336 WRITINGS OF JOSEPH HENRY. [1855- 



that we can study the fluid while it is actually in motion, and 

 from the results deduce inferences as to the mode in which 

 some of the effects are produced in the discharge of frictional 

 electricity. The two classes of phenomena however, though 

 referable to the same cause, are in many respects so differ- 

 ent in character that considerable caution is required in 

 drawing inferences from analogy. The phenomena of ordi- 

 nary electricity are characterized by an intensity of action 

 W'hich indicates a repulsive force between the atoms of the 

 hypothetical fluid, which is in some way — at least partially 

 neutralized, in the case of galvanism. 



Ordinary electricity in a state of equilibrium appears 

 to produce but a very feeble effect upon bodies in which 

 it is accumulated. However great may be the quantity 

 present, no effect is perceived by a person when insulated 

 on a glass stool, and charged either positively or nega- 

 tively, so long as the electricity remains at rest. If how- 

 ever it is drawn from him in the form of a spark, then a 

 disagreeable pricking sensation is experienced at the point 

 of rupture. Dr. Faraday constructed a small metallic house 

 or room, which he suspended by silk ropes in mid air, 

 and charged it so strongly that long sparks could be 

 drawn from the outside, yet not the least effect was perceived 

 by the persons within : even when the air of the interior of 

 the house was strongly electrified, the excitement was only 

 perceptible on the outside. 



It is fully established by the most satisfactory experiments 

 that in all cases in which a discharge of electricity takes 

 place by breaking through a stratum of non-conducting sub- 

 tance like air, there is an actual transfer of matter each way 

 between the two ends or sides of the opening in the conduc- 

 tor along the path which the spark traverses. If two con- 

 ducting rods be employed having the end of each termi- 

 nated by a brass ball, one of which is covered with gold leaf, 

 and the other with silver, a transfer in opposite directions 

 of these two metals will be observed. A similar effect is pro- 

 duced in the discharge of lightning from ihh clouds, and 

 there are several well authenticated cases on record, in 



