232 CLASSIFICATION [CH. X 



For convenience of practical study we may consider 

 nitrogenous plastic substances as divisible into 



(1) Proteids insoluble in water but soluble in dilute 

 1 — 2 p. c. alkali, [e.g. the gluten of cereals.] 



(2) Proteids soluble in water. 



Substances which give all the characteristic proteid 

 reactions and are precipitated by potassium ferrocyanide 

 and acetic acid, trichloracetic acid, cupric acetate, and 

 normal lead acetate, etc., etc. 



They may or may not be precipitated by boiling after 

 addition of acetic acid, or by adding excess of 90 p. c. 

 alcohol. [e.g. the ' soluble proteids ' of cereals and legu- 

 minous seeds.] 



(3) Peptones amd albumoses. 



Soluble in water but not precipitated by any of above 

 reagents : give a characteristic reddish tint with the biuret 

 reaction: are completely precipitated from neutral solu- 

 tions by alcoholic mercuric chloride, and from slightly 

 acid solutions by sodium phosphotungstate ; are slightly 

 dif3fusible through membranes, [e.g. vegetable peptones 

 and albumoses of leguminous seeds.] 



(4) Amides. 



Amido-derivatives of organic acids, [e.g. asparagin, 

 glutamin, betain, etc.] 



(5) Ammonia, nitrates and nitrites. 



The examination of these constituents is made on 

 special extracts of original material obtained as described 



