FATS AND FIXED OILS 45 



butter, and many vegetable oils. It is soluble in ether and 

 other fat solvents, and also in alcohol. It has the peculiar 

 property of uniting both with acids and with bases (p. 50) 

 to form salts. It is often described as a phosphorised fat, 

 because, on saponification, it yields stearic and palmitic acids, 

 choline and glycero-phosphoric acid. It is, therefore, re- 

 garded as a compound type, i.e. a glyceride of stearic and 

 palmitic acids, in which the third hydroxyl of the glycerine 

 is replaced by the phosphoric acid radicle in ethereal com- 

 bination with choline (p. 56). This constitution is indicated 

 by the following formula : 



CH 3 (CH 2 ) 16 CO.C\ / O.N(CH 3 ) 3V 



>C 3 H 5 .O.PO< >C 2 H 4 



CH 3 (CH 2 ) 14 CO.O/ X) 



(Lecithin.) 



Kancidity. Fats, when exposed to air, exhibit a tendency 

 to become rancid. This tendency is not confined to any one 

 group or class, but is common to those of vegetable and of 

 animal origin alike. The tendency is, however, much-fflore 

 pronounced in some, e.g. palm oil, butter, etc., than in others. 



Primarily, no doubt, this change consists in the partial 

 saponification of the fat by the action of micro-organisms or 

 of enzymes. The free fatty acids produced have an extremely 

 disagreeable taste, and those which are volatile, e.g. butyric 

 acid, have also a very offensive odour. It is probable, how- 

 ever, that secondary changes also occur, resulting in the 

 oxidation of the unsaturated portion, if any, and the partial 

 oxidation of the glycerine to aldehydes and acids of a more 

 or less volatile character. 



Natural Fats. There is no essential difference between 

 animal and vegetable fats except as regards their origin. 

 Both consist of the glycerides of fatty acids mainly oleic, 

 stearic, and palmitic acids. In the animal, fat forms part of 

 the tissue substance. It occurs in layers or masses in various 

 parts of the body, but especially in the region of the kidneys 

 which it surrounds and encloses. Small quantities of fat are 

 intimately associated with the muscular tissues. Beefsteak, 

 free from all visible fat, was found to contain from i to 3 per 



