ALTERNATIONS OF APERTURE. 29 



2?. Oscillations at fixed pressure differences. Effectively the case 

 of oscillation is one in which the large, sparsely distributed fog par- 

 ticles emit more nuclei and the ver}^ abundant small fog particles 

 fewer nuclei, i. e., the phenomena may be looked upon as though 

 the nuclei were generated during the growth of the fog particles. 

 This plausible result, however, is not to be maintained ; for the emis- 

 sion would have to be as the growth of surface in other words, as the 

 volume and the number of particles varies inversely as their volume. 

 A counter supposition may be hazarded to the effect that the fog par- 

 ticles of large coronas absorb more nuclei because of their abundance 

 than the fog particles of small coronas. But the period of suspension 

 of particles is too short a period to be cf moment. 



If negative ions are more active as condensation nuclei than posi- 

 tive ions, the results observed may be tentatively grouped in accord- 

 ance with the following scheme : 



< > 



V V 



L,et the ions be originally neutral as a whole, and suppose, as in 

 case i , that the negative ions are first precipitated. In the interval 

 between this and the next exhaustion fresh ions are generated or taken 

 in through the filter, as shown in case 2. If these negative ions 

 partially neutralize the positive ions left over in case i , the second 

 precipitation takes place on the positive ions. Thereafter, case 3, the 

 first is repeated, etc. But if the coronas are taken as a measure of the 

 number of particles, the number of effective nuclei must be eight times 

 larger in the first case than in the second, whereas the ions should be 

 present in equal numbers. Hence there is serious objection to this 

 hypothesis at the outset, quite apart from the absolute numbers in 

 question, which are enormously too large to be referred to ions. 



28. Undersaturation. Some mechanism of this kind is nevertheless 

 probable, and it will suffice if the Undersaturation produced by the 

 precipitation of fog particles is not rapidly made up by diffusion and 

 convection, or, even better, if water nuclei are produced by evaporation 

 of fog particles. Of the above hypotheses that of undersaturatiou has 

 broad bearings and accounts qualitatively for most of the phenomena, 

 as will presently be pointed out in detail. True, the large coronas 



