CRYSTALLIZATION PRODUCED BY VOLTAIC ACTION. 425 



in contact with the air. These products are undoubtedly the result of 

 the decomposition of the subacetate of lead arising from actions ana- 

 logous to those which take place during the production of the protoxide 

 of copper. 



Zitic. — The surface of this metal when in contact with the air is usually 

 covered with a very thin layer of suboxide which is opposed to electro- 

 chemical action. This compound is slowly dissolved by the acids, and 

 it is to its presence that we are to ascribe the feeble intensity of the cur- 

 rents produced by a pile which is charged with a non-acid solution. 



The following is a veiy simple mode of obtaining crystallized oxide of 

 zinc : we take two small bottles, one containing a solution of zinc in 23ot- 

 ash, and the other a solution of nitrate of copper. The communication 

 between them is established by means of a bent tube filled with potter's 

 clay moistened with a solution of nitrate of potash. A plate of lead com- 

 municating with the positive pole of a pile composed of two or three 

 elements, is immersed in the solution of zinc, and a plate of copper, in 

 communication with the negative pole, is immersed in the solution of 

 nitrate of copper. 



A pile may even be dispensed with, if the plate of lead and the plate 

 of copper be put in metallic communication with each other. 



The nitrate of copper is decomposed in consequence of the action of 

 the current proceeding from the action of the alkali on the lead : the 

 oxygen and the nitric acid are transferred to the plate of lead, and there 

 produce nitrate of potash and oxide of lead which is dissolved in the al- 

 kali. After the experiment has been continued for some days there are 

 found deposited on the plate of lead small clear crystals having the shape 

 of flat prisms and so disposed as to form rosettes. These crystals are 

 formed during the slow precipitation of the oxide of zinc by the oxide of 

 lead, which gradually saturates the solution of potash. In contact with 

 the air, they become gradually translucid. Exposed to the action of heat 

 they take a yellow tinge without being melted, and become white when 

 cooled again ; a property which is characteristic of the oxide of zinc. 

 Subjected to the action of acetic acid they gave an acetate of zinc which 

 the sulphuret of potash throws down a white precipitate, — a proof that 

 these ci-ystals contain no lead. 



If we substitute a plate of zinc for a plate of lead, the only deposit ia 

 a white substance, which is a combination of zinc and potash. 



If we continue to let the pile operate with the plate of lead, a yellow 

 powder is precipitated, which is probably a combination of anhydrous 

 protoxide of lead with potash. 



By substituting for the plate of zinc a plate of copper or platina, or 

 any metal not easily oxidable, there will be found deposited on the up- 

 per side a tritoxide of lead in very simple layers which are easily detached 



