gold leaf or tin-foil. It must be carefully freed from all ints and asperities; and if perforations are made in for the purpose of attaching wires and other kinds of txtures for the purposes of experiment, they should be ade about the size of a quill, and should have their edges well rounded and sinoothed off. The pressure of the cushion against the cylinder is regulated by an justing screw, adapted to the wooden base at E, on hich the glass pillar that supports the conductor is fied. From the upper edge of the cushion there proeds a flap of thin oiled silk D, which is sewed on the hion about a quarter of an inch from its upper edge. extends over the upper surface of the glass cylinder within an inch of a row of metallic points, proceeding, de the teeth of a rake, from a horizontal rod, which is xed to the adjacent side of the opposite conductor. The motion of the cylinder, which is given by a single handle, or by a multiplying wheel, must always be given in the direction of the silk flap. That part of the cshion which comes in contact with the glass cylinder should be coated with an amalgam composed of a little -fool and mercury, mixed like a paste by means of gs' lard. The amalgam should be placed uniformly wer the cushion, until level with the line formed by the seam which joins the silk flap to the face of the shion. No amalgam should be placed over this line, on the silk flap; and it is even requisite to wipe silk flap clean whenever the continued motion of machine shall have soiled it, by depositing dust or sigam on its surface.

This machine acts in the following manner:-When cylinder is driven round by the handle, the friction the cushion upon it produces a transfer of the electric ted from the latter to the former; that is, the cushion comes negatively, and the glass positively, electrified. by the revolution of the cylinder, the fluid adhering to the glass is carried round, and its escape is at first ented by the silk flap which covers the cylinder, til it arrives near to the metallic points, which absorb st of the electricity, and convey it to the prime consector. This being positively electrified, the conductor ected with the cushion being deprived of this electy, is negatively electrified; so that light balls sus ended by threads at F, being oppositely electrified, will aract each other. After the action has gone on for ve time, the cushion and its conductor become exted of their electricity; so that a new supply must e brought from the earth, the great reservoir of the This is easily done, by establishing a communin between the cushion and the ground by means a metallic chain or wire. In this manner, a constant Can of positive electricity flows to the prime contor. Negative electricity is obtained by insulating conductor to which the cushion is attached, and ecting the prime conductor with the ground, so as carry off the fluid collected from the cylinder. If person who works the machine be supported upon ool having glass legs, and connected with the conor by means of a metallic rod, or if he touch it with hand, he is found to be in the same state of electri7. and another person standing upon the ground can sparks from him by presenting his knuckles to body. By using the electrical machine in the above manner, arenabled to collect a considerable quantity of tity, and thus perform experiments upon an ple scale. A pith ball, or a fragment of gold leaf, ery strongly and immediately attracted by the elecconductor; and, the instant after it has come ntact with it, it is repelled; but it is now at ted by the other bodies in its neighbourhood, to *tch it communicates its own electricity, and then is certain conditions of the atmosphere, electrical sparks are d in abundance from paper as it issues from the cylinders paper-making machine, the friction of the dry material of the apparatus being the apparent cause of the phenoIf a battery of Leyden jars were employed to collect these trical experiments might be readily performed with

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again in a state to be influenced by the conductor, and to be again attracted; and this alternation of effects will continue as long as the conductor remains charged. This alternation of attractions and repulsions accompanying the transferring electricity by moveable conductors, is also illustrated by the motions of a ball suspended by a silk thread, and placed between two bells, of which the one is electrified, and the other communicates with the ground. The alternate motion of the ball between the two bells will keep up a continual ringing. This amusing experiment has been applied to give notice of changes taking place in the electrical state of the atmosphere.

The mutual repulsion of bodies that are similarly electrified gives rise to many interesting experiments. A small figure in the shape of a human head, covered with hair, when placed upon the conductor, and electrified, will exhibit the appearance of terror, from the bristling up and divergence of the hair.

The intensity of the electricity which bodies may contain, is measured by a delicate instrument, called an Electrometer, of which there are several invented by various distinguished individuals. Our limits, however, will not admit of our giving a minute account of them. They all depend upon the repulsive property of electrified bodies, and the distance to which the one is repelled by the other is indicated by an index of one kind or another.

We have already observed, that upon the extent of the surface of a body, its capacity for receiving electricity principally depends. Electricity is therefore supposed not to spread throughout the whole mass of a body, at least equally, but to remain principally, if not altogether, at the surface. This has been proved by experiments for trying the distance to which the electricity extended beyond the coating of the Leyden jar.

Several remarkable phenomena occur when electricity is drawn off by means of a conductor from those bodies in which the electrical equilibrium has been destroyed. A sharp snapping sound is heard, accompanied by a vivid spark, whilst intense heat is evolved in the path which the electric fluid takes. A perfect conductor offering no impediment to its course, it is unattended with light during its passage through such a body, light only appearing when there are obstacles in its path, such as imperfect conductors. Of the velocity with which it is transmitted, we have already spoken. It is so great, that in experiments performed with a chain of considerable length, each link became apparently instantaneously luminous. There are various methods of showing the intensity and colour of electrical light. Conductors having a rounded form give the longest and most vivid sparks, which are sometimes seen to take a zig-zag course, similar to that of a flash of lightning. This deviation in its course is supposed to be occasioned by the fluid darting to minute conducting particles, such as those of moisture floating in the air. Electrical light is similar to light obtained from other sources, and its brilliancy depends upon its intensity. Sir David Brewster found that it was capable of polarisation. It displays every shade of colour, that quality being dependent upon the nature of the substance through which the fluid passes.

An interesting question arises-Whence comes the light-is it the electric fluid which thus renders itself visible? This was really supposed to be the case by the early electricians, but later philosophers have substituted other theories to account for the phenomena. That of M. Biot, a celebrated French philosopher, is, that electric light has the same origin as the light disengaged from air by mechanical pressure; " and that it is purely the effect of the compression produced on the air by the explosion of electricity." This hypothesis has been objected to, however, on the ground that electrical light is produced in the best vacuum that can be formed; and although he has replied to the objection, that no perfect vacuum can exist, yet his arguments, though they carry weight, do not bring conviction to

the mind.

We have already observed, that various sounds accompany the various modes of transference of the electric fluid; a peculiar odour has also sometimes been felt near a machine which has been sharply wrought; but whence its origin, is unknown. All sharp-pointed bodies, we have said, concentrate most of the electric fluid at their apex, from whence it has a powerful disposition to escape; and every discharge is accompanied by currents of air. Upon this principle, many ingenious experiments are founded. An apparatus, consisting of wires terminating in points, and having balls annexed to them to represent the planets, may be constructed so as to revolve when electrified, and thus to imitate the planetary motions. We cannot enter further into this subject, but may state in general terms, that the appearances of the electric spark depend upon the nature of the surface from whence it issues and towards which it is directed. When it escapes from a pointed body, the luminous appearance is that of diverging streams, resembling the filaments of a brush, and forming what is termed a pencil of light; but when the fluid goes to a point, the light concentrates at the point itself, and assumes the appearance of a star.

The most convenient mode of obtaining an accumulation of electricity arising from induction, is by the employment of coated glass; that is, of a plate of glass on each side of which is pasted a sheet or coating of tin-foil. Care must be taken to leave a sufficient margin of glass uncovered with the metal, for preventing the transfer of electricity from one coating to the other, round the edge of the glass; and all sharp angles or ragged edges in the coatings should be avoided, as they have a great tendency to dissipate the charge.

The form of coated glass best adapted to experiments is that of a cylindric jar; this is coated, within and without, nearly to the top. The cover consists of baked wood, and is inserted with sealing-wax, to exclude moisture and dust. A metallic rod, rising two or three inches above the jar, and terminating at the top in a brass knob, is made to descend through the cover till it touches the interior coating. The name of the Leyden phial, or jar, is applied to this instrument. It is used in the following manner :-The outer coating being made to communicate with the ground, by holding it in the hand, the knob of the jar is presented to the prime conductor when the machine is in motion; a succession of sparks will pass between them, while, at the same time, nearly an equal quantity of electricity will be passing out from the exterior coating, through the body of the person who holds it, to the ground. The jar, on being removed, is said to be charged; and if a communication is made between the two coatings, by a metallic wire extending from the external one to the knob, the electric fluid which was accumulated in the positive coating, rushes, with a sudden and violent im

Fig. 3.

petus, along the conductor, and passes into the negative coating; thus at once restoring an almost complete quilibrium. This sudden transfer of a large quantity accumulated electricity is a real explosion; and it

gives rise to a vivid flash of light, corresponding in intensity to the magnitude of the charge. The effect of its transmission is much greater than that of the simple charge of the prime conductor of the machine; and it imparts a sensation, when passing through any part of the body, of a peculiar kind, which is called the electric shock. The arrangement of the parts in a Leyden jar is shown in the foregoing figure, in which the simple bent discharging rod, for establishing a direct coinmunication between the inner and outer coating a jar or battery, and restoring the electrical equili brium, without the operator receiving the charge of the jar, is exhibited. E represents the insulating handle, and A the bent rod of brass reaching from the ball to the external coating. When opened to a proper degree, one of the balls is made to touch the exterior coating and the other ball is then quickly brought into contact with the knob of the jar, and thus a discharge is effected. By uniting together a sufficient number of jars, are able to accumulate an enormous quantity of elec tricity. For this purpose, all the interior coatings

the jars must be made to communicate by metallic rods, and a similar union must be established among the exterior coatings. When thus arranged, the whole series may be charged, as if they formed but one jar and the whole of the accumulated electricity may be transferred from one system of coatings to the other by a general and simultaneous discharge. Such a com bination of jars is called an electrical battery. An ar rangement of this description is here represented, in which twelve jars are united in one box, and the whole series connected together by wires and balls.

If we wish to send the whole charge of electricity through any particular substance which may be the subject of experiment, we must so arrange the connect ing conductors as that the substance shall form a neces sary part of the circuit of the electricity, as it is termed With this view, we must place it between two good con ductors, one of which is in communication with the outer coating; and the circuit may then be completed by connecting the other conductor with the inner coat ing by means of a discharging rod, to one branch o which, if necessary, a flexible chain may be added.

In forming arrangements for directing the passag of accumulated electricity, it should be borne in min that the electric fluid will, on these occasions, alway pass through the best conductors, although they may be more circuitous, in preference to those which ar more direct, but have inferior conducting power; an it must also be recollected, that, when different path are open for its transmission along conductors of equa power, the electricity will always take that which is the shortest. Thus, if a person holding a wire between hi hands discharges a jar by means of it, the whole of th fluid will pass through the wire without affecting him but if a piece of dry wood be substituted for the wire he will feel a shock; for the wood being a worse con ductor than his own body, the charge will pass throug the latter, as being easier, although the longer circun During its transit through the human body, in lik manner, the shock is felt only in the parts situated u

the direct line of communication; and if the charge be made to pass through a number of persons, who take ene another by the hand, and form part of the circuit between the inner and outer coatings of the jar, each will feel the electric shock in the same manner and at the same instant; the sensation reaching from hand to hand, directly across the breast. By varying the points of contact, however, the shock may be made to pass in ther directions, and may either be confined to a small part of a limb, or be made to traverse the whole length f the body from head to foot.

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By accurate experiments, it appears that the force of the electric shock is weakened, or its effects are minished, by employing a conductor of great length for making the discharge. A retardation in the passage ef electricity also takes place if the conductor is not of a sufficient size; and when this is the case, as well as in those instances where the conductor is not a good (one, the discharge will not be effected so instantaneously or so completely. It has also a tendency to diverge from the direct line of its course, being drawn towards conducting bodies which may attract it. The motion of electricity through perfect conductors is attended with no perceptible alteration in the mechanical properties of the conducting bodies, provided they be of hcient size for the charge of the electric fluid transitted. On the contrary, very considerable effects are produced when a powerful charge is sent through a wire ich is too small to allow the whole quantity to pass with perfect freedom, or through an imperfect conduc, though of large size, as is proved when a tree is ruck by lightning.

ELECTRICITY APPLIED TO INORGANIC BODIES AND ANIMALS. The effects of electricity passing through various bstances are both of a mechanical and chemical rature. The former resemble those which would be produced by a material agent driven with great velocity through the substance of the body. But there are many changes induced by electricity, such as cannot be attriAated to mechanical agency, and are undoubtedly of a chemical nature. Some of the mechanical effects have Aiready been noticed. Dr Priestley discovered that it expanded bodies. This is proved by passing a stream the fluid through a capillary or thermometer tube aded with mercury; the latter will be so much expanded as to break the glass to shivers. The tenency to expand will of course be greater as the concarting power of the body which transmits it is less. Although we know nothing of the nature of electricity, yet it has been found convenient to speak of it as a d. Its action upon bodies which either obstruct its action or afford it a ready passage, renders its analogy with a fluid very striking, and the laws of its equilibrium are just those of an imponderable fluid. Solid bodies capable of being diffused into vapour by passing ctricity through them, as is shown by the following periment:-Take three strips of window-glass, each ut three inches long and one wide, and having pared two narrow strips of gold leaf or leaf brass been them, so that the ends of the gold leaf project a te beyond the glass, transmit the charge of a large Leyden jar through the gold leaf. The gold leaf will found to be melted by the shock, and driven into the trface of the glass. The outer plates of glass are genenly broken in this experiment, and the middle one, rich frequently remains entire, has an indelible metale stain upon each of its surfaces. This stain is bviously the metallic vapour of the gold driven into the pores of the glass.

The metallic colours thus obtained have been employed for impressing ornamental figures upon paper er silk. In order to do this, trace the outline of the figures on thick drawing-paper, and having cut it out as in stineil plates, place it on the silk or paper intended to be ornamented. When a gold leaf is laid upon it, and a card above the gold leaf, the whole is placed in a press or beneath a weight, and an electrical charge sent through it; the metallic stain is limited to the portion of

the drawing-paper that is cut away, and, consequently any outline figure may be readily impressed upon the ground employed to receive it.

CHEMICAL CHANGES OF ELECTRICITY.

The effects of electricity as a chemical agent are strikingly displayed in its power of evolving heat, and, consequently, of inflaming and fusing bodies, and its power of promoting chemical composition and decomposition. Combustible bodies, such as a common candle, can be lighted in various ways, by passing the electric fluid through them. The heat evolved by electricity, like most other of its effects, is in proportion to the resistances opposed to its passage. Nor is its heating power in the smallest degree diminished by its being conducted through any number of freezing mixtures which are rapidly absorbing heat from surrounding bodies. Sparks taken from a piece of ice are as capable of inflaming bodies as those from a piece of red-hot iron. Amongst the more striking chemical effects of electricity, or electro-chemistry, are the decomposition of water, the oxidation of metals, and the restoration of the oxides to their metallic state.

Many experiments have been made for the purpose of ascertaining the changes effected in phosphorescent bodies by electricity, and the results are not without importance. It has been discovered, for instance, that substances not naturally phosphorescent, such as statuary marble in its natural or calcined state, were not only rendered phosphorescent by heat after being strongly electrified, but acquired this property with a beauty, a variety, and an intensity of colour, superior to those which occur in specimens that possess natural phosphorescence. It has also very recently been discovered, that electricity exercises a curious influence upon odoriferous bodies. When a current of the fluid is made to traverse camphor, the odour gradually disappears. After being withdrawn from electrical influence, it remains odourless for some time, and then slowly resumes its former properties.

There are certain mineral bodies, which, from being in a neutral state at ordinary temperatures, acquire electricity simply by being heated or cooled. This property is possessed only by regularly crystallised minerals; and of these the most remarkable is the tourmalin. It is a stone of considerable hardness, and the form of its crystals is generally that of a nine-sided prism, terminated by a three-sided pyramid at one end, and by a six-sided pyramid at the other. When heated to between 100 degrees and 212 degrees, the latter extremity becomes charged with positive electricity, whilst the former remains negative. On cooling, the electric states are generally reversed, that end becoming positive which was formerly negative. Other gems possess similar properties, such as the topaz, some species of diamonds, &c. There are a great many substances which become electrified by passing from the liquid to the solid form, such as sulphur, gum-lac, and in general all resinous bodies. The conversion of a body into the aëriform state, is also generally attended by some change in its electrical condition.

There are some bodies which are rendered electrical by pressure. The substance which possesses this property in the most remarkable degree, is that variety of the carbonate of lime known by the name of Iceland spar. Cork, bark, hairs, paper, and wood, also possess the property of producing electricity by compression. A number of substances, when reduced to powder, exhibit electricity, if they are made to fall upon an insulated metallic plate. The relation subsisting between electricity and the chemical properties of matter, is the most important branch of this inquiry. It is observed by Sir Humphry Davy, that most of the substances that act distinctly upon each other electrically, are also such as act chemically when their particles have freedom of motion; this is the case with the different metals, with sulphur and the metals, with acid and alkaline substances. Of two metals in contact, the one which has the greatest chemical attraction for oxygen acquires