CRYSTALLIZATION PRODUCED BY VOLTAIC ACTION. 



421 



Q 



ence when they are employed as jjositive electrodes : but as thLs 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. — In 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 phsenomena 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 witli 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 cojDper should take that direction if the cur- 

 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 phaenomena which take place will vary. Let us 



