io FATS, OILS, AND WAXES 



and are able to form additive compounds with many substances, 

 notably the halogens. 



Thus, while the saturated hydrocarbon will only react with 

 chlorine or bromine by the replacement of one atom of hydro- 

 gen for each atom of halogen introduced into the molecule, 



C 2 H 6 + Br 2 = C 2 H 5 Br + HBr 

 Ethyl bromide 



an unsaturated compound, such as ethylene, will add on the 

 halogen directly 



C 2 H 4 + Br 2 = C 2 H 4 Br 2 



Ethylene 

 dibromide 



the resulting additive compound being saturated. 



It will thus be seen that it requires two atoms of bromine 

 to saturate an unsaturated compound containing one double 

 bond, and similarly it requires four atoms of halogen to satu- 

 rate a compound containing two double bonds. In this way 

 it is shown that since the acids of the oleic, linolic, and lino- 

 lenic series require two, four, and six atoms of halogen respec- 

 tively for saturation, they must contain respectively one, two, 

 or three double bonds. 



A measure of the degree of unsaturation of a given acid 

 may accordingly be obtained by determining how much bro- 

 mine it will absorb ; as, however, the interaction with bromine 

 is liable to be violent it is found more convenient to employ 

 iodine, which, in addition to being less violent in its action 

 than bromine, is also easier to handle. 



A description of the method employed in determining what 

 is known as the " iodine value" of fats is given below (p. 31). 



EXTRACTION OF FATS. 



The isolation of fats from admixture with other substances 

 may be effected by extraction by means of fat solvents. 



The principle of the extraction is to treat the dried mixture 

 with a solvent which will dissolve only the fat and leave the 

 other substances unchanged. The solvents most commonly 

 used for this purpose are ether, light petroleum, carbon tetra- 

 chloride and carbon disulphide, the two latter being used 

 chiefly on a commercial scale. 



