ELECTRICITY. 91 



ated air a discharge may be made to pass across six or seven 

 feet of space, while in air of the ordinary density it would 

 not pass across an inch. An observer regarding the beauti- 

 ful phenomena exhibited by this electric discharge in attenua- 

 ted gas, which, from some degree of similarity in appear- 

 ance to the Aurora Borealis, has been called the electric Au- 

 rora, would have some difficulty in believing such effects 

 could be due to an action of ordinary matter. The amount 

 of gas present is extremely small ; and the terminals, to a 

 cursory examination, show no change after long experiment- 

 ing. It is therefore not to be wondered at that the first ob- 

 servers of this and similar phenomena, regarded electricity 

 - as in itself something as a specific existence or fluid. Even 

 in this extreme case, however, upon a more careful examination 

 we shall find that a change does take place, both as regards the gas 

 and as regards the terminals . Let one of these consist of a high- 

 ly-polished metal a silver plate is one of the best materials for 

 the purpose and let the discharges in attenuated atmospheric 

 air take place from a point, say a common sewing needle, to the 

 surface of the polished silver plate ; it will be found that this 

 is gradually changed in appearance opposite the point it is ox- 

 idated, and gradually more and more corroded as the discharge 

 is continued. 



If now the gas be changed, and highly-rarefied hydrogen 

 be substituted for the rarefied air, all other things remaining 

 the same, upon passing the discharges as before the oxide 

 will be cleared off the plate, and the polish to a great extent 

 restored not entirely, because the silver has been disinte- 

 grated by the oxidation and the portion which has been af- 

 fected by the discharge will present a somewhat different ap- 

 pearance from the remainder of the plate. 



A question will probably here occur to the reader : What 

 will be the effect if there be not an oxidating medium pres- 

 ent, and the experiment be first performed in a rarefied gas, 

 which possesses no power of chemically acting on the plate ? 



