52 THE FEEDING OF ANIMALS 



We see that the number of elements ordinarily found 

 in the proteins is five, nitrogen and sulfur being those that 

 chiefly distinguish these bodies from all others which 

 make up the mass of combustible matter. Two other 

 elements are found in certain of these bodies, as, for 

 instance, phosphorus in casein and ovovitellin and iron 

 in a constituent of blood. 



65. Familiar examples of proteins. Proteins are 

 familiar objects, and their properties are matters of 

 common observation. The tenacious cud of gum from 

 wheat gluten, the strings of coagulated albumin which 

 separate from the cold-water extract of fresh lean beef 

 that is brought to the boiling-point, the hardening of the 

 white of an egg into a tough mass as it is dropped into 

 boiling water, the stiffening of the muscular tissue of the 

 slaughtered animal or the rapid formation of strings of 

 fibrin in the cooling blood in all these instances there 

 are manifested certain chemical or physical properties 

 which pertain to these most important and widely uti- 

 lized compounds. 



SIMPLE PROTEINS 



66. The albumins. The albumins have several sources. 

 They are found in the juice of plants, in certain liquids 

 of the animal body such as the serous fluids, in the cell 

 substance of muscular tissue, in blood and milk, 

 and abundantly in eggs. Unlike other proteins, 

 these compounds are freely soluble in pure cold water, 

 and when such a solution is heated to the boiling- 

 point, they separate from solution by coagulation and 

 become insoluble unless acted upon by some strong 

 reagent. 



