16 HUMAN PHYSIOLOGY. 



potassium hydroxid, sodium hydroxid, the acid produced combines 

 at once with the alkali to form a salt known as a soap, while the 

 glycerin remains in solution. The reaction is as follows : 



3 KHO + (C u H M O a ), = 3(KC^H n 2 ) + 3 H 2 O. 

 Potassium. Oleic Acid. Potassium Oleate. Wator 



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, of 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 combina- 

 tion 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^HgoN.POj,, is found in blood lymph, red and white cor- 

 puscles, nerve tissue, yolk of eggs, etc. When pure, it presents itself 

 generally under the form of a white, crystalline powder, though some- 

 times as a white, waxy mass. Lecithin is easily decomposed, yielding, 

 with various reagents, glycero-phosphoric acid, cholin and stearic acid. 



Protagon, QeoHgosNgPOas, 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. 



