Two Fluids, and of Two Metals not in Contact. 491 
on long use in acids^ so that plates which fresh were nearly ho- 
mogeneous, acquired with time a considerable heterogeneity. 
Filing the surfaces thoroughly bright was the sole means of 
restoring them to their original state. Nevertheless I em- 
ployed in the further progress of my inquiries, and on the repe- 
tition of experiments, only distilled zinc, to be more certain. 
For a similar reason I also had recourse to amalgamated 
zinc. Plates of ordinary zinc, possessing a very considerable 
degree of heterogeneity, lose it in fact by amalgamation (pro- 
duced by rubbing them with dilute sulphuric acid and mercury), 
and become almost perfectly homogeneous ; but they remain so 
only immediately after this operation, so long as the surface of 
the metal is, as it were, fluid. After some time the amalgam 
hardens to a crystalline mass, generally sooner or to a greater 
extent on one plate than on the other, and then the hetero- 
geneity again makes its appearance. The plate with the fluid 
surface is in acids positive to that with the crystalline dull 
appearance. By rubbing the latter with mercury the hetero- 
geneity may again be suppressed. Although it is in our power 
to bring amalgamated zinc plates to any degree of homogeneity 
previous to each experiment, yet it did not appear advisable to 
employ solely such, as amalgamated zinc must in a certain degree 
be regarded as a different metal from ordinary zinc, and its easy 
liability to change might produce disturbances. The sequence, 
however, showed that in general it affords the same results as 
the non-amalgamated zinc, and in numerous cases possesses 
considerable advantages over the latter. 
Besides this, no precaution was neglected that is indispen- 
sable in this kind of experiments*; for instance, the clean- 
sing of the plates after each experiment, by immersion in water. 
* Here among other things might be reckoned the order of immersion, for 
it is a well-known fact, that of two plates of one and the same metal, on 
being immersed in a like fluid, that dipped in last is always negative to the 
one first inserted. I have found this to be perfectly true, but have likewise 
observed that on the contemporaneous immersion of two zinc plates in water 
slightly acidulated with sulphuric acid, several reversions in the direction of 
the current are produced ; for instance, in the first moment a westerly deflection 
of 10°, followed by an easterly of 70°, which was soon succeeded by a perma- 
nent easterly deflection of 20°, that gradually sunk down to 0°, and passed into 
a westerly deflection of 38° ; sometimes a couple more reversions occurred. 
After several contemporaneous takings out and immersions only the westerly 
deflection was evident. A mere knocking one of the plates against the 
bottom of the vessel, which produced an elevation of about 1 line, rendered this 
plate negative, or, causing with respect to the deflection, an increase or diminu- 
tion in this deflection, although but transitory. The plates employed for this 
purpose were recently scoured, and at the termination of the experiments, 
which lasted nearly an hour, they had lost from the weakness of the acid little 
or nothing of their brightness. Both became covered with bubbles of hydro- 
