10 FATS, OILS, AND WAXES 



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. 29). 



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. 



It must be borne in mind that besides extracting fats, ether 

 will also dissolve out lecithin, cholesterol, essential oils and the 

 somewhat indefinite group of substances known as lipoids.* 



Moreover, other substances which are of themselves insol- 

 uble in ether may become soluble in the presence of fats. 



Whatever solvent is employed must be tested before use 

 to see that it leaves no residue on evaporation and is free from 

 moisture. 



A rough and ready method of extracting fat from a given 

 sample is to place the finely divided and dried material on 

 a filter paper folded into a funnel and to pour the fat-solvent 

 on to it. The filtrate will contain most of the fat which may 

 be recovered by evaporating off the solvent. 



When it is desired to extract the fat quantitatively, the 



* This term has no chemical significance, and comprises a variety of different 

 substances, which owing to their exhibiting the same solubilities as fats were 

 called by Overton Lipoids (see p. 44). 



