346 WRITINGS OP JOSEPH HENRY. [1855- 



a non-conductor, and is capable of confining electricity in 

 a condition of accumulation or of diminution, and of pre- 

 venting the restoration of the equilibrium that without the 

 existence of this insulator, would otherwise take place. 



The hypothesis was at first advanced that the earth at- 

 tracts the setherial medium of celestial space and condenses 

 it in a hollow stratum around the whole globe ; that the 

 electricity of the atmosphere is due to the action of this 

 exterior envelope. Dr. Hare, our countryman, has presented 

 this hypothesis with considerable distinctness. Without 

 denying the possibility or even probability of such a distri- 

 bution of electrical excitement, we may observe that if this 

 electrical shell were of uniform thickness, and we see no 

 reason to suppose it should vary in different parts in this re- 

 spect, 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 particle of matter 

 remaining, as we have seen, at rest or un-afFected at any 

 point within a hollow sphere. This fact appears to militate 

 against the truth of this assumption. 



Another hypothesis attributed the electricity of the at- 

 mosphere 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 air on solids or 

 liquids does not develop electrical phenomena. 



The next hypothesis — advanced by Pouillet, referred 

 the electricity of the atmosphere to the evaporation of 

 water, particularly that containing saline ingredients. But 

 when pure water is carefully evaporated in a space not ex- 

 posed to the sky, no electricity is produced except by the 

 friction with the sides of the vessel in the act of rapid ebulli- 

 tion ; and when the experiment is made with salt water the 

 electrical effects observed are found to be produced by an 

 analogous friction of the salt against the interior of the ves- 

 sel. When pure water is evaporated under a clear sky the 

 vapor produced is negativelj'' electrified; but this state is 

 contrary to that in which the atmosphere is habitually 

 found. 



