with a large voltaic battery. 367 
conducts best, is not heated red when connected with any of- 
the other metals. Should it be objected, that if the electri- 
city meet with greater resistance in one body than in the 
other, equal quantities cannot be transmitted in equal times 
by the two substances, (a circumstance essential to electrical- 
action,) I answer, that a body may be propelled through two 
media of different densities, with equal velocity, if the pro- 
pelling forces be proportionate to the resistances ; and it is a 
necessary consequence that whatever effect the passage of the 
bodv be capable of producing in the least resisting medium, 
it will produce it in a still greater degree in the most resisting; 
and if that effect be heat, the greatest portion will be developed 
in the latter instance. In the case in question, indeed, there 
is but one propelling force; but as it is sufficient to overcome 
the greater resistance, the analogy is unshaken. That the 
ignition of the wire is generally first perceptible at the 
point of contact next the pole of the battery (to which- 
ever pole it be presented) is in favour of the hypothesis. 
I once thought the phenomena might be owing to the joint 
effect of difference of conducting power, and inequality of the 
capacity of different metals for heat ; but the experiments of 
Crawford, Leslie, Dalton, Irvine, and others, militate 
against that idea; for, according to them, the capacities of iron 
and platina exceed those of all the other metals, whereas, on 
the supposition alluded to, they ought to be inferior. From the 
foregoing results, the order of the conducting powers of the 
metals tried is silver, zinc, gold, copper, iron, and platina. 
Tin and lead fuse so immediate! v at the point of contact, that 
they cannot be placed. Between gold and copper the dif- 
ference is trifling ; and with regard to platina and iron, their 
