SECT. I. CARBON. 15 



it not for vegetables which decompose it, assimilate the 

 carbon and set the oxygen free to mingle with the air 

 and make it again fit for respiration. Carbon has a 

 greater power- ef-eegtbmation than any other simple 

 substance except hydrogen. 



Mr. Faraday compressed carbonic acid gas into a 

 liquid by the pressure of its own elasticity when dis- 

 engaged from combination. in close vessels, a force equal 

 to the weight of thirty-five times that of our atmosphere ; 

 and the liquid was reduced to a solid by M. Thilorier 

 by rapid evaporation, during which the heat was given 

 out so quickly by one part of the liquid, that the re- 

 mainder was condensed into a substance like snow, 

 which could be touched with impunity, but when mixed 

 with sulphuric ether its temperature was reduced to 

 166 below zero of Fahrenheit's thermometer. 



Carbon appears naturally under a great variety of 

 forms, and exhibits one of the most striking instances 

 of allotropism, the same substance showing the greatest 

 contrast in appearance and physical properties. The 

 diamond, the most resplendent, transparent, and hardest 

 of gems, is identical with carbon, which is black, dull, 

 opaque, and brittle. Both are combustible ; carbon is 

 easily ignited, but it requires a heat of I860 to consume 

 the diamond. 



However numerous the crystalline forms assumed by 

 substances either naturally or artificially may be, they 

 are all capable of being grouped into geometrical sys- 

 tems ; each system possessing its own allied and deriva- 

 tive forms capable of mutual variations among them selves, 

 but the forms of one system never assuming those of 

 the other. With that law, however, carbon and a few 

 other substances are completely at variance. The 

 diamond crystallizes in octohedrons, while graphite^ 

 which is also carbon, crystallizes in six-sided plates, 

 two forms that belong to different systems quite irrecon- 



