considerable quantity of this oxide is contained in that glass used for lustres, for artificial gems, and for most optical purposes. The superior appearance of Guinand's Swiss flint-glass is attributed to the quantity of oxide of lead which it contains.

ELECTRICITY.

Why is electricity so called?

Because of its origin from electron, the Greek name of amber, the phenomena displayed by rubbing a piece of amber, being the first physical fact recorded in the history of science.

Electricity investigates the attractions and repulsions, the emission of light and explosions, which are produced not only by the friction of vitreous, resinous, and metallic surfaces, but by the heating, cooling, evaporation, and mutual contact of a vast number of bodies.

Why are certain bodics called conductors of electrcity?

Because they suffer electricity to pass through their substance. The metals are all conductors; according to Mr. Harris, (Phil. Trans. 1827) silver and copper are the best conductors; then gold, zinc, and platinum, iron, tin, and lead. Well burned charcoal and plumbago also conduct.

Why are other bodies called non-conductors?

Because they only receive electricity upon the spot touched. Dry air, glass, sulphur, resins, and oils, are non-conductors. Water, damp wood, spirit of wine, damp air, some oils, and most animals and plants, are imperfect conductors.

Why have different bodies various conducting powers for electricity?

Because their degree depends on the quantity of electricity which traverses them; so that of two conducting bodies, that which is the best for one electric current, may be the worst for either a stronger or a weaker current.—Professor Delarive, of Geneva.

Why are the phenomena of electricity called negative and positive?

Because, Franklin, observing that the same electricity was not inherent in the same body, but that glass sometimes exhibited the same phenomena as wax, and wax as glass, instead of regarding the phenomena as dependent upon two electric fluids, referred them to the presence of one fluid, in excess in some cases, and deficient in others. To represent these states, he used the terms plus and minus, positive and negative. Thus, when glass is rubbed with silk, a portion of electricity leaves the silk and enters the glass; it becomes positive, therefore, and the silk negative: but when sealing-wax is rubbed with flannel, the wax loses and the flannel gains; the former, therefore, is negative, and the latter positive. All bodies in nature are thus regarded as containing the electric fluid; and when its equilibrium is disturbed, they exhibit the phenomena just described.-Brande.

Why does an electrical machine produce flashes and sparks of light, when the plate or cylinder is turned?

Because, it is conjectured, of the sudden compression of the air, or medium, through which the electricity passes: it is, probably, always attended by a proportionate elevation of temperature, as is shown by the power of the spark to influence spirits of wine, fulminating silver, and other easily inflammable compounds. -Brande.

Why will a feather adhere to rubbed or excited seatingwax, and then fall off?

Because it is attracted by, and remains in contact with, the wax, till it has acquired its electricity, when it will be repelled, and in that state of repulsion, it will be attracted by an excited glass tube.

Why does the appearance of electric fire vary?

Because of the different density of the medium

through which it passes. Thus, in common air, short sparks are straight, or nearly so, and long ones zig-zag; the former are brilliant, especially at their extremities, the latter usually of a paler or redder hue. In condensed air, the electric spark is bright and white; in rarified air it is of a reddish tinge, and faint and divided; in the more perfect vacuum of a good airpump, it is of a purplish hue, and only visible in a dark room. In the most perfect vacua which can be obtained, it is scarcely visible, and of a greenish tint. In different gases the electric spark appears most brilliant in those which are most dense; in hydrogen gas it is faint and red; in carbonic acid it is vivid and white.--Brande.

Why is the electrical shock produced at the joints of the fingers, the elbows, shoulders, and chest, by holding the discharger of a Leyden phial?

Because there is an accumulation of electricity within the phial, which consists of a thin glass jar, coated internally and externally with tin-foil, to within a short distance of its mouth. When the inner surface is rendered positive by union with the conductor of the electrical machine, the exterior, being connected with the ground, becomes negative by vicinity or position. When the inner and outer surfaces are united by a conductor, all electrical accumulation is annihilated by a powerful spark, and the two opposite states are found to have been precisely equivalent. Metallic wires, with balls at their ends, bent or jointed, and fixed to a glass handle, are generally used to transfer the electric charge, and these instruments are called dischargers. A pane of glass, coated upon both sides to within an inch of its edges, with tin-foil, exhibits precisely the same phenomena as the phial; but it is a less convenient form of the apparatus.— Brande.

Why does the aura (or wind of an electrical machine) blow out a candle?

Because of the rapid succession of the electrified air. This effect is felt upon holding the hand near the point of a wire affixed to the conductor, when a peculiar coldness is perceived. It communicates motion to light bodies, as is seen in electrical fly-wheels, and in models of mills, orreries, and other amusing electromechanical apparatus.

Why is an arrangement of several Leyden jars called an electrical battery?

Because by a communication existing between all their interior coatings, their exterior being also united, they may be charged and discharged as one jar.

The discharge of the battery is attended by a considerable report, and if it be passed through small animals, it instantly kills them; if through fine metallic wires, they are ignited, melted, and burned; and gunpowder, cotton sprinkled with powdered resin, and a variety of other combustibles, may be inflamed by the same means.

Why is light produced in the empty part of a barometer tube, when it is shaken in a dark room?

Because the glass becomes electrified by the friction of the mercury. Even the friction of air upon glass is attended by electrical excitation: for it has been found, that by blowing upon a dry plate of glass, with a pair of bellows, it acquired positive electricity.

Why is a plate of zinc, when brought into contact with one of copper, or silver, found, after removal, to be positively electrical, and the silver or copper left in the opposite state?

Because of the electrical excitation by contact of the different metals, the most oxidizable (liable to rust) metai being always positive in relation to the least oxidizable metal, which is negative; and the more opposite the metals in these respects, the greater the electrical excitation; and if the metals be placed in the following order, each will become positive by the con

tact of that which precedes it, and negative by the contact of that which follows it, and the greatest effect will result from the contact of the most distant metals. Platinum Gold, Silver, Mercury,

Copper,

Iron,

Tin,

Lead,

Zinc.

Why are brass cocks in leaden cisterns corroded at the junction?

Because of the chemical effects of the contact of the metals. In like manner, the places where solder is applied are liable to depositions from the water. Iron railings are apt to be decayed and dissolved, where lead is used to fix them in stone cavities; and where iron is employed in fixing a bronze statue, my friend Mr. Chantrey (observes Mr. Brande) informs me that it prevents the acquisition of the desirable green rust.

Why have copper been substituted for iron nails and pins in fastening sheets of copper to ships' bottoms?

Because the galvanic action produced by the union of the two metals, iron and copper, was a great cause of destruction; and copper nails and pins, although not so strong, are not attended with the same inconvenience.

The last experiments which engaged Sir Humphry Davy's attention to any extent, were on this application of electrical combinations, for the purpose of preserving the copper sheathing of ships' bottoms. To this subject Sir Humphry Davy gave much of his time, and personally inspected all the boats and vessels on which the trials were made. Although the theory upon which they were conducted proved eminently correct, no advantage could be ultimately taken of the plans which it suggested. The saving of the copper