34 
M. DE LA RIVE’S RESEARCHES ON THE VOLTAIC ARC. 
The deposits of the transported matter, form upon the plate, when it is negative 
and the point positive, a species of very regular ring, the centre of which is the pro- 
jection of the point upon the plate. This takes place equally, whether the plate be 
vertical or horizontal, plainly indicating a determinate direction in the transfer of 
the substance from the positive to the negative electrode ; in the air and with 
metallic electrodes, the deposits always consist of the oxidized dust of the metal, of 
which the positive electrode is composed. 
I shall here enter into some details. A plate and a point of platinum have been 
used as electrodes in a vacuum, in air and in hydrogen. In a vacuum with a Grove 
battery of fifty pairs of plates, which had previously been used, I had only a very 
feeble effect, and particularly when the plate served as the positive electrode. The 
point was hardly removed a millimetre* from the plate when the arc broke ; to re- 
establish it, it became necessary to renew the contact between the point and the 
plate, by touching another point of the plate, the first point which was touched ap- 
pearing to have undergone such a modification as to prevent the re-formation of the 
arc. The same effect is produced when the experiments are made in the air, but it 
ceases when the power of the battery is increased : this is probably due to an aug- 
mentation of cohesion consequent on the increase of temperature in that part of the 
plate which acts as the positive electrode. Besides, when the experiment is made in 
air, the voltaic arc is more marked and of greater length than when it is made in 
vacuo, at least if the battery be weak ; for when the battery is powerful, composed, 
for example, of fifty pairs of plates freshly charged, it appeared to me that the con- 
trary obtained. I did not, however, perceive any great difference ; but the vacuum 
in which I experimented was far from being perfect ; it was that of a pneumatic 
pump, enclosing therefore highly rarefied air. 
In the latter case, that is to say, with the pile composed of fifty pairs strongly 
charged, and in highly rarefied air, a bluish spot, perfectly circular and presenting 
the appearance of a coloured ring of Nobili, was formed on the plate of platinum 
when it served as the positive electrode. The same spot appeared in atmospheric 
air, but its diameter was one-half less, and its colours much less vivid. In 
hydrogen, no coloured spot was formed ; its formation is therefore evidently the 
result of the oxidation of the platinum at a high temperature when acting as a posi- 
tive electrode in the ordinary atmosphere, and still more so, perhaps, in rarefied air-}-. 
When the same plate of platinum was made use of as a negative electrode, the point 
being positive, it became covered with a white circular spot, formed of a vast number 
* 1 millimetre = 003937 inch. — T rans. 
t This effect may possibly have been owing to the action of the oxygen brought by the voltaic current into 
that particular state which Schonbein first described under the name of ozone. Indeed, in this state the 
oxygen may attack those metals which are supposed to be inoxidizable ; and M. Marignac and I have shown 
that this may be effected by causing a succession of electric discharges to pass through the oxygen, even when 
very dry, with which the phenomenon of the voltaic arc has a great resemblance. 
