CRYSTALLIZATIGN PRODUCED BY VOLTAIC ACTION. 425 
Some clay, very much divided and moistened with 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 the two solutions 
took place rapidly in the first few instants at the 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, we 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 per- 
fect or a confused crystallization, or a total absence not only of erystal- 
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 the sulphurets. We expected therefore to be able to 
obtain the insoluble iodides by the same process which has served to 
obtain the sulphurets. In thinking thus we are but generalizing the 
principle. 
Lodide 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, the decomposition commencing at the 
* About .’. of an inch. 
