32 president's address— SECxiojir a. 



I shall state the value of tlie mobility as that of the A'clocity, in 

 ■centiinetres per second, due to an electric force represented by a 

 potential gradient of one volt per centimetre. 



Two types of ion are recognised as existing naturally in the air, 

 the small ion, with, a mobility of about I'o under normal conditions, 

 and another, discovered by M. Langevin,* and called by him the large 

 ion, which is characterised by the very small mobility of only 1/3000. 

 To these I now add a third, which has a mobility of about 1/100 

 under normal circumstances. It may be called, for the present at 

 least, the ion of intermediate mobility, or the intermediate ion. 



M. Blochf finds in air bubbled through water ions of mobility of 

 the order of O'l or 0'2; these seem to form a fourth class of ions, and 

 it would be interesting to know if they exist in air not specially 

 treated. 



The small atmospheric ions are identical with those artificially 

 produced in air by ionising agents which have been made the subject 

 of such numerous researches as described by Professor Bragg in his 

 address. There is now considerable knowledge, resumed in the 

 beautiful kinetic theory of gases, of molecular movements and dimen- 

 sions, and when it is thought that an ion moves more slowly in an 

 electric field than would a single molecule if charged, as the ion must 

 be made of the stuff of the gas in which it is formed, what more 

 natural than to consider it a cluster of a few molecules? This idea 

 has been generally adopted. The small ions are thus assumed to be 

 of somewhat greater size than their fellow molecules; but, as the 

 mobility notably increases with decrease of pressure, and with rise 

 ■of temperatm-e, their diameter is appai-ently not a constant quantity. 



The direct argument, Avhich is used to support this view, con- 

 siders that in the numerous collisions which occur between the 

 •charged and uncharged molecules, in many cases the kinetic energy 

 of the latter will not be great enough to carry them away, after 

 impact, from the attraction of the charge. Tlie charged molecule 

 will thus collect other molecules around it; but, as the effect of the 

 charge on the outer members of the cluster diminishes as the collec- 

 tion of molecules increases, the growth will cease when the size is 

 such that the attraction of the charge at the surface of the cluster, 

 in grazing im])act of ion and molecule, is just insufficient to hold the 

 latter as a permanent member of the ionic system. The principle 

 involved, in calculating the value of the limiting radius, is similar to 

 that whicli determines whether a comet, in its close approach to the 

 sun, shall become a permanent member of the solar system or wander 

 into the space from which it came. The calculation of the ionic size 

 which has been made on these lines assumes the ions as charged, the 

 molecules as uncharged conducting spheres, and taking the radius of 

 the molecules as 10 "^ centimetres, reaches the conclusion that the 

 radius of the ion cannot exceed three times this value. 



* Langevin, C.R., t. 140, p. 232, 1905. 

 + Blccli. C.R., t. Hn, p. 54, 1907. 



