164 
MR, DAVY ON ASCERTAINING THE PRESENCE OF METALS. 
the union of a chemical and an electrical action, and seem to he comprehended 
in the principle developed by Sir Humphry Davy relating to the changes and 
transitions by electricity, viz. that “the metals are attracted by negatively 
electrified metallic surfaces, and repelled by positively electrified metallic sur- 
faces, and these attractive and repulsive forces are sufficiently energetic to 
destroy the usual operation of elective affinity 
If the decompositions which I have noticed were effected by the mere contact 
of the zinc with acid solutions of the metals, there would, indeed, be reductions, 
but no depositions on the platina. The electrical action arising from the con- 
tact of the two metals, in cases when small slips of zinc and platina were em- 
ployed, was quite sufficient to destroy or suspend the usual operation of che- 
mical affinity. Thus, in numerous instances of the decompositions of metallic 
compounds, the platina remained covered with the reduced metal during the 
contact of the zinc ; but on removing the zinc, the coating was readily re- 
dissolved by the slight excess of acid present. The transition of the reduced 
metal to the surface of the platina, is the effect that gives to the method a 
beauty, simplicity, and delicacy, exceeding, in my opinion, all other modes of 
detecting the metals already referred to, together with a number of others 
which remain to be noticed. 
As this paper has already exceeded the limits at first intended, I shall defer 
noticing a number of useful applications of the facts, until I shall have the 
honour of submitting to the Society the remaining part of the subject. 
* Phil. Trans. 1807. 
