-1859] WRITINGS OP JOSEPH HENRY. 347 



Pouillet also supposed that the process of vegetation was 

 a source of disturbance of the electrical equilibrium, but this 

 has not been supported by critical experiments. 



The discovery accidentally made a few years ago of the 

 great amount of electricity evolved in blowing oflf steam 

 from the boiler of a locomotive, seemed to afford a ready 

 explanation of the electrical state of the atmosphere. It was 

 then attributed to the condensation of the aerial vapor. 

 Faraday proved however by one of his admirable series 

 of model experiments, that this effect was due entirely to 

 the friction of the water (which escaped in connection with 

 the steam) on the side of the orifice through which the 

 discharge took place. When dry steam, or that which is 

 so heated as to contain no liquid water, was blown out, all 

 electrical excitement disappeared ; and when condensed air 

 — even at elevated temperatures, was discharged from an in- 

 sulated fountain, no electricity was produced. 



The celebrated physicist of Geneva, Professor De la Rive, 

 refers the electricity of the atmosphere to thermal action. 

 It is well known that if the lower end of a bar of iron (or of 

 any other metal not readily melted) be plunged into a source 

 of heat while the upper end remains cool, a current of elec- 

 tricity will flow from the heated to the cooled end, the former 

 becoming negative and the latter positive, and that these 

 different states will continue as long as the difference of tem- 

 perature is maintained. Now according to Professor De la 

 Rive a column of the air is in the same condition as the bar 

 of metal — its lower end is constantly heated by the earth 

 and its upper cooled by the low temperature of celestial 

 space. Unfortunately however for this ingenious hypoth- 

 esis, a column of air is a non-conductor of electricity, while 

 a bar of metal is a good conductor, and it still remains to 

 be proved that such a distribution of electricity as that we 

 have described relative to the bar of metal can be produced 

 in a column of air. 



The foregoing are the principal hypotheses which have 

 been advanced to account for what has been considered the 

 free electricity of the atmosphere. After an attentive study 



