C. 

 would if it were proportional to the absolute temperature be 

 12 cm./sec, which is only one-fifth of the actual value. The rapid 

 increase in mobility with temperature is what we should expect if 

 the positive charge could, like the negative, pass from one molecule 

 of the gas to another. 



