NUCLEI OF POSITIVELY AND NKCATIVF.l.Y CHARGED IONS. 301 



wntrr condense on them ; a fact in agreement with the results of the present investi- 

 gation, since negative ions were, in this case, plainly in question. The absence of fog 

 when the expansion was made with the field reversed was, however, as we now see, 

 no proof that no ions escape from the positively charged plate, for the expansions used 

 were insufficient to cause water to condense on positive ions had they lieen present. 

 Another case of interest is that of the nuclei produced by the discharge from a point. 

 Even when positive electricity was escaping from the point, fogs were obtained with 

 expansions which are now seen to have been insufficient to catch positive ions. We 

 are therefore driven to the conclusion that the positive discharge does not consist 

 simply in the escape of positive ions from the point of the wire, but that negative ions 

 (or nuclei of some other kind than ions) are present as well, possibly produced by the 

 action upon the moist air of the radiation from the glowing point of the wire. 



Indications had already been noticed of an increase in the number of drops, produced 

 in ionised air as v t /i 1 was increased beyond the point now shown to be that at which 

 the positive ions first begin to act as condensation nuclei. Professor THOMSON was 

 indeed led to make the suggestion contained in the words quoted at the beginning of 

 this paper by noticing indications of such an increase in the neighbourhood of the 

 point t'j/v, = 1 '3. My own observations had also previously led me to believe that 

 there was an increase at the point v^v l = 1'31 ; for example, my notes for March 4, 

 1898, contain the remark, "Many experiments with air, as well as H, seem to show 

 that there are nuclei requiring expansion =1*31 to catch them in addition to those 

 appearing at 1'25." 



Positive and negative ions (at least those produced in air by Rontgen rays) have 

 now been proved to differ in their efficiency as condensation nuclei ; they also differ, 

 as ZELENY has shown, in the velocity with which they move in an electric field of 

 given strength. The negative ions move the faster and are the more efficient as 

 nuclei for the condensation of water vapour. 



A possible way of accounting for both differences is to suppose that the charge 

 carried by the negative ions is greater than that carried by the positive, the number 

 of the latter being, of course, correspondingly greater. We might, for example, take 

 the view that the ionisation consists in the breaking-up of the neutral molecules into 

 a certain number of positive ions and half as many negative ions, each carrying twice 

 as large a charge as the positive, a process which we can readily imagine to take place 

 in the case of water molecules. 



The experiments already described make this view hardly tenable, for we have 

 seen that when the expansion is sufficient to make water condense on all the ions 

 (vi/Vi = 1'35 or more) the fogs in the two halves of the apparatus are indistinguish- 

 able in appearance and in the rate of fall of the drops. We have still to consider 

 to what extent this proves equality in the number of positive and negative ions 

 produced. 



The velocity with which the drops fall is proportional to the square of the radius, 



