THE HYDROCARBONS AND THEIR DERIVATIVES. FLAME 347 



give compounds of higher molecular weight. Thus, methane 

 gives ethylene and acetylene, along with hydrogen : 



2CH 4 -+C 2 H 4 + 2H 2 . 



At a white heat all the hydrocarbons decompose into hydrogen 

 and free carbon. 



The latter is deposited in a dense form called gas-carbon, which 

 is used in making carbon rods for arc lights and electric furnaces, 

 and carbon plates for batteries, and for the electrodes employed 

 in electrolysis. The carbon is ground up, moistened with petro- 

 leum residues, subjected to hydraulic pressure and finally heated 

 strongly to expel volatile matter. 



Derivatives of the Hydrocarbons. Although the hydro- 

 carbons are themselves almost inert chemically, yet many impor- 

 tant classes of organic substances may be regarded as their deriv- 

 atives, one or more atoms of hydrogen in the graphic formula 

 (p. 344) being replaced by other elements. The following tab- 

 ulation should be carefully studied, and the graphic formula 

 of each compound mentioned should be written down by the 

 student. 



1. Halogen Derivatives. When a mixture of methane and 

 chlorine is exposed to sunlight several successive changes occur: 



1 CH 4 + C1 2 ->HC1 + CHaCl (methyl chloride) 

 CH 3 C1 + C1 2 ->HC1 + CH 2 C1 2 (methylene chloride) 

 CH 2 C1 2 + C1 2 -HC1 + CHC1 3 (chloroform) 

 CHC1 3 + C1 2 ->HC1 + CC1 4 (carbon tetrachloride). 



Chloroform CHC1 3 , used as an anaesthetic, and carbon tetra- 

 chloride CC1 4 (p. 332) are familiar substances. lodoform CHI 3 

 is employed in surgical dressing. These substances are not salts, 

 and are not ionized in solution. 



