302 



CARBON. 



paste with water, and well mixed with 20 parts of tar and 500 

 of coal finely pulverized, have been found to afford, after the 

 mass was dried and ignited out of contact with air, a charcoal 

 which was little inferior to bone-black in quality. When char- 

 coal which has been once used in such a filter, is calcined again, 

 it is found to possess very little discolouring power. This is 

 owing to the deposition upon its surface of a lustrous charcoal, 

 of the lamp black variety, produced by the decomposition of 

 the organic colouring matters, which has little or no discolouring 

 power. But if the charcoal of the sugar filters be allowed to 

 ferment, the foreign matter in it is destroyed ; and if afterwards 

 well washed with water and dried, before being calcined, it 

 will be found to recover a considerable portion of its original 

 discolouring power. 



Bussy has constructed, from observation, the following table 

 of the efficiency of the different charcoals. These substances 

 are compared with ivory black, as being the most feeble species, 

 although this is superior by several degrees to the best wood 

 charcoal. The relative efficiency, it will be observed, is not 

 the same for two different kinds of colouring matter : 



This remarkable action of charcoal in withdrawing matters 

 from solution is certainly an attraction of surface, but it is ca- 

 pable notwithstanding, of overcoming chemical affinities of 

 some intensity. The matters remain attached to the surface 

 of the charcoal, without being decomposed or altered in nature, 

 For if the blue sulphate of indigo be neutralized and then fil- 



