of Electrification by Evaporation. 399 



2. From solutions of alkaline solids the water evaporated 

 carries away a negative charge. 



» 3. From solutions of gases, acids, or salts the water carries 

 off a positive charge. 



Tait and Wanklyn repeated Pouiilet's experiments with a 

 more sensitive electrometer. They detected electrification 

 where Pouillet had failed to do so, e. g. in the evaporation of 

 distilled water ; and in some other cases the effects obtained 

 were totally different in kind and degree from those obtained 

 by Pouillet and earlier experimenters. 



In these experiments a few drops of liquid were dashed on 

 a red-hot platinum dish insulated and connected with an elec- 

 trometer. So long as the spheroidal state continued, little or 

 no electrification was observed; but as soon as violent vapori- 

 zation began, decided deflections were obtained. This fact 

 and other circumstances about the experiments indicate that 

 friction was the chief source of electrification, and make it 

 doubtful whether evaporation had any part in producing the 

 observed electrification. Tait and Wanklyn recognized the 

 fact that to friction was due a large part of the electrification 

 which they obtained. Still, in his lectures on Thunder- 

 storms*, Professor Tait, speaking of atmospheric electricity, 

 says: — "In calm clear weather the atmospheric charge is 

 usually positive. This is very commonly attributed to evapo- 

 ration of water ; and I see no reason to doubt that the pheno- 

 mena are closely connected." After reading these lectures, 

 the writer of this article decided to investigate whether eva- 

 poration alone and under normal conditions causes electrifica- 

 tion, and whether such electrification, if any, is sufficient to 

 account for the phenomena of atmospheric electricity. In this 

 connexion it will be remembered that Faraday traced the elec- 

 tricity, produced by the escape of steam or water from orifices, 

 to friction, and concluded it was not due to evaporation, nor 

 did it have any bearing on atmospheric electricity. 



The investigation was begun with the expectation, on the 

 part of the writer, of finding electrification ; Prof. Rowland, 

 to whom the author is much indebted, believed that no elec- 

 trification would be found. In any case the subject seemed 

 important enough to warrant experiments, which were free 

 from the objections which hold against those already described. 

 At the very beginning of the investigation it was found that 

 at most the electrification due to evaporation would be ex- 

 tremely small, and very great caution would be necessary to 

 eliminate various sources of error. The apparatus finally used 



* 'Nature,' vol. xxiii. p. 340. 

 2H2 



