146 Galvano-magnetism. 
ment made according to the European plan, with eighty feet of cop 
per wire covered with silk. 
It is well known that of two metals of different susceptibility of ox- 
idizement, after contact with each other, that will be found positive, 
which is most oxidizable, and that negative, which is the least at- 
tractive of oxygen. In this sense copper is said to be negative in re- 
lation to zinc. It should however be recollected, that since the more 
oxidizable metal becomes positive by a discharge from the other, 
during the existence of a ‘galvanic circuit, the’ metal which is nega- 
tive in the sense above mentioned, forms the positive pole. Thus, 
if we constitute a circuit of zinc, moistened paper, and copper, the 
copper is positive; and if we connect it with the end of the coil 
which enters over the needle, and stand so as to look in that direc- 
tion, the north pole moves to the left. 
Having supplied the bottom of a saucer with a stratum of mercury 
from my pneumatic cistern, covered by water and paper, a disc of 
copper was placed over it onthe paper. Under these circumstances 
I was surprized to find that when a wire proceeding from one pole of 
the multiplier, was held in contact with the copper, and the other 
wire dipped into the mercury, the same deflection took place as when 
a-simiar circuit was made, substituting zine for mercury, the same 
wire being in both cases kept in contact with the copper. On sub- 
stituting successively iron, tin, lead and tin plate, for the copper, the 
same wire being in contact with the mercury as in the first instance, 
I found this metal to have the same relation to all of them as zinc. 
Its relation to zinc was found to be feebly of an opposite kind. 
Subsequently, I procured an adequate quantity of pure merculy; 
by precipitating the protonitrate, by copper. This I found to have 
a polarity with copper and all the other metals above named, the op- 
posite of that which the impure metal had with them. The impure 
metal had the same relation to it as zinc. Thus we have a convel 
ient method of testing the purity of mercury, since a very slight im- 
purity renders this metal in the circuit negative with copper, unless 
the impurity be of one of the precious or less oxidizable metals. 
Possibly we may in this way have the means of testing gold and ail- 
ver, by amalgamation with mercury. 
Having the keeper, and a weight of about fifty-six pouuds, sU> 
pended by a galvanic magnet, of which the coils were in the circu! 
of a galvanic pair of about a square foot of surface, I attached one 
pole of my calorimotor, of fifty square feet, to the keeper, and the 
