M. DE LA RIVE’S RESEARCHES ON THE VOLTAIC ARC. 
35 
of minute grains of platinum, which, having been raised to a high temperature, re- 
mained adhering to the surface. The white spot, like the blue one, was much larger 
in rarefied air than in a vacuum. If the experiment be prolonged for a minute or 
two when the plate is negative, the rod of platinum terminating in a point, which is 
positive, soon becomes highly incandescent ; its end is fused and falls on the plate 
in the form of a perfectly spherical globule. When the plate is positive and the point 
negative, the latter is less heated, and does not become fused ; but the plate, unless 
it be very thick, is liable to be perforated: besides, as may easily be imagined, the 
phenomenon lasts much longer in the latter case. The light is less brilliant, but it is 
accompanied by a reflexion of a superb blue, which maybe seen when the experiment 
is made in the interior of a bell, whether the air be rarefied or not. This blue re- 
flexion is observed on the side of the bell, and is to be seen whatever may be the 
nature of the electrodes, or the colour of the light to which these give rise in the 
centre of the bell ; only when this central light is very brilliant, it becomes slightly 
paler by the effect of contrast. 
I substituted for the platinum point a point of coke, but the plate of platinum re- 
mained; this being positive and the point negative, I obtained a luminous arc more 
than double the length of the arc produced by the point of platinum. With respect 
to the arc, instead of its being a cone of light, having its base on the plate and its 
apex at the point, as was the case when the latter was platinum, it was composed 
of a multitude of luminous jets diverging from different points of the plate, and 
tending to various parts of the point of coke. This fact shows clearly the influence 
that may be exercised by the negative electrode, the function of which is very far from 
being a merely passive one. Let me add, that although the strength of the pile was 
precisely the same as when the point was of platinum, not only was the luminous 
arc much longer with the point of coke, but the heat developed in the plate of pla- 
tinum was so much greater that it was soon melted and perforated. The coke being 
positive and the plate negative, the length of the arc was less than in the preceding- 
case, and particularly so in air, where it was sensibly less than in a vacuum. The heat 
generated was however still very great, the point of coke becoming quickly incan- 
descent throughout. I ought to add, that with the point of coke, the luminous arc 
was so brilliant that the blue light which I have mentioned almost entirely disap- 
peared, which was not the case with any other kind of point. 
Leaving the plate of platinum, I adjusted a zinc point. The effects were most 
brilliant, but of short duration, the point speedily melting. In common air, a deposit 
of white oxide was precipitated upon the platinum plate ; in highly rarefied air (the 
vacuum of an air-pump), a black deposit was formed : in both cases it commu- 
nicated with the positive pole. An iron point being substituted for that of zinc, 
equally produced in common air a brownish red deposit of oxide of iron, and in 
rarefied air a deposit of black oxide. 
I call the attention of chemists to these two facts, as well as that of the oxidation 
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