NUCLEI OF POSITIVELY AND NEGATIVELY CHARGED IONS 307 



drojw in negative ; in other words, that solutions of hydrogen peroxide in water, 

 surrounded by air, attract negative electricity more than positive. 



The view here suggested is, therefore, that the nuclei requiring the definite expan- 

 sions 1'25 or 1'31 to make water condense on them are always really ions, the cases 

 in which an electric field is without influence upon the result of expansion being 

 explained by supposing that the ionisation does not in such cases take place until 

 supersaturation is produced. 



The separation of the positive and negative electricity by the formation of drops on 

 the negative ions only, as soon as the supersaturated state reaches the necessary 

 limit, will take place equally well whether the ions exist before the supersaturation is 

 produced or are the result of the supersaturation. Moreover, if the initial growth of 

 a drop, as above suggested, is able to cause it to acquire a charge equal to that of one 

 ion, the further growth of the drop may result in an increase of the charge. The 

 drops may thus acquire a charge considerably exceeding that of one ion, even if there 

 be no coalescence of small drops to form large ones. 



Further experiments on this point are evidently required. 



I do not propose to discuss here the meteorological bearings of the results obtained. 

 The questions with which the experiments are concerned are, I think, fundamental 

 ones in connection with the electrical effects of precipitation. From this point of 

 view the principal results of this investigation are : 



(1.) To cause water to condense on negatively charged ions, the supersaturation 

 must reach the limit corresponding to the expansion v 2 /t>, = T25 (approximately a 

 fourfold supersaturation). To make water condense on positively charged ions, the 

 supersaturation must reach the much higher limit corresponding to the expansion 

 v 2 /v, = 1/31 (the supersaturation being then nearly sixfold). 



(2.) The nuclei, of which a very small number can always be detected by expan- 

 sion experiments with air in the absence of external ionising agents, and which require 

 exactly the same supersaturation as ions to make water condense on them (as well as 

 the similar nuclei produced in much greater numbers by the action of weak ultra- 

 violet light on moist air) cannot be regarded as free ions, unless we suppose the ionisa- 

 tion to be developed by the process of producing the supersaturation. 



We see, then, that if ions ever act as condensation nuclei in the atmosphere, it must 

 be mainly or solely the negative ones which do so, and thus a preponderance of 

 negative electricity will be carried down by precipitation to the earth's surface. 



The experiments described in this paper were carried out at the Cavendish 

 Laboratory. 



NOTE ADDED SEPTEMBER 25, 1899. 



In order that the results of these investigations should have a direct bearing on 

 the subject of atmospheric electricity, it is necessary to assume that condensation in 

 the atmosphere frequently takes place from the supersaturated condition. There is 



2 R 2 



