with a large Voltaic Battery. 411 



ceding phenomena, and oifer the following conjecture with dif- 

 fidence. When a perfect communication is established between 

 the poles of the battery, the electricity circulates without pro- 

 ducing anv visible effect ; but if it meet with resistance in its 

 passage, it manifests itself bv chemical action, by the evolution 

 of heat, or both. Thus, if a bar of metal be connected with 

 one pole of the battery, and its extremity immersed in a basin 

 of mercury connected with the other pole, a.t the instant the 

 surfaces come in contact, heat and light are evolved, which 

 cease as soon as the l)ar, if it be of sufficient size, is fairly 

 plunged beneath the surface of the quicksilver. If the circuit 

 be completed bv two pieces of charcoal, the evolution of heat 

 and light is permanent, as long as their surfaces remain in con- 

 tact, because that contact can nover be so perfect as to oppose 

 no resistance to the electricity; whereas, in the case of the bar 

 of metal and the mercury, it soon becomes complete, and the 

 current is then uninterrupted. Resistance, therefore, appeal's 

 to occasion the development of heat (whatever be the ultimate 

 cause of the pheenomenon); and as this nmst be inversely as the 

 conducting power, when any two of the wires connected con- 

 tinuously are placed in the circuit, that which is the worst con- 

 ductor mui't be most heated ; and thus platina, having the lowest 

 conducting power, is ignited before all the rest ; and silver, whicH 

 conducts best, is not heated red when connected with any of 

 the other metals. Should it be objected, that if the electricity 

 meet with greater resistance in one body than in the other, equal 

 quantities cannot be transmitted in equal times by the two sub- 

 stances, (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 propelling forces be propor- 

 tionate to the resistances ; and it is a necessary consequence that, 

 whatever effect the passage of the body be capable of producing 

 in the leaht 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 develOj)ed in the latter instance. In the 

 case in question, indeed, tiicre 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 gent'rallv first 

 perceptible at tlie point of contact next the pole ot tlie battery 

 (to whichever pole it lie presented) is in favour of the hypothesis. 

 1 once thought the phcenomena might be owing to the joint 

 effect of difference of conducting power, and inequality f the 

 capacity of different metals for heat; but the experiments of 

 Crawford, Leslie, Dalton, Irvine, and others, militate against 

 tliat idea; for, according to them, the capacities of iron and 

 platina exceed those of all the other jnetals, whereas, on the 



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