808 ON CONDENSATION NUCLEI OF POSITIVELY AND NEGATIVELY CHARGED IONS. 



very little direct evidence of the existence of supersaturation in the atmosphere, but 

 there is at least an equal lack of evidence against its existence above the lower cloud 

 layers even as a normal accompaniment of precipitation. That supersaturation 

 occurs in connection with thunderstorms is held by v. BEZOLD and others (' Sitzungsb. 

 Akad. d. Wissenschaft. zu Berlin,' 1892). 



In the lower dust-charged layers of the atmosphere supersaturation is not to be 

 expected. When there is an ascending air current, however, the dust particles may 

 be retained in the lower cloud layers, through each becoming loaded with water, and 

 ceasing to rise as soon as a certain critical size, depending on the upward velocity of 

 the air, is attained. Supersaturation will exist under these conditions in the air 

 which has left its dust particles behind, and if the ascending current reaches a 

 sufficient elevation a second condensation will take place at a higher level, as was 

 pointed out in a former paper (' Phil. Trans.,' A, vol. 189, p. 286, 1897) ; the conditions 

 will then be such that the experimental results obtained in the present investigation 

 may be applied. 



Whether the drops will from the first be too large to be supported by the upward 

 current and therefore at once begin to fall as raindrops, growing rapidly as they fall 

 through the supersaturated layers, or will still continue to be supported by the 

 ascending current and form an upper cloud layer, depends on the upward velocity of 

 the air, the number of nuclei (negative ions ?) and other conditions. In either case 

 we should expect the drops to be negatively charged, the air rising above them 

 carrying a corresponding excess of positive ions. 



