74 Professor CumMinG on the Developement 
APPENDIX. 
Sixce this paper was read to the Society, it occurred to me, that, 
as the juxta-position of two particles of the same metal at different tem- 
peratures, was the sole condition requisite for eliciting electro-magnetism, 
it might be exhibited by the minutest metallic specimens. Portions of 
bismuth and antimony, each weighing one grain, were therefore placed 
on a silver disk connected with the galvanoscope; on touching the upper 
surfaces of each separately, with one end of a heated silver wire, the other 
extremity of which was placed in the other cup of the galvanoscope, the 
needle deviated through 90°, positive and negative respectively. By this 
method I was enabled to examine the compound ore of iridium and osmium, 
of which the largest specimen did not exceed * of a grain, and to verify, 
in a few minutes, results, for which the laborious process of casting bars 
of the different metals had been previously requisite *. 
As the effect of the electro-magnetism developed by heat, is perfectly 
analogous to that caused by galvanic excitation, in its tendency to produce 
the rotation of a magnetic bar, it is evident, that, if the magnet be fixed 
and the apparatus at liberty to revolve, it will, in like manner, exhibit 
the converse experiment. The instruments formed of Platina and Silver 
wires, which are represented by Fig. 12 and 13, were constructed for this 
* If a heated metallic wire be applied to a plate of the same metal, there is in all cases 
a deviation; the nature of which seems to depend upon some peculiar property in the metal 
itself. Copper, Zinc, Bismuth, for instance, are positive; Platina, Silver, Iron, Brass, 
Antimony, are negative. 
