438 ANALOGIES OF 



and repulsions take place in vacua as well as 

 under the pressure of the atmosphere. See 

 Philosophical Transactions for 1822, where the 

 experiments of Davy are recorded. 



It is very true, that electricity in the diffused 

 state, moves more freely over the surface of 

 bodies, than through their substance ; and that 

 atmospheric air, when perfectly dry, assists in 

 preventing its escape from other bodies, because 

 it is a bad conductor. It was before stated, that 

 an electric spark expands in an exhausted re- 

 ceiver into a diffused lambent light and Mr. 

 Harris of Plymouth, has proved, that the striking 

 distance of the spark varies inversely as the 

 density of the air through which it passes, the 

 charge being the same. But these experiments 

 afford no proof that electricity is not attracted 

 by ponderable matter. The important practical 

 application of metallic conductors, as lightning 

 rods by Franklin, was founded wholly on their 

 attraction for the electric fluid. It never entered 

 into the imagination of this eminent philosopher, 

 that flashes of lightning are determined to the 

 earth and its conductors by the pressure of the 

 atmosphere. 



That electricity, when concentrated, is capable 

 of passing through the substance of bodies, was 

 proved by Jallabert of Geneva 70 years ago, 

 who found that it might be readily transmitted 

 through metals, covered over with pitch. (Priest- 



