If the snow were not placed in front of the mirror, the thermometer would receive as much caloric as it emits, and hence its temperature would remain constant; but as the temperature of the snow is lower than that of the thermometer, the latter receives less than it imparts, and its temperature falls.

Having stated the most important general facts connected with heat, we must refer the reader to other articles in the work for more particular information, under the words Expansion, Thermometer, Pyrometer, Combustion, etc. 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 a piece 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 versa. Bodies that are electrified by glass are said to be positively or vitreously electrified; and bodies submitted to excited wax are called negatively, or resinously 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 nonconductors or insulators are as follows: -

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 nonelectrics, and the best insulators electrics, 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 resinously electrified. In the following table the several substances acquire vitreous electricity when rubbed with those which follow them, and resinous when rubbed with those that precede them:-

The skin of a cat.

Polished glass.

Woollen stuff, or worsted.

Feathers.

Dry wood.

Paper.

Silk.

Lac.

Roughened glass.

The early experimenters used only the glass tube for the excitation of electricity; but the labour and insufficiency of this process soon gave rise to a machine for the same purpose. There are two kinds of electrical machines now in use, which are called the plate machine, 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 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 extended towards the cylinder, for the purpose of collecting the electricity generated by the friction of the cylinder against the rubber. The cushion is mounted on a glass leg for the purpose of procuring resinous electricity; but when only vitreous electricity is required, 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 quantity of electricity is required than can be furnished 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 represents 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 brass wire. When the knob b is brought near the prime conductor of the electrical machine, sparks pas3 into the jar, until it has become charged. A discharging rod c, furnished with a glass handle d, 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.