Upon tho introduction of foreign substances into the carbon rods, a number of experirnents have been made. The materials ehosen have been phosphate of lime, borate of lime, silicate of lime, chloride of calcium, phosphate of magnesia, borate of magnesia, magnesia, silicate of alumina, and puro precipitated silica, with the following observed results :— Phosphate of lime is completely decomposed, reduced calc.um 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.
Borate and silicato of lime, and chloride of calcium are all decomposed ; the boracic and silicic acids are volatilized, and escape electric action. The light does not equal that from phosphate of lirne.
Magnesia salts are decomposed ; the magnesium burns with a white flame, while the acids are vaporized. The light is less.than frotn lime salts.
Silicate of alumina, and alurnina, require a very strong current to effect their decomposition, and burn with a blue flame of small illuminating power.
Silica melts and volatilizes without undergoing decomposition.
M. Gaudoin has proposed two distinct methods of preparing carbon for electric rods. According to the first, he decomposes by heat, organic matters capable of yielding pure carbon after decom position, e. g. pitches, fats, &c. The dtcomposition is effected in closed retorts, or in graphite crucibles, at bright red heat. In the bottoms of the latter, are provided a tube for the liberation of volatile matters, and a second tube for feeding purposes. The gaseous products of decomposition are led into a condensing chamber, for recovery and utilization. The more or less compact carbon remaining in the retort is finely pulverized, and with it are mixed certain proportions of lampblack, and of the carbides of hydrogen previously produced by the decomposition process. These, being quito free from iron, are much superior to commercial hydrocarbons. The draw-plate or moulding
apparatus employed by Gaudoin differs from that commonly used, in the following important particulars. The carbon is made to issue horizontally, at a descending angle of about 503, and is guided by tubes, and supported so tbat the mould can be emptied without interruption and the carbon does not break under its own weight. Gaudoin's second plan is to take dried wood, shaped in the form of the rod, and to carbonize it and soak it in carbonaceous liquids. The wood is then subjected to a slow distillation process, in ender to drive off the volatile matters ; it is then washed in acids or alkalies, to remove impurities ; and is finally desiccated in a reducing atmosphere at very high temperature. The pores of the wood are closed by submitting it to the aotion of chloride of carbon and various hydrocarbons under heat. This process promises to afford carbons which will burn at a slow rate, and give a steady light.
The advantage derived from closing the pores of carbons has been further attested by the success of the Sawyer and Mann rods, which are prepared in the following manner :—The carbon rod is immersed in olive oil until it has become thoroughly saturated ; while in this condition, it is included in a powerful electric current, the effect of which is to carbonize the oil in the pores and ou the surface. Rods thus prepared are extremely hard, of steel grey colour on the surface, and give very constant light.
Bad carbons are undoubtedly rendered more uniform conductors by covering them with a coating of metal. A great increase of light is also secured by a slight coating of metallic bismuth, or by saturating with a solution of nitrate of bismuth. It has been proposed to attain the same cnd by incorporating powdered copper or iron with tho carbon ; also by inserting a wire core in the rod, and by winding a thin strip of metal around it.
Electric Light,' Dr. Paget Higgs. (Sec I3lacks ; Gems ; Graphite.)