58 Biological Chemistry. 



substances like the paraffins when a halogen cannot enter 

 into combination with the carbon compound without 

 replacing another element. The formula of ethylene is 

 generally represented in the following way : 



CH 2 



|| , or CH 2 :CH 2 



CH 2 



The two carbon atoms are said to be united together by 

 means of a double bond. It is not necessary to enter 

 into any discussion here as to the physical meaning to 

 be attached to this expression, or the kind of combination 

 between two carbon atoms which are united in this way ; 

 this point is still a subject of some controversy. It will 

 be sufficient for the present to remember that the double 

 bond is used as a graphical method to represent certain 

 facts. When a compound is represented as containing 

 a double bond, it will usually be more unstable than 

 those substances in which it is absent, and will readily 

 form certain addition products. If the double bond is 

 regarded as representing two valencies, the carbon atom 

 will still appear as quadrivalent. When the addition pro- 

 duct has been formed (in the case when only one double 

 bond exists) the unsaturated character of the compound 

 disappears. Thus, in the case of ethylene, the addition of 

 bromine may be represented graphically 

 CH 2 CH 2 Br 



CH 2 CH 2 Br 



Further action of bromine can only take place by 

 substitution to form such compounds as CHBr 2 CH 2 Br, 

 CBr 3 CH 2 Br, etc. It is to be noticed in the above equa- 

 tions that the bromine atoms are represented as being 

 attached to two different carbon atoms, and the substance 

 obtained thus from ethylene is different from the sub- 



