Abbildungen der Seite
PDF
EPUB

inspissated vegetable oil, very elastic, not solable in water. I placed one on a glass spindle, and fitted to its shape two narrow rubbers, one of naked brass, the other of brass covered with sealing-wax. With a very small motion of the winch, that bead became negative, and the naked brass rubber became positive; but the same bead became positive when the brass rubber was covered with sealing-wax, and this rubber was made negative.

36. These experiments afford a true analysis of the electric effects of friction. Its general effect, as I have stated, is to set in motion the electric fuid residing on the surface of the bodies which are rubbed; and the consequence of that effect is, that the boily which recedes from the point of friction carries along with it a part of that electric fluid; which effect extends even to the case when the rub. ber and the body rubbed are of the same substance, when the latter suddenly recedes, and meets in its way a conductor, before returning to the rubber.

37. These experiments remove the difficulty which Mr. Donovan found (p. 339) to reconcile the (supposed) equal distribution of the electric fluid with the impermeability of glass. The equal distribution is gratuitously supposed, since glass is but an imperfect nonconductor when not covered with some resinous varnish.

38. Mr. Donovan says (p. 345): “ As to the fact whether glass is actually impermeable, many experiments have been made; but they all appear to have been of doubtful force, and may be explained in some manuer, without supposing that electricity passes through.”

39. I have made in this respect some decisive experiments, proving that when a glass rod transmits the electric fluid it is only by its surface. I used for that purpose three glass rods of the same glass and the same diameter; one remaining naked, another covered all over with sealing-wax, the third covered with sealing-wax, with only an interruption in the middle of its length. These rods were supported horizontally on insulating pillars which left their extremities accessible to the knob of a Leyden vial. I used three very sensible electrometers, each having a long brass conductor in order to connect them with the rods.

Exp. 1. Having placed the naked rod on the pillars, with an electrometer at each end and one in the middle, and applied the Leyden vial to one extremity, the electrometer near it first diverged, then that in the middle, and soon that of the other extremity, showing that some time was required for the propagation of the electric fluid, even on the naked surface of glass.

Exp. 2. Having used the rod entirely covered with sealing-wax, a small motion was produced in the next electrometer, by the influence of the vial, but none in the remote electrometer.

Exp. 3. The rod with an interruption of the sealing-wax in the middle of its length was to be the test. If the glass were permeable the electrometer applied to that part ought to have been put in motion; but if it moves only along the surface of the glass, being stopped by the sealing-wax, the electrometer applied to the naked part in the middle cannot be put in motion. The result of the experiment was, that whatever time the Leyden vial remained applied to that extremity, no motion was produced in that electrometer any more than in that of the other extremity. It is therefore demonstrated that glass is absolutely impermeable to the electric fluid.

40. A very specious objection of Mr. Donovan against Franklin's doctrine is thus expressed, p. 349 : “Franklin supposes that no electricity can be received on one surface, unless the other can part with an equal quantity. In the case of excitation of the common cylioder, the inner surface having no communication with conductors, can part with none. How then can its outer surface receive the the great quantity we find in it ?” The answer is however obvious, for this is the common effect of friction. The cylinder receives constantly electric fluid from the rubber, especially when this is covered with a metallic amalgama : for it is in communication with the ground. In that case it is not necessary that the opposite surface of the glass should part with any electric Auid; the whole process takes place on the outward surface. The rubber gives electric fluid to the glass cylinder, which parts constantly with it in meeting the prime conductor; but the rubber communicating with the ground, furnishes also constantly a new quantity of the Auid.

41. It remains only to state a very essential point in electric phenomena, namely, How does the eleciric fluid communicate itself through space? This was the object of an experiment which I made in 1774, in Mr. Walsh's laboratory, which experiment in the first volume of my work, “ Idées sur la Météorologie," p. 521, I left under Mr. Walsh's name, because he had published it, without my knowledge, in the “ Philosophical Transactions," for 1785. But these experiments were concerted between Dr. Franklin and myself, and only in the house and in presence of Mr. Walsh, with an apparatus of which the description will show the purpose and its origin.

42. In my experiments for producing a truly comparable barometer, repeating those of a French academician, Du Fay, for producing light in the vacuum of barometers, an experiment related in the “Mém. de l'Académie des Sciences de Paris," for 1723, I found, as has been explained in my work, “Recherches sur les Modifications de l’Atmosphère,” tom. i, p. 43, that when the Torricellian vacuum was procured by making the mercury boil in the tube, no light appeared at the top of the barometer; whence I concluded that a perfect vacuum was not a conductor of the electric fluid.

43. Haviug expressed this idea to Dr. Franklin, he proposed to me an experiment, very difficult to execute, but which he encouraged me to undertake. The apparatus consisted of a glass syphon, the legs of which were about three feet distant from each other; the curve began about three inches above the point at which stood the common barometer, at that moment. When the syphon was filled with mercury it was inverted, with its two legs plunged into separate cups, each resting on an insulating stand : these cups received the mercury descending from the upper curve, and the

column thus separating, there were two barometers with a common vacuum.

44. In that situation, when a spark was given with a Leyden vial to one of the cups, a luminous arch was seen filling the top of the syphon, and a spark could be drawn from the other cup, thus showing that it was not a conductor of the electric fluid.

45. There remained to be performed the second part of the experiment and the most difficult, namely, to have the mercury boil in that long syphon. I succeeded in this operation; and when the syplion was placed with its leys in the cups, that complete vacuum ceased to be a conductor. The rain sparks were given to one cup, none was drawn from the other.

46. This experiment convinced Dr. Franklin of my system, that the electric fluid was a sort of parasite substance, which, on our globe and its atmosphere, was distributed on all other matters, and nowhere accumulated so as to produce lightnings, and their common attendants thunders; but that these phenomena proceed from the decomposition of the atmospheric air by a certain process, which manifests that the electric fluid enters into the composition of that air, a Auid sui generis, and not a mixture of different kinds of air, as it was supposed in the new chemical theory. Which conclusion of my long course of experiments serves as the basis of my work under the title of “Introduction à la Physique terrestre par les Fluides Expansibles.”

Opinions and Conjectures concerning the Properties and Effects of

the Electrical Matter, arising from Experiments and Observations made at Philadelphia, 1794.*

1. The electrical matter consists of particles extremely subtle, since it can permeate common matter, even the densest metals, with such ease and freedom as not to receive any perceptible resistance.

2. If any one should doubt whether the electrical matter passes through the substance of bodies, or only over and along their surfaces, a shock from an electrified large glass jar, taken through his own body, will probably convince him.

3. Electrical matter differs from common matter in this, that the parts of the latter mutually attract, those of the former mutually repel each other. Hence the appearing divergency in a stream of electrified effluvia.

4. But though the particles of electrical matter do repel each other, they are strongly attracted by all other matter. +

5. From these three things, the extreme subtilty of the electrical matter, the mutual repulsion of its parts, and the strong attraction

• From “ Franklin's Experiments in Electricity.”

| See the ingenious “Essays on Electricity," in the 6 Transactions," by Mr. Ellicot.

between them and other matter, arise this effect, that when a quantity of electrical matter is applied to a mass of common matter, of any bigness or length, within our observation (which hath not already got its quantity), it is immediately and equally diffused through the whole body.

6. Thus common matter is a kind of spunge to the electrical fluid. And as a spunge would receive no water if the parts of water were not smaller than the pores of a spunge ; and even then but slowly, if there were not a mutual attraction between those parts and the parts of the spunge; and would imbibe it faster, if the mutual attraction among the parts of the water did not impede, some force being required to separate them; and fastest, if, instead of attraction, there were a mutual repulsion among those parts, which would act in conjunction with the attraction of the spunge. So is the case between the electrical and common matter.

7. But in common matter there is (generally) as much of the electrical as it will contain within its substance. If more is added it lies without upon the surface, and forms what we call an electrical atmosphere; and then the body is said to be electrified.

8. 'Tis supposed that all kinds of common matter do not attract and retain the electrical matter with equal strength and force, for reasons to be given hereafter. And that those called electrics per se, as glass, &c., attract and retain it strongest, and contain the greatest quantity.

9. We know that the electrical fluid is in common matter, because we can pump it out by the globe or tube. We know that common matter has near as much as it can contain, because, when we add a little more to any portion of it, the additional quantity does not enter, but forms an electrical atmosphere. And we know that common matter has not (generally) more than it can contain, otherwise all loose portions of it would repel each other, as they constantly do when they have electric atmospheres.

10. The beneficial uses of this electric fluid in the creation we are not yet well acquainted with, though doubtless such there are, and those very considerable; but we may see some pernicious consequences that would attend a much greater proportion of it. For had this globe we live on as much of it in proportion as we can give to a globe of iron, wood, or the like, the particles and other light matters that can get loose from it, would, by virtue of their separate electrical atmospheres, not only repel each other, but be repelled from the earth, and not easily be brought to unite to it again ; whence our air would continually be more and more clogged with foreign matter, and grow unfit for respiration. This affords another occasion of adoring that wisdom which has made all things by weight and measure !

11. If a piece of common matter be supposed entirely free from electrical matter, and a single particle of the latter be brought nigh, it will be attracted, and enter the body, and take place in the centre,

or where the attraction is in every way equal. If more particles enter, they take their places where the balance is equal between the attraction of the common matter and their own mutual repulsion. 'Tis supposed they form triangles, whose sides shorten as their number increases, till the common matter has drawn in so many, that its whole power of compressing those triangles by attraction, is equal to their whole power of expanding themselves by repulsion, and then will such piece of matter receive no more,

12. When part of this natural proportion of electrical Auid is taken out of a piece of common matter, the triangles formed by the remainder are supposed to widen by the mutual repulsion of the parts, until they occupy the whole piece.

13. When the quantity of electrical fluid, taken from a piece of common matter, is restored again, it enters ; the expanded triangles being again compressed till there is room for the whole.

14. To explain this : take two apples, or two balls of wood or other matter, each having its own natural quantity of the electrical fluid. Suspend them by silk lines from the ceiling. Apply the wire of a well charged vial held in your hand to one of them, and it will receive from the wire a quantity of the electrical fluid; but will not imbibe it, being already full. The fluid therefore will flow round its surface, and form an electrical atmosphere. Bring A into contact with B, and half the electrical fluid is communicated, so that each has now an electrical atmosphere, and therefore they repel each other. Take away these atmospheres, by touching the balls, and leave them in their natural state : then, having fixed a stick of sealing-wax to the middle of the vial to hold it by, apply the wire to A, at the same time the coating touches B. Thus will a quantity of the electrical fluid be drawn out of B, and thrown on A. So that a will have a redundance of this fluid, which forms an atmosphere round it, and B an exactly equal deficiency. Now, bring these balls again into contact, and the electrical atmosphere will not be divided between A and B into two smaller atmospheres as before, for B will drink up the whole atmosphere of a, and both will be found again in their natural state.

15. The form of the electrical atmosphere is that of the body it surrounds. This shape may be rendered visible in a still air, by raising a smoke from dry rosin dropt into a hot tea-spoon under the electrised body, which will be attracted, and spread itself equally on all sides, covering and concealing the body. And this form it takes because it is attracted by all parts of the surface of the body, though it cannot enter the substance already replete. Without this attraction, it would not remain round the body, but dissipate in the air.

16. The atmosphere of electrical particles surrounding an electrified sphere, is not more disposed to leave it, or more easily drawn off from any one part of the sphere than another, because it is equally attracted by every part. But that is not the case with bodies of any other figure. From a cube it is more easily drawn at the corners

« ZurückWeiter »