90 
MR. GROVE ON THE ELECTRO-CHEMICAL POLARITY OF GASES. 
an inch of mercury ; and except where otherwise stated, a similarly attenuated me- 
dium was employed for all the following experiments. 
Two small cells of the nitric acid battery, each plate exposing 4 square inches of 
surface, were used to excite the coil machine, and the discharge from the secondary 
coil was taken between the steel point and the silver plate. The distance between 
these was generally =0T of an inch, but this may be considerably varied. When 
the plate formed the positive terminal, a dark circular stain of oxide rapidly formed 
on the silver, presenting in succession yellow, orange and blue tints, very similar to 
the successive tints given by iodizing in the ordinary manner a Daguerreotype plate. 
Upon the poles being reversed and the plate made negative, this spot was entirely 
removed, and the plate became perfectly clean, leaving, however, a dark, polished 
spot occasioned by molecular disintegration, and therefore distinguishable from the 
remainder of the plate. 
The experiment was repeated a great many times, and with varying proportions of 
gas, and I found that with proportions varying from equal volumes of hydrogen and 
air to those of one volume of the former to two and a half of the latter, the experi- 
ments succeeded ; better, I should say, when there was rather an excess of hydrogen 
as compared with the equivalent of oxygen in the atmospheric air ; about one volume 
of hydrogen to one and a half of air succeeded well ; when excess of air was present, 
oxidation took place whether the plate was positive or negative, and when excess of 
hydrogen was present no oxidation took place. 
2nd. I experimented with an air vacuum (to borrow an expression of Dr. Faraday), 
and found that oxidation took place whether the plates were positive or negative, 
but in different degrees; when the plate was positive, a small circular spot was 
rapidly formed, quickly deepening in colour, and apparently eating into the plate ; 
when the plate was negative, a large diffuse spot was formed, the oxidation was more 
slow, and the plate not so rapidly corroded. 
3rd. I now operated with a hydrogen vacuum ; when the plate was clean no dis- 
coloration took place, the plate retained its polish, though after a long continuance 
of the discharge a molecular change was perceptible, producing a frosted appearance 
similar to the mercurialized portions of a Daguerreotype. 
When the plate had been previously oxidated by the discharge in an air vacuum, 
the oxidation was rapidly and beautifully cleared off by the discharge in the hydrogen 
vacuum, and this whether the plate was positive or negative, the effect being, how- 
ever, better and more rapidly produced in the latter case. 
4th. I substituted respectively for the steel needle, wires of copper, silver and 
platinum, and found the effect produced by all and with nearly equal facility; if 
there were any difference, the platinum point was the least efficient; this maybe 
due to the peculiar effect of platinum in itself combining the gases, or to its in- 
oxidable character, the oxygen being thrown off from its surface, and not uniting 
with it as with the more oxidable metals; the flame or luminous appearance which 
