What still further proves, that the electricity, in this way collected, had no dependence whatever, for its production, upon the application of the metals to the frog, but had been merely the portion of electricity, naturally possessed by the frog, in common with other conducting substances, is, that when the electricity, which was collected from its first contractions, had been drawn off from the plate, no more could afterwards be collected, although the contractions, excited by the metals, still continued as vigorous as ever.
6. When the electropherus was charged with electricity, as highly as it was possible to charge it by friction, the contractions produced by the insulated metals in a prepared frog, laid upon the metal plate of the electropherus, were not at all affected by raising it from the varnished surface. A proof that the phenomena in question are not affected, either by the condensation or rarefaction of the electricity, in either the animal or the metals, by which they are exhibited.
I have not found, that any quantity of electricity, which I could accumulate in the metal plate of the electropherus, did ever, when discharged into the nerve of a frog, excite contractions nearly so strong as what are excited by the application of zinc and silver; nor could I, at any time, collect a sufficient quantity of electricity, from five insulated frogs, sufficient to excite contractions in a single leg of a frog, though recently separated from its body, and consequently excitable by stimuli of very weak powers.
The politeness of the very learned Mr Robison, Professor of Natural Philosophy in this University, enables me to lay before the public the following communication; which, independent of its intrinsic merit, affords an additional gratification, by evincing the great interest taken in the subject, by one whose abilities and extensive knowledge so well qualify him for giving it a full investigation.
Edin. May 28. 1793.
SIR,
About a fortnight ago, my son told me of a curious experiment, with a piece of zinc and a piece of silver applied to the tongue, by which a strong irritation, resembling taste, was produced, and that a luminous flash was excited, by applying one of them to the eye. I immediately repeated them according to his directions, and my curiosity was greatly excited to prosecute them in a variety of circumstances. I understand, that these experiments have originated from the curious discoveries made some time ago in Italy, of which I was informed last winter. But I have been so much out of the world for some years past, that I have had no opportunity of knowing what was going forward.
Being informed, that you have been long engaged in experiments on this subject, and are about to favour the public with an account of them, I have taken the liberty of communicating to you, a few facts which have occurred to me, some of which, perhaps, may be new to you.