THE FATS 55 



Of the long list of fatty acids given above only a few occur to any extent 

 in the animal body. In milk, although the greater part of the fat consi 

 of the tnglycendes of oleic, palmitic, and stearic acids, other members of the 

 series given above are present in small amounts. On the other hand the 

 adipose tissue, strictly so called, consists almost exclusively of the fats de- 

 rived from the fatty acids palmitic, stearic, and oleic, i.e. tripalmitin, tri- 

 stearin, and triolein. The great differences in the appearance of the fat of 

 different animals are due to the varying amounts in the relative quantities of 

 these three fats which may be present. While triolein is liquid at C., tri- 

 stearin and tripalmitin are solid at the temperature of the body. According 

 to the relative amounts of these three substances, therefore, we may have a 

 fat which, like mutton suet, is solid at the body temperature, or a fat con- 

 taining much olein which is still fluid and runs away when the body is opened 

 after death, even when it has already cooled. 



PROPERTIES OF THE FATS. The fats are colourless substances 

 devoid of smell. They are insoluble in water, in which they float. They are 

 soluble in warm absolute alcohol, but separate out into crystalline form on 

 cooling. They are easily soluble in ether. If they are strongly heated with 

 potassium bisulphate they give off pungent vapours of acrolein derived from 

 the decomposition of the glycerin of their molecule. 



C 3 H 5 (OH) 3 - 2H 2 = C 3 H 4 



If they are heated with water or steam or submitted to the action of certain 

 ferments, they undergo hydrolysis, taking up three molecules of water, and 

 are split into three molecules of fatty acid and one molecule of glycerin, e.g. t 



C 3 H 5 (C 16 H 31 2 ) 3 + 3H 2 = 3HC 16 H 31 2 + C 3 H 6 (OH) 3 



(neutral fat tripalmitm) (palmitic acid) (glycerin) 



This process may occur spontaneously when fat is left exposed to the air. 

 Fat which has been artificially split in this way is said to be rancid. Most 

 natural fats generally contain a small amount of fatty acid which gives them 

 an acid reaction. 



On boiling a neutral fat for a long time with an aqueous solution of 

 potassium or sodium hydrate, or better still with an alcoholic solution of 

 potassium or sodium ethylate, the fat undergoes saponification, giving the 

 alkaline salt of a fatty acid and glycerin. The former compound is spoken ol 

 as a soap. In water the soaps form a sort of pseudo-solution on heating whicl 

 sets to a solid jelly on cooling. From a dilute watery solution the soap 

 be thrown down in the solid form by the addition of neutral salts 

 insoluble in and non-miscible with water. If shaken up with v 

 droplets rapidly run together and rise to the surface, forming a com 

 layer of the oil or fat. The same thing happens if an absolutely neu 

 be shaken up with a dilute solution of sodium carbonate, 

 fat be slightly rancid, i.e. if fatty acid be present, the latter combines 

 the alkali with the expulsion of C0 2 to form a soap. The presence 

 in colloidal solution in the water at once diminishes or abolishes the s 



