connect the voltaic battery to the primary circuit of the induction coil through the button; and the secondary circuit of the coil is in turn connected by wires within the stem of the rod to a pair of sparking points, which are exposed to the air at the upper end

of the rod. On inserting these points into the gas jet and pressing the contact button so as to send the current through the primary, a stream of sparks traverses the space between the metal points and ignites the gas.

The battery is sometimes a single

chloride of silver cell also contained within the rod : and when a platinum wire spiral is used instead of the spark, a bichromate of potash cell having its zinc and carbon plates separated by a layer of sawdust or of soft asbestos soaked in a solution of bichromate of potash is employed. This "dry" battery can be conveniently packed in small compass in a chamber at the base of the rod which serves to reach the gas jet.

Such a battery is exposed in the chamber below the petroleum lamp Fig. 79. Outside the chamber is a press-button which closes the circuit of the battery, allowing the current to flow through wires to a fine platinum spiral, which is brought to the wick of the lamp by depressing a finger-key. The heated wire lights the lamps and the release of the key tilts the wire out of the flame. At the recent Crystal Palace Exhibition Mr. Edison exhibited a cigar lighter or electric tinder box, on the same principle, the lights being obtained by kindling a match soaked in spirits of wine at an incandescent platinum wire.

CHAPTER XII.

MISCELLANEOUS.

WHEN a current of electricity of sufficient strength, say that from two or three cells of a voltaic battery, is sent through water, it decomposes the water into its constituent gases, hydrogen and oxygen. This experiment is usually shown by means of the apparatus shown in Fig. 80. This consists of a glass vessel v

from the bottom of the cell or vessel. These plates are connected by wires to the poles of the voltaic battery c, as shown, and therefore they act as electrodes and pass the current from the battery through the water. That in connection with the

positive pole + of the battery is termed the "anode," and that in connection with the negative pole is the "cathode." Now, as the water is decomposed the hydrogen gas is found to collect on the cathode, by which the current is supposed to leave the water, while the oxygen collects on the anode, by which the current is believed to enter the water; and being a gas, and lighter than the water, it rises into the upper ends of the tubes. The volume of hydrogen at the cathode is always twice the volume of oxygen at the anode, and this agrees with the known constitution of water. Further, the quantity of water decomposed in a given time is proportional to the strength of the electric current, and hence if the tubes are graduated to show the volume of gases collected in them, the instrument becomes a voltameter, or current-measurer. This important discovery of electro-chemical decomposition was made in 1800 by Carlisle and Nicholson, and from it a great number of industries take their source.

We have, for example, the Bain electro-chemical telegraph, in which the current is caused to decompose a solution of iodide of potassium in starch and water. Paper is moistened with the solution, and the current is passed through it. The result is that the iodine is separated from the potassium and leaves a blue stain on the paper to mark the passage of the current, and be interpreted by the clerk as a signal of the message.

ELECTRO-PLATING.

The principal outcome of the discovery is, however, the art of electro-plating the baser metals with gold, silver, nickel, and other metals of a nobler kind, so as to improve the appearance of articles made from the baser metals, and at the same time prevent their surfaces from oxidising. When a metallic solutionsay, a solution of sulphate of copper-is substituted for the water in the voltameter experiment which we have described, the sulphate of copper is decomposed, and metallic copper is deposited on the cathode, while an oxide of sulphur, known as "sulphion," is deposited on the anode. The sulphur, however, soon combines with water to form sulphuric acid and free oxygen. If the electrodes are of copper instead of platinum, one copper plate becomes heavier in this process by the deposition of fresh copper on it, and the other becomes lighter because the newly-formed sulphuric acid eats it away. The rate at which this process of building and wasting goes on is a measure of the strength of the current, and Mr. Edison has constructed on this plan an ingenious current-meter for measuring the electricity consumed in electric lamps, just as gas is measured

now.

The important fact, however, is that metals can be deposited in this way from solutions, and hence we pass at once to the apparatus employed in electro deposition. It was in 1836 that Mr. De la Rue observed that in a Daniell's cell the copper deposited on the copper plate took on its under-side the impress