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which is acted upon by the contact of the plant. In general, we are very little advanced in the knowledge of the subtile agents operating in terrestrial phenomena ; and as we cannot make any real progress in this knowledge but by endeavouring to increase, by observation and experiments, the number of the phenomena which have analogies with each other in some respect, it might be useful to follow an attentive comparison at different times and in different arrangements of circumstances, between the effects of contact on the gold leaves of the column, and on the leaves of the mimosa sensativa, and even contact with different bodies.
I come to greater symptoms of the motion of the electric fluid in the column, beginning by experiments, which will prove what, in the former paper, I have concluded, from my theory, namely, that by the cause assigned to this motion, the negative effect goes on increasing from the zinc to the copper extremity of the column, at the same time that the positive effect increases from the latter to the former ; and that the electric state of each point of the insulated column is the sum of the correspondent terms of two inverse series of progress represented by determined, though in some respects variable, numbers, in a table given in that paper.
For these experiments a third electroscope is used: in the figure it is seen connected with the middle point C of the column, where is a thick brass plate with a projecting loop, 4. This immediate connexion of the electroscope with the middle point of the column serves for some experiments ; but every other point of the column may be made to communicate with it, by the interposition of a soft wire held in the middle by an insulating handle. When this is used, the communication of the electroscope with the middle point is taken off; and, by bending the wire, it is easily made to connect, as conductor, the necessary parts of the column with this electroscope,
Exp. 7. At a time when there are simple divergences in the electroscopes at the extremities of the column, if they be equal, positive at A, and negative at B, there is no divergence in the electroscope at C; this is neutral, which is the case expressed in Table I, of the former paper ;* and if at this time any point of the column, at a distance from the point C, on the negative or positive side, proportional to one of the terms of the table, be tried with the insulated conductor, the divergence which it produces will be found, as exactly as can be expected in such experiments, correspondent to the number expressed in the table, with its sign.
Exp. 8. In this situation of the column the states expressed in Table II, and Table III,+ may be also observed by placing alternately its extremities in communication with the ground; but by a wire, because the metallic chains, commonly used for this purpose, do not transmit completely such small quantities of electric fluid, probably on account of some dust getting between the links. The following are the two cases of this experiment. 1. When the communication with the ground is made at B, the
• Volume viji, p. 449. + Ibid.
electroscope at the middle point C diverges positively, in the same degree as did before the electroscope at A ; and the divergence of the latter is now nearly double. I say nearly, because equal increases in the electric state produce smaller increases in the angular motion of the gold leaves in proportion as the angle increases. By using then the small insulated conductor, it is found, that the whole column, (except the very extremity B, which, communicating with the ground, is neutral) is in a positive state, increasing towards A : which is expressed in Table II.
2. When the communication with the ground is made at A, the electric states of the column are all reversed. The electroscope at the middle point C has now a negative divergence, equal to that of the electroscope at B in the insulated state of the column; and the divergence at B is nearly double. Then, by observing the state of the other parts of the column with the insulated conductor, the negative state is found increasing toward B, from A, the only point not negative, but neutral. This is the case represented in Table III.
In the three different cases above described, the indicated positive and negative states are, in every part of the column, common to zinc and copper. There is no doubt, in every binary association of the metals, that difference between them which their nature requires; but it is insensible in them individually, as it is when they are single ; and their electric state, embracing both metals, is determined, according to their position in the column, by the motions of the electric fluid resulting from these insensible elements; and that they follow the laws determined in my paper from the cause assigned, is verified by these experiments; which demonstrate at the same time, that there are no positive or negative states belonging to any part of the column (nor consequently of the pile): since each part may change from positive to negative, or inversely, according to circumstances, by the different motions impressed on the electric fluid; which motions may be concluded from these phenomena themselves, but will be directly perceived in the following experiment.
Exp. 9. A necessary condition of this class of experiments is, that the state of the ambient air be such, that alternately, at each extremity of the column, one of the gold leaves strikes the side of the electroscope, and at last sticks at one of the extremities. The following are three different cases in these phenomena.
1. When the strikings are alternate at the extremities, these instantaneous communications of the column with the ground at each side, by the contact of the gold leaf with the tin foil, produce in the former a flux and reflux of electric fluid. When the gold leaf strikes at the copper extremity, some fluid ascends from the ground into the column, and repairs the deficiency on this side ; but this additional quantity of electric fluid in the column occasioning a striking at the zinc extremity, the new quantity of fluid returns that way to the ground. These rises and ebbs of the electric fluid in the column are observed at the middle electroscope, but only when some time elapses between the strikings ; for, on account of the slowness of motion of the fluid, directly shown above, when the strikings rapidly alternate, before one of the effects has extended itself in the column, the contrary effect begins ; in the same manner as the rise and ebb of the water are not sensible within the Mediterranean Sea and the Baltic, on account of their narrow entrance ; for before the flux has extended itself some way up these seas, the reflux operates in a contrary direction. But these motions of the electric fluid are very sensible at the middle point, in the following cases.
2. When the gold leaf comes to adhere at the copper extremity, thus placing it in a continued communication with the ground, the strikings, which become more frequent at the zinc side, produce a pulselike stream of the electric fluid in the column, manifested by the motions of the gold leaves in the middle electroscope : their divergence is positive, the whole column being now in this state (Exp. 8, 1), but they fall in part, when the gold leaf strikes at the zinc side, and rise in the intervals of the strikings ; thus pointing out clearly a current flowing from B to A, at a higher level than the standard, which level alternately rises and falls.
3. When the adhesion of the gold leaf takes place at A, the zinc side ; which circumstance, producing a continued communication of this side with the ground, renders the column negative in the whole (Exp. 8, 2); while the gold leaf at B, the copper side, goes on striking: a current of electric fluid is also produced, but at a lower level than the standard : the divergence of the gold leaves in the middle electroscope is consequently negative, and, as in the former case, they fall in part at every striking, and rise in the intervals ; but while in that case they fell by the lowering of level of the cur. rent, and rose when it came higher; now diverging as negative, they fall at every striking, because some fluid, ascending from the ground, makes the column less negative ; and they rise again while this fluid flows into the ground by the zinc side, and thus prepares another striking.
I have made the same experiments on the motions of the electric fluid within the pile itself; they are more confused and less lasting on account of the calcination of the metals : but the column, being in fact the electric machine of the pile, shows clearly and permanently these motions, which I shall now follow in the circuit, or when the two extremities of the column (or the pile) are connected together by some intermediate body. In this case the motion of the electric fluid is manifested by more or less retardation of its current, according to the degree of conducting faculty of the body employed.
For this class of experiments (see the figure) brass hooks, 5 and 6, are fixed to the small brass plates, terminating the column at each extremity, and against which press the screws : these hooks project a little more than an inch, and serve for different purposes. The following experiments will relate to the conducting faculty of that kind of glass tube filled with water, entered by wires on both sides, in which chemical effects are produced when it is applied to the pile; but with the column, these effects do not take place.
Exp. 10. The tube of the above kind, which serves in this experiment, is represented in the figure, as suspended at the point 7; its wire, 8, having a hook, held up by a silk thread, which, passing over the pulley, 10, descends to a thin brass plate, 11, fixed to the base of the instrument. This brass piece bends forward at the top, and the silk, entering into a notch of this projection, is stopped there by a bead fixed to it. The other wire, 9, of the tube, is hooked on the projecting wire, 6. In this situation of the tube, it does not affect the electroscopes of the column, they continue to diverge as if the tube were not connected with one of them ; but when, the silk being disengaged, the hook of the wire, 8, comes to rest on the hook, 5, of the column, the circulation of the electric fluid produced through it between the extremities A and B is so rapid, that the divergence entirely ceases in the electroscopes; and it returns only, when the extremity of the wire, 8, is again lifted up. This shows, that the glass tubes of this kind are sensibly as good conductors as metals.
The different conducting faculties of bodies proceed from different degrees of adhesion of the electric fluid to them : but beside this difference among bodies, there is another, which relates to permeability. All the bodies, which I have tried, are permeable to the whole of the electric fluid, except those that can be charged; which are impermeable to the electric matter, and permeable only to the vector. This operation, called charge, as I have explained and proved in my works, consists in accumulating the electric matter only on one side of the laminæ made of these substances, by occasioning a proportional diminution of its quantity on the opposite surface, which is an operation of the vector ; and the reason why other bodies cannot be charged is, that, the electric matter pervading them, though slowly in some of them, no sensible difference in the quantity of electric matter can be produced between their opposite surfaces.
Glass, in this respect, is a remarkable substance. It is absolutely impermeable to the electric matter, and, being a solid body, it is used for insulating pillars in our electric apparatusses; but the electric matter moves easily along its surface, as I have visibly shown by the Lichtenberg figures produced on its naked surface, where they dissipate in a little time, while they last many days on resinous surfaces. This is the reason of covering the glass pillars destined to insulate with some resinous varnish ; but all these varnishes are not equally fit for the purpose, and this is one of the objects of the following process, as well as the trial of the different conducting faculties of other bodies.
For these experiments, the bodies to be tried must be reduced into slips or rods, which are to be laid on the hooks, 5 and 6, of the column, in order to observe the effect produced on its electroscopes ; but there are necessary precautions to be used in laying them on the hooks. For instance, in respect to the bodies with which I shall begin, those which have different conducting faculties, belonging mostly to the vegetable and animal kinds, when they are placed on the hooks with the fingers, as it is almost impossible to lay them on both hooks at the same instant, the end which touches first disturbs the equilbrium of the electric fluid in the column ; and I have shown above, that it is but slowly restored. In order to obviate this defect, and for another purpose that will follow, two brass wire brackets, 12, 12, are fixed in the front of the base of the instrument, on which the slips are first laid, and there taken up by two glass hooks covered with sealing wax, with which they are placed on the hooks of the column. I shall give a general idea of these trials under the following head.
Exp. 11. The substances of this class having more or less conducting faculty, they lessen in different degrees the divergence in the electroscopes, by transmitting more or less electric fluid from A to B. This is a curious kind of experiment, but as the particulars are not here my object, I shall relate only one, concerning the physiology of vegetables, which may lead to others of the same kind. Having repeated in presence of Dr. Lind these experiments on the different conducting faculties of various bodies, I showed him a phenomenon, which had surprised me. A thin slip of deal, cut along the fibres, being applied to the column, there remained but little divergence in the electroscopes ; while a slip of the same wood, of the same thickness and breadth, but cut across the fibres, produced much less diminution in the divergence. Dr. Lind found probably the cause of this difference, assigning it to the situation of the resinous substance within that wood: it does not belong to the fibres themselves, since they transmit easily the electric fluid ; it is lodged between the fibres, and thus forms an impediment to the passage of the fluid from fibre to fibre in the slips cut across them.
When these experiments are made with the view of trying the insulating property of bodies, still more precaution is required in placing them on the hooks of the column : for the bodies fit for this use being fundamentally impermeable to the electric matter, their electric state is changed, more or less permanently, by friction; and this in the manner which I intend to explain in a future paper. As, however, they can hardly be handled without some friction, they act upon the column by their influence (an effect which I shall show directly hereafter), and their insulating property cannot be observed, on account of the disturbance which they produce in the state of the column itself. These bodies therefore must remain a little time untouched on the brackets, and be there breathed upon, in order that the moisture of the breath may dissipate their electrization ; serving as a conductor for their whole surface to the ground, through the