simply indicating its presence are denominated ELECTROSCOPES-the pithball arrangement is about the simplest of the latter. Another simple electroscope can be made by attaching to a stick of sealing-wax (as an insulating handle) a narrow strip of gold-leaf: a very little moisture from the lips will be sufficient to attach it. This form of electroscope for delicate experiments is usually enclosed in a glass bottle, as gold-leaf is very susceptible to the slightest motion. In this case it is necessary to attach the gold-leaf to a brass rod, or some conductor projecting beyond the mouth of the bottle.

Amongst the most interesting electrical phenomena we must place that of induction, for it is everywhere, and no experiment can be carried on without it. In the experiments previously shown, induction played its part; but in order to show it more effectually, let us try the following experiment:

On an insulated stand of glass let us support a small metal cylinder (brass is the best, but wood covered with tinfoil will answer the purpose); upon the end of it place our electroscope (a pith-ball arrangement, similar to what we used before); excite the glass tube or ebonite, and present it towards the end opposite to the pith balls. Immediately the tube comes near enough, the cylinder becomes charged inductively, causing the pith balls to diverge; remove the glass tube-the balls converge, and show the absence of electricity. This may be repeated, and

the same results will follow so long as any charge remains in the glass tube. If the cylinder be tested whilst under the action of induction, it will be found that the end nearest to the glass tube is oppositely (negative) electrified, whilst the other end is similarly electrified, showing that a disturbance has taken place of the natural electricity of the conductor under induction

FIG. 4.

the opposite electricity to the end near the excited body. Similar electricity to the opposite end (following the before-mentioned fundamental law of oppcsite electricities) are attracted, and similar are repelled.

Whilst the cylinder (Fig. 4) is under induction, let it be touched with the finger: the pith balls will immediately collapse; remove the finger and suddenly withdraw the tube: the pith balls will diverge, and remain so for a length of time. If they be examined, the electricity will be found to be negative or opposite to what it was before.

For any experiments relative to induction with statical electricity, the ELECTROPHORUS is about as good an instrument as can be used. The following is a description of this useful instrument:

B is a metal sole upon which A, a resinous plate of equal parts of resin, shellac, and Venice turpentine, is placed ;* c is a metallic cover with an insulated handle. The plate A is to be excited by striking it briskly with dry silk: place the cover on. This cover, being insulated, does not take away the electricity from the plate, but is simply electrified by induction; the upper surface is

A polished plate of ebonite will be easier to prepare, and equally efficacious.

charged similarly to the plate; the under, being next to the plate, is charged oppositely. Now, if the top of the plate be touched with the finger or other conductor, the negative induced electricity will escape, but the positive on the lower surface will remain. Remove the cover: the electricity will spread over

the surface, and the cover be found to be charged positively. The plate remains in the same state, not having lost any of its electricity, as the cover was electrified by induction only. It will thus be seen how useful this instrument may become; in fact, it has been termed a perpetual electrical machine. The discovery is due to Professor Volta.

ELECTRICAL MACHINES.

An electrical machine is simply the glass tube and rubber on a large scale, and is "an apparatus by means of which electricity can be developed and accumulated in a convenient manner for experiment;" in fact, any instrument for the excitement of electricity is a machine. The following illustration will explain the form now usually adopted: it is generally termed a cylinder machine. The old machines were usually excited by hand, but in the modern the great improvement of rubbers has been introduced.

The hollow cylinder of glass, having open ends fitted with caps of wood, is fixed on two upright supports. At one end of the cylinder is fixed a crankhandle. In front is placed the rubber on an insulated stand; this rubber consists of a cushion of soft leather, stuffed with horse-hair. When in action it is necessary to smear the surface with a metallic amalgam,* the object being to increase the excitement. On the opposite side to the rubber, and also on an insulated stand (this is frequently made separate from the machine), is placed the conductor, or prime conductor, as it is called: this is a cylinder of brass or wood covered with tinfoil, provided next the machine with a number of metal points, and with a metal ball on the opposite side. The addition of

Amalgam. -The following is given as the method of making it: "Melt together five parts of zinc and three parts of tin; pour gradually on the melted mixture nine parts of metallic mercury previously warmed; the whole is shaken briskly till cold in an iron or thick wooden box: it is then reduced to a fine powder in a mortar, sifted through muslim, and mixed with lard in sufficient quantity to reduce it to the consistency of paste."-Noad.

It can be procured of any philosophical and electrical instrument maker.

a flap of oiled silk (oiled on one side only) from the rubber, covering the glass cylinder, completes the machine. The advantage of this silk is the prevention of the electricity escaping from the excited cylinder into the atmosphere.

It is very essential that the glass cylinder should be kept free from dirt and moisture; before being used it should be well warmed: placing a well-heated flat iron under it is a very good plan. These machines can be constructed very cheaply, and with but little trouble, the only great expense being the purchase of the cylinder; the remaining portion can be easily put together, A small machine of this kind is very useful, for with it all the experiments connected with frictional or static electricity can be performed.

It is generally necessary that the machine should be connected with the earth; for this purpose a chain or wire to the table or floor should be in connection with the rubber. Attach this to the rubber, and turn the machine; the electricity of the rubber (according to Noad) is decomposed by the friction, the positive is carried off on the glass round to the conductor, whose induction comes into play. The negative electricity of the conductor is thus brought towards the points where the negative of the conductor and the positive of the cylinder unite; this is the cause of the bright sparks constantly passing between the two. The result is, that the conductor is left powerfully positive, and the rubber proportionately negative. On applying the hand, or anything metallic in connection with the earth, to the ball on the conductor, vivid sparks, accompanied by loud cracking noise, will constantly pass from one to the other; this is termed the positive spark. On the application of the knuckles to the conductor, the spark will be accompanied by a slight shock. Take away the earth connection from the rubber, the effect will be that but a small quantity of electricity will be given off: the supply of the rubber and glass being small, it is necessary to recruit them from the earth. By connecting the conductor with the earth, a constant supply of negative electricity can be obtained.

Besides the cylinder, there is also a powerful machine made of glass, called

the plate machine, from the disc being of plate glass. The glass plate is fixed between two uprights; attached to it is a handle for rotation. At top and bottom it passes between two pieces of wood, with amalgamated rubbers and silk flaps, as in the cylinder machine. Its conductor is formed of hollow brass, fixed on to one of the uprights, and brought round at each end close to the machine, where, at the place the flaps leave off, it is fitted with points. The action is similar to the cylinder. Mr. Hearder estimates that cylinder machines of equal surface are four times superior to plate machines. These latter have also been made of ebonite, but in that case it is necessary that the amalgam should be softer.

Many and very beautiful experiments may be made with a cylinder machine. Relative to the variety of the sparks, Professor Faraday says:

"In air they have, when obtained with brass balls, a well-known intense light and bluish colour, with frequently faint or dark parts in their course, when the quantity of electricity is not great.

FIG. 8.

"In nitrogen they are very beautiful, having the same general appearance as in air, but more colour of a purple or bluish character.

"In oxygen they are brighter, but not so brilliant as in common air.

"In hydrogen they are of a fine crimson colour, but have very little sound, in consequence of the physical character of the gas.

"In carbonic acid gas they have the same general appearance as in air, but are remarkably irregular. Sparks can be obtained under similar circumstances much longer than in air, the gas showing a singular readiness to pass the discharge.

"In muriatic acid gas, when dry, they are nearly white, and almost always bright throughout.

"In coal gas they are sometimes green, sometimes red; occasionally one part is green and another red. Black parts always occur very suddenly in the line of the spark; i.e., they are not connected by any dull part with bright portions, but the two seem to join directly the one with the other."

For observing the peculiarity of the spark under the above varying circumstances, the following apparatus, consisting of a glass globe about four inches in diameter, provided at each end with a brass cap. Through the upper of these projects a brass rod, terminating at each end with a brass ball; at the lower cap is arranged a stop-cock, to which is attached a short brass rod and ball, slightly projecting into the globe; the upper rod should be so arranged that by a well-fitting collar it may be lengthened or shortened, in order to increase or decrease the distance between the two balls. (Fig. 8.)

To put this in action, it is necessary to exhaust the air from the globe by an air-pump or condensing syringe; and for observing the discharge in rarefied air only, connect one ball with the positive, the other with the negative conductor. Immediately the machine is turned, "a current of beautiful light passes from the positive to the negative ball, on which it breaks, and divides into a luminous atmosphere entirely surrounding the ball and stem.”

For experimenting with the spark in different gases, first well exhaust the Connect the wires, and on globe, and then charge it with the required gas. turning the machine the sparks will be observed of the colours mentioned above.

Very many beautiful experiments can be performed with the machine by bringing the sparks into play. A pretty experiment may be tried by pasting on a sheet of glass pieces of tinfoil cut out into various patterns, separated from each other by a slight distance: letters may be so cut out and arranged. On connecting the one end with the conductor, and the other with the earth, the whole is inmediately lighted up, and the design becomes apparent.

In the experiments that I have mentioned, the power of the electricity obtained has been, comparatively speaking, slight; but in order to obtain clectricity of greater power, we make use of what is termed a Leyden jar, an instrument that plays a very important part in static experiments. Its discovery was made under peculiar circumstances in about the years 1745 and 1746. Some Dutch philosophers at Leyden turned their attention to try and find some method of enabling an insulated conductor to retain an electric charge, as they found the conductors rapidly discharge themselves. In their experiments a small glass phial was filled with water, establishing electric connection with the water by a nail through the cork; this was attached One of the experimenters, in trying to take away to the prime conductor. the phial, received a shock across his arms and breast that shook his whole frame.

A similar result was obtained about the same time by Von Kleist. In a letter, found in the register of an academy at Berlin, he communicates his discoveries.

"When a nail or a piece of brass wire is put into a small apothecary's phial and electrified, remarkable effects follow; but the phial must be very dry and warm. I commonly rub it over beforehand with a finger, on which I put some pounded chalk. If a little mercury, or a few drops of spirits of wine, be put into it, the experiment succeeds the better.