88 HOW CROPS GROW. 



Three familiar representatives of this class of bodies 

 are, albumin, or the white of egg ; fibrin, or the clot of 

 blood, and casein, which yields the curd of milk. 



Oeneral Characters. — Many o|^ these substances occur 

 in two very distinct modifications, one form being soluble 

 in water, or in highly-diluted acids or alkalies, or in salt- 

 solutions, the other insoluble in these liquids. 



Some of the solulle proteids we find naturally dissolved 

 in the juices of living plants and animals. Some may be 

 obtained in the solid form by evaporating ofE at a very 

 gentle heat the water which is naturally associated with 

 them. They then appear as nearly colorless or yellow- 

 ish, amorphous solids, destitute of odor or taste, which 

 dissolve again in water, but are insoluble in alcohol. 



Soluble compounds of proteids with magnesium or 

 iron occur in plants, or may be obtained from the blood 

 of animals, in the form of white or red crystals. 



Solutions of most of theialbuminoids are readily coagu- 

 lated by heat and by concentrated mineral acids, the 

 albuminoids being thereby themselves chemically changed 

 and made insoluble. Some coagulate spontaneously. 



The insoluble albuminoids, some of which also occur 

 naturally in plants and animals, are, when purified as 

 much as possible, white, flocky, lumpy or fibrous bodies, 

 quite odorless and tasteless. 



The albuminoids, when subjected to heat, melt and 

 burn with a smoky flame and a peculiar odor — that of 

 burnt hair or horn — while a shining charcoal remains 

 which is difficult to consume. 



Tests for the Albuminoids.— The chemist employs the behavior of 

 the albuminoids towards a number of reagents* »s tests for their pres- 

 ence. Some of these are so delicate and characteristic as to allow the 



* Reagents are substances commonly employed for the recognition 

 of bodies, or, generally, to produce chemical changes. All chemical 

 phenomena result from the mutual action of at least two elements, 

 which thus act and react on each other. Hence the substance that 

 excites chemical changes is termed a reagent, and the phenomena or 

 results of its application are called reactions. 



