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power of which could be increased by a greater number of groups.

It would have been too expensive to proceed in that atteinpt with silver plates, and I was going to undertake it with copper plates, pasting the paper over them, when fortunately I thought of a paper, called Dutch gilt paper, on which a kind of copper is ready laid. At órst I did not find the paper of that sort which is plain, but having tried that which is flowered, and finding much effect, I made the following experiment.

Exp. 27.—I cut seventy-six pieces of this paper to the size of my zinc plates, 1.6 inch diameter, and haviug formed a pile of seventysix groups, with only the zinc plates separated by the pieces of this paper,

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side of all of them turned the same way, I found more electric effects at its extremities, than with the former pile of the same number of groups of zinc, silver, and wet cloth.

This experiment increased my hope of obtaining a natural electric machine, not indeed of a great power, but new in experimental philosophy, and which might lead to some farther discovery; but unluckily for speed, being sedentary at Windsor, I was not yet informed that, by a mode rendering zinc ductile, thin plates could be obtained of it, which would auswer my purpose, since in this use, calcination was not to be apprehended. Wishing however to proceed some way in this attempt, and having found by trial with my condenser, that tinned iron plates had sensibly the same effect as pewter plates, I procured as many plates of the former, 1-6 inch diameter, as would fill up the two columns of my frame, separated as above by pieces of flowered Dutch gilt paper. There were four hundred in each column, thus composing a pile of eight hundred groups.

Exp. 28.—The first object I shall mention of this experiment strongly corroborates the former conclusions concerning the mode of action of the galvanic pile. Though this pile had at its extremities greater electric effects than I have ever seen at those of any pile producing the strongest chemical effects in the glass tubes with water ; yet these tubes being applied, there was not the least appearance of such effects.

But at the same time a new scene was opened to my view. This natural electric machine soon gave me reason to hope that it might become a new and very essential meteorological instrument. Having no motive to dismount it, I left it a long time in the same state, observing the electroscopes, and the following are the general phenomena which they offered to me.

1. The quantity of electric fluid put in motion by this pile was too great for the gold leaves to remain with a simple divergence; one of them struck the tinfoil on its side, then fell, and struck again; which effect became the object of observation.

2. At times these strikings were at both extremities of the pile, alternately. When the gold leaf struck on one side, it placed this extremity for an instant in communication with the ground; the gold leaf fell, but this communication with the ground increased the

opposite electric state at the other extremity, where one of the gold leaves struck and fell; and this continued by turns.

3. At other times the strikings were only on one side, and continued perhaps one day or two; then that state changed, and the strikings were at the other side.

4. At which ever side the strikings were going on there was a great difference in their frequency: sometimes the gold leaf seemed to beat seconds, while at other times it struck but once in a minute, or a longer time.

5. When the frequency of the strikings was upon the whole nearly equal many consecutive days, it increased succesively from the morning till some time in the afternoon, and then went on diminishing till night.

6. Sometimes I connected one side of the pile with the ground, by a metallic rod, which made the gold leaves fall on this, and increased the frequency of the strikings on the opposite side; but this increase was neither always the same, nor equal in the same time, when the communication with the ground was changed from one side to the other.

7. The beginning of my observations of this kind was in the winter of 1808, and in this season the strikings were upon the whole very frequent; but as the spring advanced, and perhaps as vegetatation increased on the ground, there was a gradual diminution in the motion of the electroscopes, and a change in the effects on the pile from the communication of its extremities with the ground.

8. Lastly. These changes could not be attributed to those of either heat or moisture, at least in my room, and as influencing directly the pile; for the room remained nearly at an equal temperature, and there were but small changes in my hygrometer. I also frequently observed the barometer, and found no connexion between its variations and these great changes in the strikings of the gold leaves.

From the whole together of these observations, I could attribute the variations in the electric phenomena of the pile only to changes in the electric state of the ambient air ; or those in the effects of the communication with the ground, to changeable differences between the electric state of the latter and that of the air ; an object on which we owe to Sig. Volta a great and fundamental truth. This justly celebrated experimental philosopher has made it evident that air possesses the electric fluid as well as all the bodies which it surrounds, and on this principle he has founded the only real explanation of the electric motions of free bodies, such as the pairs of balls and the gold leaves of our electroscopes. The electric fluid tends to an equilibrium among all bodies, including the air, and there is a mutual attraction between it and all of them, weak with conductors, but strong with non-conductors, among which is air. The proportional quantity of electric fluid which is actually possessed by air, is the standard of plus and minus in the electroscopes ; and the proportional quantity of this fluid, which is actually possessed by

the ground, is the standard of plus and minus concerning the differences in the electric state of insulated bodies compared with its own: these standards are sometimes similar, but they are often different. I shall not enlarge here on this beautiful theory, which I have fully explained in my works, and demonstrated by direct experiments;* but applying it to the pile, the immediate effects of the ambient air must be to give some electric fluid to its negative side, which has proportionally less than itself, and to take some from the positive side, which has more ; and as this influence must change according to the changes in the electric state of the air, the modifications of the electroscopes at the extremities of the pile may lead to unfold the latter, but not without farther discoveries.

I was stopped in the progress of these researches by an incident which has occasioned me much labour, and still retards them. This new pile is certainly in itself a meteorological instrument of great importance, as may be already judged, and shall be farther explained in the following paper; but in the state above described it was not fit for regular observations, and till the present momentt I have not yet surmounted all the difficulties. I foresaw them, and it was the reason why I wished that the beginning of this new career should be soon known to experimental philosophers through the Phil. Transactions, in order that it might be followed by others, and probably with more success than by myself; but I am reduced to give the history of my own progress,

From what has been above explained, the strikings of the gold leaves in the electroscopes were become the object of observation: for this purpose, therefore, they ought to have regularly continued; however, after one of the gold leaves, at either side, had alternately struck and fallen for some time, it at last stuck to the tinsoil. The side, therefore, to which this happened, was placed in permanent communication with the ground, which made the strikings to begin at the opposite side; but there also the gold leaf stuck. These adhesions continued, till, by a stroke on the top of the electroscope, the the gold leat fell; the scillations were then renewed, but again stopped in the same manner. I tried various methods for preventing this adhesion, especially by substituting for the tinfoil, which has a rough surface, polished laminæ of many sorts and forms, keeping them even at some distance from the glass, which might contribute to this effect; but all was to no purpose, still the gold leaf would stick. This was a great disappointment, and the only method I could devise was, to increase the power of a pile, till it could move small metallic balls, in hope that these would not stick,

Two ways presented themselves to my mind for this increase of power in a pile; one by using plates of a larger size, the other by

Idées sur la Météorologie, published in London by Elmsley in 1786 and 1787.

+ Two years have elapsed since I communicated to the Royal Society the experiments and observations which are here my object.

increasing the number of the groups. The first method would have been cumbersome; but for a reason which I shall explaiu, I did not expect that it would have the desired effect. However, even for the verification of my conjecture, I made the following experiment.

Exp. 29.-I procured 10 tinned iron plates, 4 inches square; 10 others round, of only 0.5 inch diameter; and I took 10 of my plates of 1.6 inch diameter. I cut pieces of Dutch gilt paper, the size of each of them, and compared their respective effects on my condenser. The result was beyond my expectation; I had only conjectured that the increase of size would not increase the divergence of the gold leaves; now this was not only verified, but the largest plates produced the smallest effect. However, this unexpected difference probably proceeded from some accidental cause, which I had no time to investigate, and I considered this experiment, which I have often repeated under various forms, only as ascertaining the following proposition: that for the electric motions, considered solely with respect to the quantity of divergence in the electroscope, the size of the plates is indifferent; though, for the frequency of the strikings of the little pendula, and the intensity of the effects when the extremities of the pile are connected together, with the same number of groups, these effects increase with the size of the plates. This distinction, to which I shall return in the following paper, constitutes a part of the theory, which, as it made me foresee the result of the above experiment, I shall now explain.

I am indebted for the ground of this theory to Sig. Volta, wbo, when in 1782, he showed me, at Paris, bis then new-invented admirable condenser, explained to me, that it could not serve to nauifest minute degrees of deviation from the electric standard, when belonging to small bodies, but only to bodies of such an extent or nature, that the application of the condenser (by this taking its share of that deviation) does not sensibly lessen it. As an example of the first case, he gave me the atmosphere; and with respect to the nature of bodies, he took a Leyden phial discharged without a continued contact, the residuum of which, from its nature, may affect a moderate sized condenser without heing much lessened : and to show me the necessity of this condition, he made use of the following analogy. When a piece of ground, by being swampy, indicates some stagnant water, if a well be dug there, the water will not bill it up to the level of the stratum of earth whence it proceeds, unless this stratum be of such an extent, that the quantity of water which gathers in the well bas no sensible proportion with that contained in the stratum ; so that the subtraction of this quantity cannot affect the level at which the water stands in the stratum. We have also an example of this case in the subject of electricity : when an insulated electrified body is small, we cannot know its real degree of electrification by applying a common electroscope ; because this, sharing the deviation of ihe state of the body from the electric standard, lessens it too much for expressing what it was before that application.

I shall use the former of these examples in explaining my theory concerning the difference of effects of the size of the plates, and the number of the groups, according to the use of the pile ; and this explanation will chiefly consist in fixing the points of analogy between the two objects.

1. I compare the number of groups in the pile to the elevation of a stratum whence water issues into a well.

2. The size of the plates, to the extent of this stratum. 3. The degree of divergence in the electroscopes at the extremities of the pile to the level which the water can attain in the well without over flowing.

These first analogies are sufficient to explain the case of the last experiment. When we attend to what is directly expressed by our electroscopes, we certainly do not expect that this instrument shall indicate the quantity of electric fluid possessed by the bodies to which it is applied; for this would require also to measure their surface; we expect only to know the comparative density of the electric fluid among bodies, or its power to produce certain degrees of divergence in the electroscope we use.* Such therefore are the indications of the electroscopes at the extremities of the pile: they express certain degrees of density of the electric fluid on them, which are the same whatever be the area of the plates; these degrees depending only on the number of the groups, because each group contributes to increase the density of the fluid on one of the extremities by lessening it on the other. Thus it is that the divergence, both in plus and minus, of the electroscopes, at the extremities of the pile, is proportional only to the number of groups, in the same manner that the height at which water stands in a well is proportional only to the elevation of the stratum whence it proceeds.

4. But when, in order to produce a current of water, a pipe is placed, or a trench is cut, on the side of a well, below the level at which the water stood in it, the current will be greater and more permanent, in proportion to the extent of the stratum, of the same elevation, whence the water proceeds; and also, when the divergence

• I shall take this opportunity of explaining why I use the word electroscope, and not that of electrometer; it is because there is no instrument entitled to the latter denomination, at least admitted among experimental philosopbers. Indeed, of our instruments serving to measure the degrees of intensity of physical causes, I know none absolute, except hygrometers, such as have been constructed by M. de Saussure and myself; for though these instruments are made of different substances, and differ in some other respects, we have obtained in both an absolute zero, and absolute maxima in determined cases, as well as determined degrees of intensity, of their object, namely moisture : all the other instruments intended for the same purpose, to my knowledge, are only hygroscopes, indicating variations in moisture, without determined points, or degrees common to them. Thus no kind of physical instrument has yet obtained the conditions of an absolute measure, but the above hygrometers, so little thought of by experimental philosophers, though very important in meteorology. The thermometer has obtained two fired

degrees of heat, and determined divisions of the interval between them,

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