The rods first used for the electric light were of wood charcoal, quenched in water or mercury; they burnt with brilliancy and regularity, but too rapidly. Next, the carbon which is deposited in gas - retorts was employed; its chief faults are found to be want of homogeneity and purity, causing variations in brilliancy; . liability to split; and hardness, entailing considerable cost for cutting it into " pencils " of the required size. With the sudden impetus given to electric lighting, much ingenuity has been devoted to the production of a more suitable carbon for this purpose. In some instances this has been attempted by purifying gas - retort carbon. The first plan of this kind was as follows:- The retort carbon is fused with caustic potash or soda, and the carbon rods are digested in this bath at a red heat for 15 minutes. In this way, the silica present is converted into a soluble silicate; the rods are then washed in boiling water, and are submitted for several hours to the action of chlorine at red heat, to change the earthy matters into volatile chlorides. These rods give a regular light, but the purification is costly and inefficient.

From a number of experiments on retort carbons impregnated with different salts, it seems that potash and soda double the length of the voltaic arc, render it more silent, combine with the silica, and eliminate it from the carbons during the action of the current; they also augment the light in the proportion of 1.25 to 1.

Lime, magnesia, and strontia increase the light as 1.40 is to 1; iron and antimony, as 1.60 or 1.70; boracic acid is said to lengthen the durability of the carbons by coating them with a vitreous layer, but it does not increase the light.

On the other hand, experiments have been made with a view to manufacturing a carbon from other sources. In one instance, it was endeavoured to imitate the process of formation of retort carbon with pure materials. Tars resulting from true distillation, therefore free from all non - volatile impurities, were decomposed in a tube of refractory earth in a furnace, and yielded plates of carbon which, when cut into " pencils," gave a light that was steadier, whiter, and 25 per cent. more powerful than that obtained with ordinary carbons. The hardness of the material, however, entailed great cost for cutting, and caused much waste. Another plan consisted in mixing 2 parts of pulverized retort carbon, 2 of pulverized wood charcoal or coke, and 1 of tar, rendering the mass a stiff paste, and subjecting it to great pressure. The moulded pieces were covered with a coating of syrup of sugar, placed beside each other in a vessel of retort carbon, and submitted to great heat for 20 or 30 hours. At an early date, a mixture of pulverized coke and sugar was proposed. To powdered coke a small quantity of syrup was added, and the compound was pugged, moulded, and strongly pressed. Next it was heated moderately, thrust into a concentrated solution of sugar, and finally heated to whiteness.

Curmer's carbon consists of lampblack, benzine, and oil of turpentine, calcined together, and moulded into cylinders of porous carbon, which is soaked with resins or saccharine matters, and again calcined. The objections to this are the high price of lampblack, and the difficulty of managing it. Pey - ret's carbon is prepared by soaking pieces of elder - tree pith, or other porous materials, in liquefied sugar, and decomposing the sugar by heat. By repeating this process, a dense carbon is obtained; it is then submitted to a current of carbon bisulphide vapour. In Archereau's carbon, the addition of magnesia makes the light steadier and increases its power. Carre adopts the following mixture: - 15 parts coke powder, 5 of calcined lampblack, and 7 to 8 of a syrup (composed of 30 parts cane - sugar and 12 gum). The whole is thoroughly triturated, and receives an addition of 1 to 3 parts water to compensate for that lost by evaporation. The paste is pressed, and passed through a draw - plate. The carbons are next arranged in horizontal layers in a crucible, the lowest tier lying on a bed of coke - dust, and the upper ones separated by paper to prevent adherence.

Between the top and the cover of the crucible is placed a stratum of coal - dust; and upon the joint of the cover is spread siliceous sand. In this position, the carbons are strongly heated, and are then placed for 2 or 3 hours in a concentrated boiling syrup of cane - sugar or caramel, 2 or 3 intervals of cooling being admitted in order that atmospheric pressure may force the syrup into all the pores of the carbons. These are then allowed to drain by opening a tap in the bottom of the vessel; after this, they are well washed with boiling water, to remove the sugar adhering to their surface. When dry, they are subjected to a second heating, and are passed through a repetition of the process" till the requisite density is obtained. In many respects they resemble retort carbons, but are harder, more tenacious, and better conductors.

Upon the introduction of foreign substances into the carbon rods, a number of experiments have been made. The materials chosen have been lime phosphate, lime borate, lime silicate, calcium chloride, magnesia phosphate, magnesia borate, magnesia, alumina silicate, and pure precipitated silica, with the following observed results: -

Lime phosphate is completely decomposed, reduced calcium goes to the negative carbon, and in contact with the air it burns with a reddish flame. Lime and phosphoric acid are abundantly diffused in fumes. The light, as measured by a photometer, is double that produced by similar - sized rods of retort carbon.

Lime borate and silicate, and calcium chloride are all decomposed; the boracic and silicic acids are volatilized, and escape electric action. The light does not equal that from lime phosphate.

Magnesia salts are decomposed; the magnesium burns with a white flame, while the acids are vaporized. The light is less than from lime salts.