CRYSTALLIZATION PRODUCED BY VOLTAIC ACTION. 42} 
ence when they are employed as positive electrodes: but as this effect 
presents nothing particularly worthy of remark, we shall forbear to 
dwell on it, that we may now proceed to explain some other processes 
by means of which we obtain the oxides crystallized. 
Of Metallic Oxides Crystallized by Voltaic Action. 
Copper.—tin order to obtain crystals of protoxide of copper, we take 
a glass tube closed at one end and having at the bottom some deutoxide 
of copper (See fig. 2.). This tube is filled with a solu- 
tion of saturated nitrate of copper, into which there is Fig. 2. 
plunged a plate of copper, that touches the deutoxide 
also, and the tube is then hermetically sealed. After 
an interval of ten days we begin to perceive on the 
plate of copper small bright crystals of the form of 
an. octahedron and of a deep red colour. In order to 
discover the electric phenomena by which they are 
produced, we must take two capsules of porcelain fill- 
ed with a solution of nitrate of copper and connected 
with each other by means of a cotton wick. One ex- 
tremity of a plate of copper is then plunged into each 
of them, while the other is attached to one of the 
extremities of the wire of a delicate multiplier. All 
things being now alike on one side and the other, 
there appears no current. But if we pour some deut- 
oxide of copper on that part of one of the plates which is plunged in 
the solution, a current is soon produced, the direction of which shows 
that the plate in contact with the deutoxide has received the negative 
electricity. Hence it follows that the plate in the other capsule is the 
negative pole of the small pile which produces the decomposition of the 
nitrate of copper. Now, an effect perfectly similar to this takes place 
in the tube: the part of the plate in contact with the deutoxide is the 
positive and the other part the negative pole. We shall presently re- 
vert to the cause which produces this pile. The existence of the latter 
being established, the portion of the plate of copper which is not in con- 
tact with the deutoxide, should attract the copper in a metallic state or 
its oxides, according to the force of the current. It is therefore but na- 
tural that the protoxide of copper should take that direction if the eur- 
rent possesses sufficient energy. It crystallizes, because the electric 
and (consequently) the chemical action being very slow, the molecules 
have time to arrange themselves according to the laws of crystallization, 
although the body is insoluble,—an advantage which is never obtained 
when the chemical forces are of greater intensity. 
According to the greater or less quantity of deutoxide of copper in- 
closed in the tube the phenomena which take place will vary. Let us 
