THE HYDROCARBONS AND THEIR DERIVATIVES. FLAME 351, 



Ethylene is a gas which burns in the air with a highly luminous \ 

 flame (see p. 356). It combines directly with bromine to form \ 

 ethylene bromide C 2 H 4 Br 2 . \ 



The hydrocarbons of the ethylene series are known as defines. 

 They are of value as illuminants. Their derivatives are similar 

 in character to those of the paraffin hydrocarbons, but are more 

 active chemically in view of their unsaturation. When heated 

 with hydrogen in the presence of a catalyst, such as finely-divided 

 nickel, they give the corresponding saturated derivatives (com- 

 pare pp. 433-4). 



Acet^leneC^.2- A mixture, containing acetylene, is formed 

 when any hydrocarbon is heated strongly (p. 346), air being 

 excluded. As in the case of ethylene, the formation from methane 

 by loss of hydrogen (p. 347) suggests the graphic formula: 



H H 



I I II 



H-C-H + H-C-H->H-C-C-H or H-C^C-H 



I I II 



H H 



Pure acetylene is prepared by the action of water on calcium 

 carbide (p. 333) : 



CaC 2 > 2H 2 - Ca(OH) 2 + C 2 H 2 1. 



Calcium hydroxide (slaked lime) remains. The gas burns with a 

 flame even more luminous than that of ethylene. It is there- 

 fore made in generators by the foregoing action for use on auto- 

 mobiles and for lighting buildings remote from a public supply of 

 illuminating gas. Acetylene tanks, which are also in use, contain 

 acetylene dissolved, under high pressure, in acetone. 



The Acetylene Blowpipe or Torch. Acetylene decomposes, 

 when heated, with liberation of heat: 



C 2 H 2 -> 2C + H 2 + 58,100 cal. 

 When acetylene burns with oxygen, therefore, 



