336 Ofi the Electricity of effluent Air and Steam. 



The superior intensity of the electrical development which 

 accompanies the emission of high-pressure steam, and the fact 

 of the upper or distended portion of the jet being more highly 

 electrified than the lower or unexpanded part, seem to favour 

 the supposition that the evolution of electricity in the steam- 

 cloud depends upon the dilatation of the steam. I have al- 

 ready adduced, in former communications, reasons for reject- 

 ing this hypothesis ; but feeling anxious to remove all doubt 

 upon the subject, I resorted to the following method of de- 

 termining the question. To the cock F, which terminates 

 the copper pipe, I attached a metallic cylinder, which was kept 

 sufficiently hot to prevent any condensation of steam taking 

 place within it, and was perforated at one end with a multi- 

 tude of small holes, through which the steam, after expanding 

 in the cylinder, was suffered to escape at a density scarcely 

 exceeding that of the atmosphere. Now if expansion had 

 produced the development of electricity, the steam would have 

 become electrical in the cylinder, and would have parted with 

 its electricity in passing through the holes, and thus the steam- 

 cloud would have been rendered neutral ; but so far from that 

 beinCT the case, I could perceive no diminution of electricity 

 when the steam was subjected to this treatment. We may 

 therefore confidently infer that the liberation of electricity 

 in the jet does not proceed from expansion, and that although 

 the intensity of the electrical development is so greatly af- 

 fected by pressure in the boiler, it is quite independent of the 

 density at which the steam is ejected. 



The precipitation of the steam appears then to be the only 

 cause to which the liberation of electricity in the jet can be 

 assigned. Assuming this conclusion to be correct, the next 

 question is, does the precipitation of the steam give rise to the 

 electricity of the evaporating vessel, as well as to that of the 

 steam-cloud; or is the electricity manifested in the boiler, the 

 effect of evaporation, or of some other process distinct from 

 that which excites electricity in the jet? The neutrality of 

 the boiler when the steam is confined, seems to imply, that 

 the same cause which produces electricity in the cloud oc- 

 casions also the opposite electricity in the evaporating vessel. 

 I endeavoured to resolve the question so far as regards the 

 ao^ency of evaporation, by discharging the steam which from 

 time to time remained in the boiler, after all the water had 

 been evaporated, and the boiler always became electrified, 

 provided the pressure of the steam were considerable. The 

 cessation of escape from the valve was the only indication 

 I could have of the perfect dryness of the boiler ; and if re- 

 liance may be placed upon this criterion, the conclusion I 



