344 new Process for the Detection of copper. 
for without this it is impossible to distinguish the brown 
color of the salt of copper, when mixed with the blue of the 
salt of iron. 
Iron acts by decomposing the salt of copper and precipitates 
the copper in a metallic state, an action well represented by 
the following formula: (Cu S)-fFe = Cu + (FeS), where we 
may see that the iron takes the place of the copper, and when 
the reaction terminates, we have sulphate of iron and metallic 
copper. But it is necessary that the liquid should be acidu- 
lated by a little acid; but if too much be added and the 
copper be in small amount, the iron blackens, and prevents the 
copper being readily distinguished. Moreover, the copper does 
not adhere, and the least disturbance causes it to separate. 
These uncertainties do not exist in the process which I am 
about to describe, and which is no more than the application 
of the observation of which I spoke in the commencement. 
It is proper that the liquid to be examined, if it be dilute^ 
should be somewhat concentrated, and slightly acidulated with 
hydrochloric acid; a drop is to be placed on a plate of 
platinum, and covered with a well polished plate of iron, in such 
a manner that the iron shall touch both the liquid and platinum. 
In a few seconds the platinum exhibits a very adherent covering 
of copper throughout every part touched by the liquid. The ex- 
planation of this fact depends entirely upon electro-chemical the- 
ory; it results from principles which I shall rapidly enumerate: 
1 st, when two metals are placed in contact, there is a production 
of electricity, one of the metals being positively and the other 
negatively electrified. 2d, when a solution is subjected to the 
action of the pile, the salt is decomposed, the acid passing to 
the positive, and the base to the negative pole. There are 
some salts which are not only thus decomposed into acid and 
base, but still further the base itself is decomposed into metal 
and oxygen; in this case, the metal only passes to the negative 
pole, the oxygen appearing at the positive pole. 
But, when iron and platinum are placed in contaet, there 
is a developement of electricity, which developement is the 
more active when a saline solution is present; the iron becomes 
