CRYSTALLIZATIGN PRODUCED BY VOLTAIC ACTION. 435 



Some clay, very much divided and moistened witli a .solution of 

 arseniate of potash, having been put into a glass tube, there was poured 

 over it a solution of nitrate of copper: the reaction of tiie two solutions 

 took place rapidly in the first few instants at tlie surface of contact of 

 the clay and the solution of the nitrate. But the solution having gra- 

 dually penetrated the mass of the clay, the consequence was, that the 

 reaction between the arseniate and the nitrate was very slow, and there- 

 fore favourable to the crystallization of the arseniate of copper. After 

 the lapse of some time, there were seen in the vacant interstices of the 

 clay some crystals resembling those of native arseniate of copper. 



The formation of double sulphurets and simple sulphurets being 

 subject to certain laws, Ave must not use tubes of any dimensions we 

 please, and must not employ liquids possessing too great an electric 

 conductibility. If, for example, the quantity of double hyposulphate 

 formed were too great to be completely decomposed by the acid which 

 comes from the tube containing the nitrate of copper and the nitrate 

 of silver, the operation would be incomplete, inasmuch as we should 

 not have, in that case, the reactions necessary for the formation of the 

 compounds we wish to obtain. Thus, in proportion as the circum- 

 stances have been more or less favourable, the result will be a Y>er- 

 fect or a confused crystallization, or a total absence not only of crystal- 

 lization but of the production of double sulphuret. 



The tubes employed must always be of small dimension, that is to 

 say, of two or three millimetres* ; for if the acid came in too great 

 quantities into the tube containing the double combination, it would 

 react on each of the components, and the desired result would not be 

 attained. It must not be forgotten that the hyposulphite which we 

 employed proceeded from a protosulphuret of potassium decomposed in 

 the air. 



Of Metallic Iodides. 



It is known that the metallic iodides are subject to the same law 

 of composition as tlie sulphurets. We exjjected therefore to be able to 

 obtain the insoluble iodides by the same process whicli has served to 

 obtain the sulphurets. In thinking thus we are but generalizing the 

 principle. 



Iodide of Lead. — In the electro-chemical apparatus already described 

 we substitute the iodide of potassium or soda for the alkaline hyposul- 

 phate; we then plunge a plate of copper into that branch of the bent tube 

 which contains the nitrate of copper, and a plate of lead into the other 

 branch, which contains the solution of iodide of potassium. In the latter 

 branch of the tube we obtain at first a double iodide of lead and potas- 

 sium, which crystallizes in very fine white silky needles ; this combina- 

 tion is gradually decomposed, tlie decomposition commencing at the 



• About v. of an inch. 



