138 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



often dense. It is also an explanation of those indefinite alternations 

 of large and small coronas (periodicity). 



Further experiments in the wooden fog chamber showed that 25 to 

 50 per cent of the fleeting nuclei could be caught and made stable on 

 solution. The mean datum of all results, irrespective of the time inter- 

 val (the effect of which within a few minutes is probably inappreciable), 

 is a preservation of 39 per cent on reevaporation of the originally 

 fleeting nuclei from the fog particles . condensed on them. In the 

 absence of subsidence of fog (which can not here be allowed for), the 

 result would be larger. In the glass fog chamber, which could be 

 made rigorously free from leakage, but in which the subsidence loss 

 was greater, 20 per cent persisted, while all but 2 to 5 per cent were 

 lost in one minute in the absence of solution. Persistence in the i to 

 3 minutes following evaporation was not appreciably different, indicat- 

 ing long periods of decay. (Cf. fig. 60, Chapter III.) 



95. Solutional enlargement of the nuclei of dust-free air. The most 

 interesting case observed is the marked decrement of the fog limit of 

 dust-free air producible by solution of the nuclei. (Cf. figs. 61, 62, 

 Chapter III.) Let the first exhaustion be made decidedly above the 

 fog limit, 8p=22 crn. (say here at 8^ = 23 cm.), to obtain a corona of 

 appreciable size. On evaporating the fog particle, let the second exhaus- 

 tion be made decidedly below the fog limit (say at S/> = 2i cm.). A 

 large corona, which would otherwise be quite absent, will be observed. 

 Within 3 minutes about 25 per cent of the nuclei are found to persist 

 After 10 minutes not more than 50 per cent were left. Whether, 

 without solution, air nuclei are possibly evanescent, and therefore 

 maintained by some penetrating radiation, remains to be seen. It 

 should be recalled that the nuclei in question are small, even as com- 

 pared with ions. 



96. Water nuclei Solutional nuclei in general. Apart from the 

 functions suggested in sections 94 and 98, it is clear that the water 

 nucleus must play an important part throughout all phenomena in 

 nucleation. It seems probable that immediately after exhaustion pre- 

 cipitation takes place on all nuclei, large and small, within the scope 

 of the pressure difference applied. The smaller fog particles then at 

 once begin to evaporate until the decrement of vapor pressure, due to 

 increasing concentration of the solution, is equivalent to the incre- 

 ment of vapor pressure due to decreasing size, whereupon evaporation 

 ceases. This is the condition of persistence of a water nucleus, the 

 ultimate size of which depends on the original strength of the solu- 



