6O PHYSIOLOGICAL CHEMISTRY. 



separating the fatty acids and washing out everything soluble 

 in hot water (butyric acid, etc.) it is found that the insoluble 

 residue left amounts in the mean to 88 per cent, but may be 

 more or less \vith different grades of butter. Commercial 

 butter contains in the mean about 85 per cent of fat, 10 per 

 cent of water and 5 per cent of salt. 



HUMAN FAT. This consists essentially of olein, palmitin 

 and stearin. In the fat of children the solid glycerides appar- 

 ently are in excess, while in later life the proportion of olein 

 increases, so that the separated fat may appear quite soft. In 

 the human adult the olein may amount to 75 per cent of the 

 whole fat, but the proportion varies with different parts of 

 the body. 



GLYCEROL. Since it is a constituent of all the true fats a 

 few words about this alcohol will be in order here. The sub- 

 stance was first recognized in the aqueous liquid left in the 

 preparation of lead plaster and for many years all used in 

 pharmacy and in the manufacture of cosmetics was made by 

 the same reactions. Since its importance in technology was 

 recognized it has been produced on the large scale by other 

 kinds of saponification or hydrolysis. In pure condition 

 it is a thick, sweetish liquid with a specific gravity of 1.266 

 at 15 referred to water at the same temperature. It mixes 

 with water and alcohol in all proportions, but is not soluble 

 in pure chloroform, benzene, carbon disulphide or petroleum 

 ether. It is very slightly soluble in ether. Like other alcohols 

 it may be combined with acids to form esters. With stearic 

 acid mono-, di- and tri-stearin are produced, the last being 

 identical with the natural fat. When fed to animals glycerol 

 may be oxidized to a limited extent only. Any excess of it 

 fails to be assimilated and soon produces disorders in digestion 

 and absorption. Its food value, in free form, is therefore very 

 slight. 



Recognition and Determination of Fats. In physiological chemical in- 

 vestigations fats are separated from accompanying substances through 

 their solubility in warm ether, chloroform or petroleum ether. The carbo- 



