sulated by the glass, and more effectually by passing it through a glass tube covered with a varnish of shell-lac and packed with silk. The leaves are made to diverge by contact with some body in a known state of excitement, as with a stick of sealing-wax rubbed with woollen; if the approach of the body, whose state we wish to determine, cause the leaves to diverge still more, we conclude that it is resinously electrical; if, on the contrary, they collapse upon its approach, it is a proof that it is in the vitreous state. (36)

There is another mode, which is still preferable to this, as affording not only an indication of the kind of force which is required to be tested, but also an accurate measure of its intensity. It consists of a needle of shell-lac suspended from a fine thread of spun glass. At one extremity of the needle is placed a small disc of paper or other conducting substance, which, being thus insulated, is capable of receiving and retaining a charge. When another electrified body is brought near to this, the kind of electricity which has been communicated

(36) Bennet's gold-leaf electrometer is here represented, the leaves being in a state of divergence from a charge which has been communicated to them.

The improved mode of insulation introduced by Mr. Singer

is shown in the section a. The wire to which the gold leaves are attached passes through a glass tube packed with dry silk, and is covered by a brass cap screwed to its upper end, by which it is protected from dust and damp.

to it being known, it will indicate by its attraction or repulsion the species of electricity thus brought within its sphere of action; and the intensity of the force may be measured upon a scale by the amount. of torsion in the thread which may be required to bring the two repelling bodies into mutual contact.(37)

208. Experiment has demonstrated that the same substance is not always similarly electrified by friction; but that its electrical state is determined partly by the condition of its own surface and partly by the body with which it is rubbed. Thus flintglass with a smooth surface, when rubbed with woollen cloth, becomes vitreously electric; but if its surface be roughened, and it be rubbed with the same cloth, it will be resinously electrical.

Smooth glass, rubbed with every substance which has been tried, except the fur of an animal, as a catskin, becomes vitreous: with a catskin, resin

ous.

White silk rubbed with black silk, metals, or black cloth, is vitreous; rubbed with paper or the hand, resinous.

a

(37) This cut represents Coulomb's torsion-electrometer, or electrical balance. b is a thread of silk or spun glass, from which the needle of shell-lac, c, is sus pended. It is attached to the screw, b, by which it can be twisted round its axis. The needle carries a gilt ball of pith or a disc of paper at one extremity, which is balanced by a counterpoise at the other; d is a metallic wire passing through the glass shade, and terminated by a metallic ball at each end. The ball of the needle and the interior brass ball of the wire are brought into contact by turning the screw, b, and the index then points to 0 on the scale, which is marked upon the circumference of the glass.

Sealing-wax, rubbed with the metals, is vitreous; with cloth, paper, or the hand, resinous.

209. It was first observed by Otto de Guericke and Hawksbee, that the friction of glass and resinous substances not only produced the phenomena which we have just described, but, under favourable circumstances, was accompanied by a rustling or crackling noise; and, when the experiment was made in a dark room, by flashes and sparks of light upon their surfaces. When once the attention has been directed to the observation, most persons will find that such phenomena of electrical light are familiar occurrences, and often present themselves in suddenly drawing off from the person a silk stocking or a flannel waistcoat, or in the friction of long hair by combing. How small a degree of friction is sufficient to excite electricity in the human body, is shown in a striking way by placing a person upon an insulating stool with glass legs. If in such a position he place his finger upon a gold-leaf electrometer, and another person flip him lightly with a silk handkerchief, the leaves will immediately repel each other.

210. We may now proceed to describe the means of producing these effects upon a large scale, and the different modes which have been devised of exciting, collecting, and measuring the electric fluids.

The electrical machine, as it is called, whatever form it may assume, resolves itself into a more or less perfect way of applying to glass the friction of silk or some other dissimilar substance, as in our fundamental experiment, and of drawing off the electricity as it is excited, and collecting it in a good insulated conductor. In machines of the more perfect construction, not only is the vitreous electricity collected in what is called the prime conductor but the resinous also is accumulated by in

sulating the rubber and connecting it with a metallic cylinder.(38)

Experience has shown that the most effective ex

(38) The following figure represents the cylinder electrical machine; A is a glass cylinder, properly mounted, to turn upon two stout glass pillars, в B, strongly fixed upon a wooden frame. It is made to revolve rapidly, by means of the multiplying wheels I and K. cc are two brass cylinders, supported upon two glass pillars, D D, in a position parallel to the glass cylin

der; to one of these a cushion, F, with a silk flap, G, is attached which is made to press against the glass by a spring, its dis tance being capable of adjustment by means of the screw H. The opposite brass cylinder has a row of points attached to it, to which the silk flap, G, extends. The latter goes by the name of the prime conductor or the positive conductor; the former is distinguished as the negative conductor.

citer of glass is a metallic substance; and when a soft amalgam of mercury, tin, and zinc is spread upon the rubber, it forms a flexible metallic surface which is perfectly adapted to the purpose and greatly heightens the effect. The best proportions of the metals have been found to be one part of tin and two of zinc, fused together, and mixed while fluid with six parts of mercury The mixture must then be triturated in a mortar, with a little lard, to the consistence of a thick paste. When large cylinders or plates of glass are made to rotate by convenient mechanical means, pressing against cushions covered with this metallic coating, they are capable of producing all the phenomena of attraction and repulsion in a high degree, and their state of intense excitement is accompanied by flashes and sparks of brilliant light, and a crackling sound. (39) (39) The Plate Electrical Machine is figured below. A is a