CARBON. (FR. Carbone ; GrEtt., Carbon.) Symbol, C.; atomic weight, 12.
Abundant information as to the characters, occurrence, and combinations of this element arc to be found in every handbook of chemistry. This article will be confined to a deseription of the artificial carbons employed in electric lighting.
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 ars 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 80E18 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 fifteen minutes. In this way, the silica pre sent is converted into a soluble silicate ; the ods are then washed in boiling water, and are sub mitted for several hours to the action of chlorins 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 I. Lime, magnesia, and strontia increase the light as 1.40 is to 1 ; iron and antimony, as 1.60 or 1.70; beracie 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 band, experhnents have been made with a view to manufacturing a carbon front other purees. In one instance, it was endeavoured to imitate the process of formation of retort carbon with pure materials. Tars resulting from true distillation, therefore fi.ee from all non volatile impurities, were decomposed in a tube of refractory earth in s. furnace, and yielded plates of carbon which, when cut ioto " pencils," gave a light that was steadier, whiter, and 25 per cent. moro powerful than that obtained with ordinary carbons. The hardness of the material, however, entailed great cost for eutting, and caused much waste. Another plan consisted in mixing two parts of pulverized retort carbon, two parts of pulverized wood charcoal or coke, and one part of tar ; mixing the mass to 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 earbon, and submitted to great beat for twenty or thirty 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 com pound was pugged, moulded, s.nd strongly pressed. Next it was heated moderately, thrust into a concentrated solution of sugar, and finally heated to whiteness. Curtner's carbon consists of lamp black, 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. Peyret'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. Carrd adopts the following mixture :— Coke powder, 15 parts; calcined lampblack, 5 parts ; and a syrup (composed of 30 parts cano sugar, and 12 parts gum), 7 to 8 parts. The whole is thoroughly triturated, and receives an addi tion of I to 3 parts of 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 coke dust ; and upon the joint of the cover, is spread siliceous sand. In this position, the carbons are strongly heated, and arc then placed for two or three hours in a concentrated boiling syrup of cane sugar or caramel, two or three 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 repetitiou of the process till the requisite deosity is obtained. In many respects they resemble retort carbons, but are harder, more tenacious, and better conductors.