Having stated the most important general facts connected with heat, we must refer the reader to other articles in the work for more particular infor mation, under the words EXPANSION, THERMOMETER, PYROMETER, COMBUSTION, &c. and proceed to another important agent in chemical research—electricity.
If a glass tube, or a stick of sealing-wax, be briskly rubbed with a dry silk handkerchief, and then presented towards small pieces of paper, feathers, or gold leaf, it will first attract and then repel them. Or if a glass tube be taken in one hand, and a stick of sealing-wax in the other, and each be rubbed, then if the glass rod be brought near apiece of gold leaf floating in the air, it will first attract, and afterwards repel it. While the gold leaf is repelled by the excited tube, if the sealing wax be brought near, the leaf will be attracted, and thus it will be seen that bodies electrified by glass will be attracted by the wax, and vice vend. Bodies that are electrified by glass are said to be positively or vitreously electrified; and bodies submitted to excited wax are called negatively, or resinoualy electrified. If either of the tubes are well excited, and a finger be presented to it, a crackling noise will be heard, and in the dark, sparks of light will be seen issuing from the tube; these are termed electric sparks. If to the further end of the excited tube a brass ball be attached by a wire, the ball will possess all the qualities of the tube itself; but if it be connected by means of silk, the electric virtues will not pass into it. From this circumstance bodies have been divided into conductors and nonconductors. Some bodies conduct or permit the passage of electricity 'more readily than others; hence arises the distinction of good and bad conductors. The following table contains a list of conducting substances in the order of their conducting power.
The worst insulators differ very little from the worst conductors, so that the whole list, from copper to shellac, might be considered as one series, in which different degrees of resistance are opposed to the passage of electric power. The best conductors are sometimes called nonelectric:, and the best insulators ekctrics, on the supposition that only the latter were capable of producing electricity by friction. This appears to be erroneous, as even metallic bodies may be excited if they are held by a nonconductor to prevent the electricity being carried away as soon as produced. A similar mistake was originally made with respect to the production of vitreous or resinous electricity. It was thought that the same body always produced the same kind of electricity; but it is now known that this depends on the nature of the rubber. In all cases where two bodies are rubbed together, if the one become vitreously electrified, the other will be reainowily electrified. In the following table the several sub stances acquire vitreous electricity when rubbed with those which follow them, and resinous when rubbed with those that precede them : The skin of a cat. Paper.
Polished glass. Silk.
Woollen stuff, or worsted. Lac.
Feathers. Roughened glass.
Dry wood.
The early experimenters used only the glass tube for the excitation of elec tricity ; but the labour and insufficiency of this process soon gave rise to a machine for the same purpose.
There are two kinds of elec trical machines now in use, which are called the plate ma chine, and the cylinder machine. The cylinder machine is shown in the annexed figure, in which a is a cylinder of glass, mounted in a frame, so as to be turned on its axis by means of the handle f. At e is a cushion stuffed with wool or horse-hair, and covered with an amalgam of three parts of mercury, two of zinc, and one of tin, melted together. Attached to the cushion is a I piece of silk c, which reaches over the cylinder as far as the prime conductor b. The prime conductor b is a cylinder of tin or brass, mounted on a glass leg d, and furnished with a row of points ex tended towards the cylinder, for the purpose of collecting the electricity gone rated by the friction of the cylinder against the rubber. The cushion is mounted on a glass leg for the purpose of procuring resinous elec tricity; but when only vitreous electricity is reqwred, a chain or piece of wire must be attached to the cushion. The principle of the plate machine is precisely similar to this ; but, instead of a cylinder, a circular glass plate is mounted, so as to turn on an axis passing through its centre, while the rubbers are applied near its circumference. When a greater quan tity of electricity is required than can be fur nished by sparks from the prime conductor, an apparatus is used, which, from having been discovered at Leyden, is called the Leyden Jar. In the engraving a represmits
a glass jar coated within and without with tin foil, except near the top ; b is a brass ball connected with the interior coating by means of a braes wire. When the knob b is brought near the prime conductor of the electrical machine, sparks pass into the jar, until it has become charged. A discharging rod c, ffirmshed with teems handle (I, is then applied, so that one ball touches the outer coating, and the other the brass ball b; the whole of the electricity then passes from the inside to the outside of the jar, which is then said to be discharged. If, instead of the discharging rod, a person applies his hands to the outer and inner sides of the jar, the electricity passes through him, and he receives an electric shock. If the electricity be accumulated in a large jar, or if several jars are connected, as in the accompanying sketch, they form an electric battery, and all the effects produced by lightning may be imitated by means of it. If electricity be passed through resin, phosphorus, ether, gunpowder, &c. it inflames them ; it will penetrate a thick card or a quire of paper ; and if in sufficient quantity, will destroy life in an eel, rabbit, dog, &c. Friction is not the only means of pro ducing electric indications, but it is the most energetic; and when we have occasion to test the presence of electricity excited by other means, it is necessary to use a delicate electrometer. One of these, called Bennet's electrometer, shown in the accompanying sketch, consists of two small slips of gold leaf suspended by a brass wire within a glass cylinder. In the improved form of the instrument the wire which carries the leaves a a passes through a plug of silk within a glass tube b in the cap of the electrometer. By this instrument we are enabled to perceive that electricity is excited in the fusion of inflammable bodies, in evaporation, in the disengagement of gas, by the sudden disruption of a solid body, by change of temperature, by contact of dissimilar bodies. This latter has been considered by Volta and others as the origin of voltaic electricity, while some of our first chemists consider chemical action to be the primary source of voltaic energy. Voltaic elec tricity is usually procured by an arrangement of copper and zinc plates, called a voltaic battery. There are different forms of this apparatus, but the battery of Cruikshank is, on the whole, the most convenient. This consists of a number of zinc and copper plates, soldered together at their edges, and cemented into grooves in the sides of a mahogany trough. To prepare the battery for ac tion, a liquid consisting of about two parts of sul phuric acid, one of nitric acid, and 100 of water, should be poured into the cells till they are nearly full; a wire must then be inserted at each end touching the outer plates. The wire con nected with the zinc plate will give off positive electricity, and the wire attached to the copper plate, negative electricity. The galvanic or voltaic battery is a highly important agent in chemical research, both from its energetic decom posing power, and from the intense heat which it produces. If the two wires are of platinum or gold, and are inserted into a glass of water, the water will be decomposed into its elements, oxygen and hydrogen; the oxygen will appear at the positive wire, and the hydrogen at the negative If the ends of the wires dip into two separate glasses of water, and a finger of each hand be immersed in them, a slight electric shock will be felt, the intensity of which will increase with the number of plates. If the number of plates is very great, or if of large dimen sions, the phenomena are beautiful and striking. The battery of the Royal Institution, used by Sir H. Davy, in his researches, contained 2,000 pairs of plates, containing a surface of 128,000 square inches. When pieces of charcoal about an inch long, and one-sixth of an inch in diameter, were brought within one-thirtieth or one-fortieth of an inch of each other, a bright spark was pro duced, and more than half the volume of the charcoal became ignited to whiteness; and by drawing back the points a little from each other, a constant discharge took place through the heated air, in a space equal at least to four inches, pro ducing a most brilliant ascending arch of light, expanded and conical in the middle. When any substance was introduced into this arch, it instantly became Platinum melted in it like wax in the flame of a common candle.