and still conducts internally. It is then in the best possible state. The same may be said of dry unbaked wood. Varnishing the upper surface of a piece of marble or wood is equivalent to laying a thin glass plate on it. Now this method, or covering the top of the marble, or of a book, or even the table, with a piece of clean dry silk, makes them all the most perfect condensators. This just view of the matter has great advantages. It takes away the mysterious indistinctness and obscurity which kept the instrument a quackish tool, incapable of improvement. We can now make one incomparably better and more simple than any proposed by the very ingenious inventor. We need only the simple moveable plate. Let this be varnished on the under side with a moderately thick coat of the purest and hardest vernis de Martin, or coach-painters varnish; and we have a complete condensator by laying this on a table. If it be connected by a wire with the substance in which the weak and imperceptible electricity is excited, it will be raised (provided there be enough of it of that small intensity) in the proportion of the thickness of the varnish to the fourth part of the diameter of the plate. This degree of condensation will be procured by detaching the connecting wire from the insulating handle of the condenser, and then raising the condenser from the table. It will then give sparks, though the origi nal electricity could not sensibly affect a flaxen fibre.

It must be particularly noted, that it can produce this condensation only when there is fluid to condense; that is, only when the weak electricity is diffused over a greater space than the plate of the condenser. In this way it is a most excellent collector of the weak atmospheric electricity, and of all diffused electricity. But to derive the same advantage from it in many very interesting cases, such as the inquiry into the electricity excited in many operations of Nature on mall quantities of matter, we must have condensers of vairus sizes, some not larger than a silver penny. To construct these in perfection, we must use the purest and

hardest varnish, of a kind not apt to crack, and highly coercive. This requires experiment to discover it. Spirit varnishes are the most coercive; but by their difference of contraction by cold from that of metals, they soon appear frosty, and when viewed through a lens, they appear all shivered: They are then useless. Oil varnishes have the requisite toughness, but are much inferior in coercion. We have found amber varnish inferior to copal varnish in this respect, contrary to our expectation. On the whole, we should prefer the finest coach-painters varnish, new from the shop, into which a pencil has never been dipped: and we must be particularly careful to clear our pencils of moisture and all conducting matter, which never fails to taint the varnish. We scarcely need remark, that the coat of varnish on these small condensers should be very thin, otherwise we lose all the advantage of their smallness.

201. Mr. Cavallo has ingeniously improved Volta's condenser by connecting the moveable plate, after removal, with a smaller condenser. The effect of this is evident from § 130. But the same thing would have been generally obtained by using the small condenser at first, or by using a still thinner coat of varnish.

202. It will readily occur to the reader, that this instrument is not instantaneous in its operation, and that the application must be continued for some time, in order to collect the minute electricity which may be excited in the operations of nature. He will also be careful that the experiment be so conducted that no useless accumulation is made anywhere else. When we expect electricity from any chemical mixture, it never should be made in a glass vessel, for this will take a charge, and thus may absorb the whole excited electricity, accumulating it in a neutral or insensible state. Let the mixture be made in vessels of a conducting substance, insulated with as little contact as possible with the insulating support; for here will also be

something like a charge. Suspend it by silk threads, or let it rest on the tops of three glass rods, &c.

203. After this account of the Leyden phial, electropho rus, and condenser, it is surely unnecessary to employ any time in explaining Mr. Bennet's most ingenious and useful instrument called the doubler of electricity. The explanation offers itself spontaneously to any person who understands what has been said already. Mr. Cavallo has with industry searched out all its imperfections, and has done something to remove them, by several very ingenious constructions, minutely described in his Treatise on Electricity. Mr. Bennet's original instrument may be freed, we imagine, as far as seems possible, by using a plate of air as the intermedium between the three plates of the doubler. Stick on one of the plates three very small spherules made from a capillary tube of glass, or from a thread of sealing wax. The other plate being laid on them, rests on mere points, and can scarcely receive any friction which will disturb the experiment. Mr. Nicholson's beautiful mechanism for expediting the multiplication has the inconveniency of bringing the plates towards each other edgewise, which will bring on a spark or communication sooner than may be desired: but this is no inconvenience whatever in any philosophical research; because, before this happens, the electricity has become very distinguishable as to its kind, and the degree of multiplication is little more than an amusement. The spark may even serve to give an indication of the ori ginal intensity, by means of the number of turns necessary for producing it. If the fine wires, which form the alternate connections in so ingenious a manner, could be tipped with little balls to prevent the dissipation, it would be a great improvement indeed. An alternate motion, like that of a pump-handle, might be adopted with advantage. This would allow the plates to approach each other face to face, aud admit a greater multiplication, if thought necessary.

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204. One of the most remarkable facts in electricity is the rapid dissipation by sharp points, and the impossibility of making any considerable accumulation in a body which has any such, projecting beyond other parts of its surface. The dissipation is attended with many remarkable circumstances, which have greatly the appearance of the actual escape of some material substance. A stream of wind blows from such a point, and quickly electrifies the air of a room to such a degree, that an electrometer in the farthest corner of the room is affected by it. This dissipation in a dark place is, in many instances, accompanied by a bright train of light diverging from the point like a firework. Dr. Franklin therefore was very anxious to reconcile this appearance with his theory of plus and minus electricity, but does not express himself well satisfied with any explanation which had occurred to him. From the beginning, he saw that he could not consider the stream of wind as a proof of the escape of the electric fluid, because the same stream is observed to issue from a sharp negative point; which, according to his theory, is not dispersing, but absorbing it. Mr. Cavendish has, in our opinion, given the first satisfactory account of this phenomena.

205. To see this in its full force, the phenomenon itself must be carefully observed. The stream of wind is plainly produced by the escape of something from the point itself, which hurries the air along with it; and this draws along with it a great deal of the surrounding air, especially from behind, in the same manner as the very slender thread of air from a blow-pipe hurries along with it the surrounding air and flame from a considerable surface on all sides. It is in this manner that it gathers the whole of a large flame into one mass, and, at last into a very point. If the smoke of a little rosin thrown on a bit of live coal be made to rise quietly round a point projecting from an electrified body, continually supplied from an electrical machine, the vortices of this smoke may be observed to curl in from all

sides, along the wire, forming a current of which the wire is the axis, and it goes off completely by the point. But if the wire be made to pass through a cork fixed in the bottom of a wide glass tube, and if its point project not beyond the mouth of the tube, the afflux of the air from be hind is prevented, and we have no stream; but if the cork be removed, and the wire still occupy the axis of the tube, but without touching the sides, we have the stream very distinctly; and smoke which rises round the far end of the tube is drawn into it, and goes off at the point of the wire. Now it is of importance to observe, that whatever prevents the formation of this stream of wind prevents the dissipation of electricity (for we shall not say escape of electric fluid) from the point. If the point project a quarter of an inch beyond the tube, or if the tube be open behind, the stream is strong, and the dissipation so rapid, that even a very good machine is not able to raise a Henley's electrometer, standing on the conductor, a very few degrees. If the tube be slipped forward, so that the point is just even with its mouth, the dissipation of electricity is next to nothing, and does not exceed what might be produced by such air as can be collected by a superficial point. If the tube be made to advance half an inch beyond the point which it surrounds, the dissipation becomes insensible. All these facts put it beyond a doubt that the air is the cause, or, at least, the occasion of the dissipation, and carries the electricity off with it, in this manner rendering electrical the whole air of a room. The problem is reduced to explain how the air contiguous to a sharp electrified point is electrified and thrown off.

It was demonstrated in § 130, that two spheres, connected by an infinitely extended, but slender conducting canal, are in electrical equilibrium, if their surfaces contain fluid in the proportion of their diameters. In this case, the superficial density of the fluid and its tendency to escape are inNow if, in imagination,

versely as the diameters (§ 130)

we gradually diminish the diameter of one of the spheres