« ZurückWeiter »
to me thereon, by experimental proof. Wherefore, the first point wherein I can dare to give my opinion, is in the Abbé's fourth Letter, (p. 66,) where he undertakes to prove, that the electric matter passes from one surface to another through the entire thickness of the glass. He takes Mr. Franklin's experiment of the magical picture, and writes thus of it. “When you electrize a pane of glass coated on both sides with metal, it is evident, that whatever is placed on the side opposite to that which receives the electricity from the conductor, receives also an evident electrical virtue.” Which, Mr. Franklin says, is that equal quantity of electric matter, driven out of this side, by what is received from the conductor on the other side ; and which will continue to give an electrical virtue to any thing in contact with it, till it is entirely discharged of its electrical fire. To which the Abbé thus objects; “ Tell me,” says he, (p. 68,) “I pray you, how much time is necessary for this pretended discharge ? I can assure you, that, after having maintained the electrization for hours, this surface, which ought, as it seems to me, to be entirely discharged of its electrical matter, considering either the vast number of sparks that were drawn from it, or the time that this matter had been exposed to the action of the expulsive cause; this surface, I say, appeared rather better electrized thereby, and more proper to produce all the effects of an actual electric body.”
applaudi très-sincérement aux endroits de son ouvrage qui m'ont paru solidement établis, ou ingénieusement pensés, et c'est ce que j'ai fait avec le plus de plaisir.”
Franklin never answered the Abbé's book, though he says, in a letter to Mr. Bowdoin, that he had collected and methodized short hints for that purpose. And in his autobiography he thus speaks of the subject, after alluding to the publication of his papers on electricity.
“A copy of them happening to fall into the hands of the Count de Buffon, (a philosopher deservedly of great reputation in France, and indeed all over Europe,) he prevailed with Monsieur Dubourg to translate them into French; and they were printed at Paris. The publication offended the Abbé Nollet, preceptor in Natural Philosophy to the royal family, and an able experimenter, who had formed and published a theory of electricity, which then had the general vogue. He could not at first believe that such a work came from America, and said it must have been fabricated by his enemies at Paris to oppose his system. Afterwards, having been assured that there really existed such a person as Franklin at Philadelphia, (which he had doubted,) he wrote and published a volume of Letters chiefly addressed to me, defending his theory, and denying the verity of my experiments, and of the positions deduced from them. I once purposed answering the Abbé, and actually began the answer; but, on consideration that my writings contained a description of experiments, which any one might repeat and verify; and, if not to be verified, could not be defended; or of observations offered as conjectures, and not delivered dogmatically, therefore not laying me under any obligation to defend them; and reflecting, that a dispute between two persons, written in different languages, might be lengthened greatly by mistranslations, and thence misconceptions of one another's meaning, (much of one of the Abbé's letters being founded on an error in the translation ;) I concluded to let my papers shift for themselves; believing it was better to spend what time I could spare from public business in making new experiments, than in disputing about those already made. I therefore never answered Monsieur Nollet; and the event gave me no cause to repent my silence ; for my friend, Monsieur Le Roy, of the Royal Academy of Sciences, took up my cause and refuted him,"
David Colden was a son of Cadwallader Colden, and devoted to the study of electricity. - Editor.
The Abbé does not tell us what those effects were; all the effect I could never observe, and those that are to be observed can easily be accounted for, by supposing that side to be entirely destitute of electric matter. The most sensible effect of a body charged with electricity is, that, when you present your finger to it, a spark will issue from it to your finger; now, when a phial prepared for the Leyden experiment is hung to the gun-barrel or prime conductor, and you turn the globe in order to charge it, as soon as the electric matter is excited, you can observe a spark to issue from the external surface of the phial to your finger, which, Mr. Franklin says, is the natural electric matter of the glass, driven out by that received by the inner surface from the conductor. If it be only drawn out by sparks,
a vast number of them may be drawn; but, if you
take hold of the external surface with your hand, the phial will soon receive all the electric matter it is capable of, and the outside will then be entirely destitute of its electric matter, and no spark can be drawn from it by the finger; here, then, is a want of that effect which all bodies charged with the electricity have. Some of the effects of an electric body, which I suppose the Abbé has observed in the exterior surface of a charged phial are, that all light bodies are attracted by it. This is an effect which I have constantly observed, but do not think that it proceeds from an attractive quality in the exterior surface of the phial, but in those light bodies themselves, which seem to be attracted by the phial. It is a constant observation, that, when one body has a greater charge of electric matter in it than another (that is, in proportion to the quantity they will hold), this body will attract that which has less; now, I suppose, and it is a part of Mr. Franklin's system, that all those light bodies, which appear to be attracted, have more electric matter in them than the external surface of the phial has; wherefore they endeavour to attract the phial to them, which is too heavy to be moved by the small degree of force they exert, and yet, being greater than their own weight, moves them to the phial. The following experiment will help the imagination in conceiving this. Suspend a cork ball, or a feather, by a silk thread, and electrize it; then bring this ball nigh to any fixed body, and it will appear to be attracted by that body, for it will fly to it; now, by the consent of electricians, the attractive cause is in the ball itself, and not in the fixed body to which it flies; this is a similar case with the apparent attraction of light bodies, to the external surface of a charged phial.
The Abbé says, (p. 69,) “ that he can electrize a hundred men, standing on wax, if they hold hands, and if one of them touch one of these surfaces (the exterior) with the end of his finger.” This I know he can, while the phial is charging ; but, after the phial is charged, I am as certain he cannot ; that is, hang a phial, prepared for the Leyden experiment, to the conductor, and let a man, standing on the floor, touch the coating with his finger, while the globe is turned, till the electric matter spews out of the hook of the phial, or some part of the conductor, which I take to be the certainest sign that the phial has received all the electric matter it can; after this appears, let the man, who before stood on the floor, step on a cake of wax, where he may stand for hours, and the globe all that time turned, and yet have no appearance of being electrized. After the electric matter was spewed out as above from the hook of the phial prepared for the Leyden experiment, I hung another phial, in like manner prepared, to a hook fixed in the coating of the first, and held this other phial in my hand; now, if there was any electric matter transmitted through the glass of the first phial, the second one would certainly receive and collect it; but, having kept the phials in this situation for a considerable time, during which the globe was continually turned, I could not perceive that the second phial was in the least charged, for, when I touched the hook with my finger, as in the Leyden experiment, I did not feel the least commotion, nor perceive any spark to issue from the hook.
I likewise made the following experiment; having charged two phials (prepared for the Leyden experiment) through their hooks, two persons took each one of these phials in their hand; one held his phial by the coating, the other by the hook, which he could do by removing the communication from the bottom before he took hold of the hook. These persons placed them
selves one on each side of me, while I stood on a cake of wax, and took hold of the hook of that phial which was held by its coating (upon which a spark issued, but the phial was not discharged, as I stood on wax); keeping hold of the hook, -I touched the coating of the phial that was held by its hook with my other hand, upon which there was a large spark to be seen between my finger and the coating, and both phials were instantly discharged. If the Abbé's opinion be right, that the exterior surface, communicating with the coating, is charged, as well as the interior, communicating with the hook; how can I, who stand on wax, discharge both these phials, when it is well known I could not discharge one of them singly ? Nay, suppose I have drawn the electric matter from both of them, what becomes of it ? For I appear to have no additional quantity in me when the experiment is over, and I have not stirred off the wax; wherefore this experiment fully convinces me, that the exterior surface is not charged; and not only so, but that it wants as much electric matter as the inner has of excess; for by this supposition, which is a part of Mr. Franklin's system, the above experiment is easily accounted for, as follows.
When I stand on wax, my body is not capable of receiving all the electric matter from the hook of one phial, which it is ready to give; neither can it give as much to the coating of the other phial as it is ready to take, when one is only applied to me; but, when both are applied, the coating takes from me what the hook gives; thus I receive the fire from the first phial at B, the exterior surface of which is supplied from the hand at A; I give the fire to the second phial at C, whose interior surface is discharged by the hand at D. This discharge at D may be made evident by receiving that fire into the hook of a third phial, which is done thus.