100 ANNUAL OF SCIENTIFIC DISCOVERY. 



bution of electrical excitement, we may observe that, if this electri- 

 cal shell were of uniform thickness (and we see no reason to suppose 

 it should vary in di IK- rent parts in this respect), it would follow, from 

 the law of central forces, that it could have no effect in disturbing 

 the equilibrium on the surface or in the interior of the earth ; a par- 

 ticle of matter remaining, as we have seen, at rest or unaffected at 

 any point within a hollow sphere. Tliis fact appears to militate 

 against the truth of this assumption. 



Another hypothesis attributed the electricity of the atmosphere to 

 the friction of the winds on each other and on the surface of the 

 earth ; but careful experiments have shown that the friction of dry 

 air on air or of ah* on solids or liquids does not develope electrical 

 phenomena. 



The next hypothesis was advanced by Pouillet; which referred 

 the electricity of the atmosphere to the evaporation of water, par- 

 ticularly that which contained saline ingredients. But when pure 

 water is carefully evaporated in a space not exposed to the sky, no 

 electricity is produced except by the friction of the sides of the ves- 

 sel in the act of rapid ebullition ; and when the experiment is made 

 with salt water, the electrical effects observed are found to be pro- 

 duced by an analogous friction of the salt against the interior of the 

 vessel. When pure water is evaporated under a clear sky, the vapor 

 produced is negatively electrified ; but this state is contrary to that 

 in which the atmosphere is habitually found. 



Pouillet also supposed that the process of vegetation was a source 

 of disturbance of the electrical equilibrium, but this has not been sup- 

 ported by critical experiments. 



The discovery accidentally made, a few years ago, of the great 

 amount of electricity evolved in blowing off steam from the boiler 

 of a locomotive, seemed to afford a ready explanation of the electri- 

 cal state of the atmosphere. It was then attributed to the condensa- 

 tion of the aerial vapor. Faraday, however, conclusively proved, by 

 one of his admirable series of model experiments, that this effect 

 was due entirely to the friction of the water which escaped in con- 

 nection 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, is blown out, all electrical excitement 

 disappeared ; and when condensed air, even at elevated temperatures, 

 was discharged from an insulated fountain, no electricity was pro- 

 duced. 



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 read- 

 ily melted, be plunged into a source of heat, while the upper end re- 

 mains cool, a current of electricity 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 temperature 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 hypothesis, a column of air is a non-con- 



