12 TEXT-BOOK OF PHYSIOLOGY. 



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

 acids. The sodium soaps are generally hard, while the potassium 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 fat acid. The combination of the acid 

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

 rapidly divides the oil into extremely minute globules. A spontaneous 

 emulsion is thus formed. 



THE PROTEINS. 



The proteins 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 bones, where they constitute 20 

 per cent, and 30 per cent, respectively. Though genetically related, and 

 possessing many features in common, the different members of the protein 

 group are distinguished by characteristic physical and chemic properties 

 which serve not only for their identification, but for their classification into 

 more or less well-defined groups. 



Chemic Composition. A chemic analysis of proteins shows that 

 they consist of carbon, hydrogen, oxygen, nitrogen and sulphur, though 

 the percentage of each of these elements varies somewhat in the different 

 proteins. 



A certain number of proteins contain phosphorus while almost all 

 of them contain different inorganic salts in varying amounts. The average 

 percentage composition of several proteins is shown in the following analyses: 



C. H. N. o. S. 



Egg-albumin... 52.9 7.2 15.6 23.9 0.4 (Wurtz). 



Serum-albumin 53.0 6.8 16.0 22.29 1.77 (Hammers ten). 



Casein 52.3 7.07 15.91 22.03 0.82 (Chittenden and Painter). 



Myosin 52.82 7.11 16.77. 21.90 i .27 (Chittenden and Cummins). 



The molecular composition of the proteins is not definitely known 

 and the formulae which have been suggested are therefore only approxi- 

 mative. Leow assigns to albumin the formula C 72 H 112 N 18 O 22 S, while 

 Schiitzenberger raises the numbers to C 240 H 392 N 65 O 75 S 3 , either of which 

 shows that the protein molecule is extremely complex. 



Structure of the Protein Molecule. From the large size of the protein 

 molecule as indicated by its chemic composition it might be inferred that 

 its structure was equally complex. This modern investigation has shown 

 to be the case. 



