APPARATUS FOR THE ELECTRIC LIGHT. 135 



Instead of carbonizing wood and reducing the charcoal to 

 powder, he selects a suitable piece of wood, which he cuts 

 to the shape required in the carbon ; then he converts it 

 into hard charcoal, and finally soaks it, as in the manufacture 

 we have described. The heating of the wood is conducted 

 slowly so as to drive off the volatile matters, and the final heat- 

 ing takes place in a reducing atmosphere at a very high tem- 

 perature. All impurities are removed from the wood by a 

 preliminary washing in acids and in alkalis. 



Gauduin also points out a method of stopping the pores 

 of the wood by heating it to redness and exposing it to the 

 action of chloride of carbon and various carbides of hydrogen. 

 He expects thus to produce electric carbons which will be 

 very slowly consumed and will give a light absolutely steady. 



These carbons have not, however, done what was expected 

 of them, therefore they have not become general, and are 

 little used except by the firm of Sautter and Lemonnier. At 

 the present time it is difficult to obtain them, and it is doubt- 

 ful if they can compete with those of Carre, who employs as 

 his raw material a very cheap substance, namely coke. 



Effects produced by the addition of metallic 

 Salts to the Carbons prepared for the Electric Light. 



As we have already seen, page 129, attempts have been 

 made to obtain some advantage by associating the carbons 

 with metallic salts, which might supply, independently of the 

 voltaic arc, a light due to the combustion of the metal carried 

 to the negative electrode. The following experiments have 

 been undertaken by Gauduin, Carre, and Archereau : 



Gauduin mixed the following substances with the pure 

 carbon : phosphate of lime from bones, chloride of calcium, 

 borate of lime, silicate cf lime, pure precipitated silica, mag- 

 nesia, borate of magnesia, alumina, silicate of alumina. The 

 proportions were so calculated as to leave 5 per cent, of oxide 

 after the carbons were dried. They were submitted to the 

 action of an electric current always in the same direction, 

 supplied by a Gramme machine sufficiently powerful to main- 



