CHEMIC COMPOSITION OF THE HUMAN BODY. 31 



All soaps are, therefore, salts formed by the union of alkalies and 

 fatty acids. The sodium soaps are generally hard, while the potas- 

 sium soaps are soft. Those made with stearin and palmitin are harder 

 than those made with olein. If the soap is composed of lead, zinc, 

 copper, etc., it is insoluble in water. 



Emulsification. When a neutral oil is vigorously shaken with 

 water or other fluid, it is broken up into minute globules that are 

 more or less permanently suspended; the permanency depending 

 on the nature of the liquid. The most permanent emulsions are 

 those made with soap solutions. The process of emulsification and 

 the part played by soap can be readily observed by placing on a 

 few cubic centimeters of a solution of sodium carbonate (0.25 per 

 cent.) a small quantity of a perfectly neutral oil to which has been 

 added 2 or 3 per cent, of a fatty acid. The combination of the acid 

 and the alkali at once forms a soap. The energy set free by this 

 combination rapidly divides up the oil into extremely minute globules. 

 A spontaneous emulsion is thus formed. 



In addition to the ordinary fats, there are present in different 

 tissues several compounds which, though usually regarded as fats, 

 nevertheless differ materially from them in composition, containing, 

 as they do, both nitrogen and phosphorus. These nitrogenized or 

 phosphorized fats are as follows: 



Lecithin, C 44 H 90 N.PO 9 , is found in blood, lymph, red and 

 white corpuscles, nerve tissue, yolk of eggs, etc. When pure, it 

 presents itself generally under the form of a white, crystalline powder, 

 though sometimes as a white, waxy mass. Lecithin is easily decom- 

 posed, yielding, with various reagents, glycero-phosphoric acid, cholin 

 and stearic acid. 



Protagon, C 160 H 308 N 5 PO 35 , is found most abundantly in the 

 brain tissue, especially in the white portion. It crystallizes from 

 warm alcoholic solutions, on cooling, in the form of white needles, 

 generally arranged in groups. It melts at 200 C., and forms a 

 syrupy liquid. 



Cerebrin, C 17 H 33 N.O 3 , is found largely in the brain, in nerves, 

 and in pus-corpuscles. It is a soft, white, amorphous powder, in- 

 soluble in water, but swelling up like starch in boiling water. When 

 boiled with dilute acids, it is decomposed, yielding a fermentable 

 dextro-rotatory sugar, identical with galactose. Cerebrin may, 

 therefore, be regarded as a glucosid. 



THE PROTEIDS. 



The proteids constitute a group of organic bodies which are 

 found in both vegetable and animal tissues. Though present in 

 all animal tissues, they are especially abundant in muscles and 



