president's ADDUESS SECTION A. 33 



To account for the eliang-e of mobility j,ssociated with alteration 

 of the pressure or temperature conditions, it is supposed that the 

 clusters of molecules forming the ions consist of fewer members at 

 low pressures and at high temperatures than under ordinary circum- 

 stances. As the temperature rises, for instance, the ion may be 

 ijTiagined as shedding one by one its component molecules. The 

 mobility, however, varies continuously, and not by jumps; it may, 

 therefore, be considered, in addition, that a cluster at any tempera- 

 ture does not always consist of the same number of molecules. In 

 the numerous collisions, to which an ion as a constituent of a gas is 

 subjected, a molecule of the cluster may be lost at one, to be gained 

 at another impact, the cluster acting on the whole as if it contained 

 the average number of members ; it is this average number which, 

 from this point of view, must be taken as decreasing; continuously 

 "with rise of temperature. 



From a consideration of tiie slow movement of the ions in an 

 electric field compared to that which it is assumed a single charged 

 molecule would have in the same circumstances, it is possible, with 

 the aid of the principles of the kinetic theoiy, to make an estimate of 

 the number of molecules wliich go to make an ion. The argument is 

 given in Mr. Pliillips' paper on '" Ionic Velocities in Air at Different 

 Temperatures,"* and he calculates from his results that the positive 

 ion at — 179^ C. consists, on the average, of about four and a half 

 molecules (4'63), while at + 138° C. the average number is only 

 about one and a half (1'5'2). For the negative ion slightly smaller 

 figures are obtained. 



Such an idea of the small ion, based, either on the direct argu- 

 ment in its restricted form already noted, or on the calculation jnst 

 mentioned, cannot be considered satisfactory, and it is now^ shown 

 to be unnecessary by two workers at opposite sides of the world, Mr. 

 Wellisch, at Cambridge, and Mr. William Sutherland, at Melbourne. 



In this connection it is interesting to recall another physical 

 problem which apparently also required for its exjjlanation a shrink- 

 age of the molecvdes with rise of temperature, that of the relation 

 between the temperature and the viscosity of a gas. Tlie solution of 

 the problem was finally reached in l(Si)3 by Mr. Sutherland, from a 

 consideration of the influence of molecular force in bringing about 

 collisions which would otherwise not occur, the investigation being 

 published in his paper on ■" The Viscosity of Gases and Molecular 

 Force."! The result of mutual attraction, only sensible at small 

 distances, is to make the molecules, considered forceless, behave as if 

 they had a diameter greater than the true value. As the molecular 

 force is less effective in causing collisions tlie greater the velocity 

 with which two molecules approach each other, the apparent diameter 

 to which it gives rise is less the higher the temperature. It is now 

 shown by the writers I have mentioned that there is a similar effect 

 due to the ionic charge. Owing to the influence of the electrical 

 attraction, collisions between ions and molecules take place which 



* Phillips. Proc. R.S. A, 78, p. 167, 1900. 

 t Sutherland. Phil. Mag. 3(5, p. 507, 1893. 



