a few remarks in connection with their mode of action may not be regarded as altogether useless. The electrical machine consists of a revolving cylinder or plate of glass, submitted to the friction of cushions or rubbers. It matters very

little what form of machine is employed. As a

general rule, a plate machine is, for equal size, of far higher power than the cylinder. The arrangements of the latter are, however, simple, and are, perhaps, more easily managed by the uninitiated. There is also an advantage on the score of economy, as old cylindric machines are readily to be procured at low prices, and, as a general

Fig. 17. Cylindric electric-machine.
Fig. 18.

Plate electric-machine. In both figures the same letters are affixed to the most important parts. A, the revolving electric, rotated by the handle C, and submitted to the friction of the rubber B B. The prime conductor, D, affords the means of collecting the electricity excited, in a state of high tension.

rule, a well-worn cylinder is far preferable to a new one. Plate machines are, on the contrary, less common, and consequently must generally be purchased new. Whichever form is employed, it is useless employing a plate of a less diameter than a foot, or a cylinder of less than five or six inches.

There is some little tact required to elicit the full power of an electric machine; and, from want of this, you will frequently find some persons quite fail in exciting any amount of electricity, even from the best-constructed machines. This art is, however, soon acquired. When the machine is required for use, the prime conductor and rubbers should first be removed, and the machine placed sufficiently near a good fire to become completely dry and warm. The surface of the glass should then be slightly rubbed with a piece of tow or flannel soaked in olive-oil, any adhering black spots from old amalgam being scraped off. By means of a dry and warm linen cloth, the oil should then be wiped away, and the polished surface of the glass is thus left clean and free from moisture. The cushions, if covered with amalgam, are then to be rubbed with a piece of brown paper, so as gently to remove the oxidised surface; but if not sufficiently covered, a little amalgam (made by melting together zinc two parts, tin one part, with mercury six parts, made

into a paste by triturating it in a mortar with a little lard) must be rubbed into the surface of the cushions with the handle of a knife or piece of smooth wood. The silk flaps are to be wiped clean, and the rubbers adjusted to the plate or cylinder. On revolving the latter, a rustling noise will be heard, accompanied in a darkened room by vivid flashes of blue light, whilst a strong phosphorus-like odour of ozone becomes perceptible. The prime conductor is next to be replaced, taking care that its insulating support is perfectly dry, and even slightly warm: the instrument is then fit for use. You will, however, not unfrequently find, that although you may have taken the precaution to connect the rubber with the table or floor, by means of a metallic conductor, still that little or no electricity is obtained on revolving the glass. This will generally be found to depend upon the badly conducting table or floor, by which a sufficiently ready means is not afforded for the complete restoration of the electric equilibrium of the rubber, when destroyed by the friction of the revolving cylinder or plate against its surface. This difficulty is best overcome in London and large towns, by connecting the rubber, by means of a long copper wire, with a branch of the leaden pipes through which the house is supplied with water. By this plan a ready communication is afforded by a good con

ductor with the great reservoir of electricitythe earth.

Having thus got the machine in good action, on revolving the cylinder or plate, and presenting the hand or a piece of metal towards the prime conductor, a series of vivid sparks, attended with a loud snapping noise, will pass between them. In this arena I am sensible that any remark connected with the theory of the excitation of electricity by the machine would be quite misplaced, as I feel that all I have the honour of addressing must be most fully acquainted with every thing pertaining to this branch of physics. There is, however, a popular error so generally believed, that I must venture to allude to it; the error consists in regarding the electricity of the prime conductor as derived from the revolving glass, the latter being regarded as pumping electricity from the rubber, and thence from the earth. Now, the fact is, that not an atom of positive electric matter leaves the glass to pass to the conductor. The cylinder or plate, rendered positive by friction against the rubber, merely acts upon the electricity naturally present in the prime conductor by induction, decomposing it into the component elements, attracting the negative fluid, which, accumulating in a state of high tension, or elasticity, darts off towards the cylinder to combine with the positive fluid free on its surface, reconstituting the neutral

compound: the prime conductor is thus left powerfully positive, not by acquiring electricity from the cylinder, but by the abstraction of its own negative element. Again, the sparks which appear on approaching the hand to the conductor are often called positive sparks, when, in truth, they are nothing of the kind, being, indeed, a series of luminous discharges formed by the union of the negative electricity of the body, which is held near the conductor, with the free positive electricity of the latter.

In addition to the electrical machine itself, a pair of directors, or rods of brass, furnished with balls of brass and glass handles, together with a few yards of common copper bell-wire, or brass chain, will be required to connect the patient with the machine, or to convey the discharge of a jar through his body. The jar itself need not have more than a square foot of coated surface, and indeed one much smaller is often sufficient.

There is one piece of apparatus which is very essential, being in almost constant requisition — I mean the well-known chair with glass legs, on which a patient may sit and be completely isolated from all electrical communication with the earth. This is an expensive, bulky, and fragile contrivance, and hence is the most inconvenient of all the electrical appliances. I advise you, however, not to trouble yourselves with the very clumsy