36 
M. DE LA RIVE’S RESEARCHES ON THE VOLTAIC ARC. 
of the platinum at a high temperature in rarefied air. They appear to prove the in- 
fluence which the state of greater or less density of the surrounding oxygen may exert 
on the phenomenon of oxidation and on the nature of the oxide formed. A plate 
and a point of soft iron were used as positive and negative electrodes, both in a va- 
cuum and in the atmosphere ; the same results appeared with a plate and a point of 
silver, a plate and a point of copper, and a plate and point of argentane*. The blue 
light was perceived in all the experiments ; coloured circles were likewise seen on all 
the plates when they had acted as positive electrodes in rarefied air. The silver and 
copper plates presented in this case very decided cavities, caused by the passage 
of the matter from the positive to the negative pole. The points became incan- 
descent throughout when they served as positive electrodes ; whereas when negative, 
they were heated only at their extremities. The copper point when positive became 
isolating at its extremity, and it was necessary to excite it by friction in order to renew 
the experiment. This circumstance is probably attributable to the formation of a thin 
film of oxide. The point and plate of copper gave out a luminous arc of a beautiful 
green light, which contrasted in a remarkable manner with the blue reflexion 
visible in this, as in the other experiments. Mercury was likewise employed, both 
as a positive and negative electrode. In a vacuum as well as in atmospheric air, 
the luminous effect was most brilliant. The mercury was excessively agitated, rising 
up in the form of a cone when it was positive, and sinking considerably below the 
positive point when it was negative. The quantity of vapour thrown off by the mer- 
cury during this experiment filled the bell so quickly that it was not easy to observe 
the details. 
I shall terminate this section by stating a fact which appears to me to be import- 
ant ; it is the influence which the nature of the metallic points forming the electrodes 
exercises on the temperature which they acquire in relation to the production of the 
voltaic arc. If the two points are of the same metal, both platinum, or both silver, 
the positive one alone becomes incandescent throughout its whole length. If the 
silver point be positive and that of the platinum negative, the latter becomes incan- 
descent, and the silver one is much less heated. Thus, when the voltaic arc is formed, 
the circuit must be regarded as completed, and then it is those parts of the circuit 
which present the greatest resistance to the current which become the hottest ; at 
first it is that portion forming the arc itself, and then, in the rest of the circuit, the 
metal which is the worst conductor. But if the conductors be of the same material 
on both sides of the arc, or if there be only a slight difference of conductibility be- 
tween them, then the development of heat, instead of being uniform, as it might ap- 
pear it ought to be, is much greater on the positive side. This important fact evi- 
dently proves that this portion of the circuit has to resist a much more energetic 
action than that which the other side experiences ; a fact which is confirmed by the 
molecular segregation accompanying this action at the positive electrode. This want 
* An alloy of copper and nickel : also known by the names of pachfong and melchior. 
