26 WONDERS OF ORGANIC LIFE. 



become yellowish, hard, and brittle. Protein 

 is inodorous and tasteless ; it absorbs moisture 

 rapidly from the air, and loses water at 212°. 

 It is insoluble in water, ether, alcohol, or in 

 essential oils. Nevertheless, by long-continued 

 boiling in water, it undergoes some change of 

 properties, and is rendered soluble. Protein is, 

 however, dissolved by acetic and phosphoric 

 acids, and also by hydrochloric acid, the solu- 

 tion in this case having a tint of indigo, which 

 changes to black when subjected to heat. 

 Under the action of concentrated sulphuric 

 acid it produces a jelly, which contracts in 

 water ; and this jelly, after being washed in 

 water and alcohol, retains a minute portion of 

 acid, though not sufficient to redden litmus- 

 paper. Mulder calls this compound sulpho- 

 proteic acid. When protein is boiled in dilute 

 sulphuric acid, it acquires a purple tint. 



Protein, then, is the essential nutriment which 

 animals derive from plants or grain, or from 

 other animals. It constitutes, moreover, the 

 basis of albumen, fibrin, casein, etc., substances 

 which differ both from their base, and from 

 each other, in many of their physical properties. 

 For example, albumen is soluble in water ; 

 not so, however, fibrin and casein. Albumen 

 coagulates under the influence of heat ; whereas 

 fibrin spontaneously coagulates from the fluids 

 in which it is held in solution, while casein ie 

 only precipitated from its solutions by dilute 

 acids, and may be re-dissolved by excess of 

 the same acids. These three bodies, thus dit- 



