Chapter XVI: Conclusion

I publish my experiments respecting muscular contractions produced by one metal with the greater confidence, as they were repeated different ways by the celebrated Humboldt, who has adopted my opinion. In his work on Galvanism, under the head which he entitles “Answer to the Objections made by Volta to the Experiments of Aldini,” he says, “J. Aldini of Bologna invented a very ingenious apparatus, by means of which he was enabled to refute the supposition of Professor Volta. For this purpose he had recourse to mercury: every thing relating to his experiments is very well represented in the plate which accompanies his memoir. Volta, in reply to these experiments, observes, that they can impose only on those who have not thoroughly examined them. He denies the facts related by Aldini, and persists in his opinion that the phænomena of Galvanism may all be referred to the laws of heterogeneity. In regard to the experiments made with mercury, he says that there is a great difference between the surface of this metal and the interior of its mass; because the surface becomes oxidated by the contact of the atmospheric air; that consequently, in the experiment of Aldini, the conducting arc is not really but apparently homogeneous, the organs being immersed to different depths in the metal: besides, that the mercury in these experiments produces a shock; and that, as this shock is not the same at both extremities of the arc, the result is an unequal development of electricity. Volta, therefore, opposes to the phænomena described by Aldini nothing but hypothesis. We might reply in the same manner; but as it is much better in philosophical disputes to have recourse to experiments, I undertook some researches for the purpose of removing all doubt in regard to this subject.

“I purified mercury by all the known means employed for that
purpose. I strained it several times through a piece of
chamois leather; I washed it with water and soap, with
vinegar and with alcohol. It appeared to me that it
contained neither lead nor tin; and that it was free from
oily particles and dust: it was perfectly fluid, and divided
itself into small, very round globules, which did not adhere
to each other, and which left behind them no traces. Its
surface was as brilliant as that of glass, without any
pellicle or spot; and a small quantity of it being stirred
in a mortar with water, did not communicate to it any
sensible colour. It dissolved in nitric acid without
sensible effervescence, and without giving any precipitate.
A large quantity of mercury thus purified was poured into
three porcelain vessels; and as I was aware that, if I
performed several experiments with the same quantity of
mercury, it might be objected, that the metal had contracted
some impurity by the mere contact of the animal substances,
I made only one experiment with the mercury in each of the
vessels.

“I prepared several legs of frogs in such a manner that a
portion of the crural nerve and a part of the muscle of the
same length were left dependent. I placed a glass tube in a
horizontal direction above a vessel containing mercury; made
fast to it two silk threads, and suspended from it the thigh
of the leg in such a manner that the nerve and muscle could
be made to descend at pleasure. I brought the leg of the
frog within two lines of the vessel, and then lengthened the
thread till the nerve touched the surface of the metal. No
contractions took place; but as soon as the nerve also was
brought into contact with the metal, by lengthening the silk
thread, the whole limb experienced a convulsive shock. I
repeated this experiment with the same precaution, employing
the two other vessels, and the same results were obtained.

“The muscle and nerve touched the mercury only at the
surface; they were in no manner immersed in the metal; and
care had been taken to lower them so gently, that it was
impossible to suspect there could be any impulse, as in the
experiment where Aldini had caused the mercury to flow down
by employing a vessel shaped somewhat like a syphon.”