94 ELEMENTARY AGRICULTURAL CHEMISTRY 



The constitution of proteicls is not yet fully known, though 

 recently they have been shown to consist of amino-acids. 



As illustrating the complexity of albumin, for example, the fol- 

 lowing empirical formula, among many others, has been proposed 

 as most nearly representing its composition : C240H392N65O75S3. 

 That any definite compound really has such a composition is 

 extremely improbable. "When hydrolysed, proteids split up 

 into their constituent amino-acids. 



Proteids all give a yellow colouration when heated with strong 

 nitric acid. This coloured substance becomes orange when 

 treated with ammonia. They also give a red colour when heated 

 with an acid solution of nitrate of mercury (Millon's reagent). 

 In analysis it is usual to assume that proteids contain 16 per 

 cent, of nitrogen. The amount of nitrogen in a substance is 

 determined, and, by multiplying the percentage of this element 

 by ^^- or 6-25, the product is taken to represent the percentage 

 of albuminoids. The result can only be approximate, since, as 

 already stated, the percentage of nitrogen varies in different 

 albuminoids. The albuminoids are, as a rule, non-crystallisable, 

 colloidal bodies. 



VII. Amides and Amino-acids. — These, also nitrogenous 

 compounds, have a much simpler constitution than the 

 albuminoids. An amide may be regarded as derived from an 

 organic acid by the replacement of the -OH by -NHj. Thus 

 from acetic acid,CH3.C00H, is derived acetamide, CH3.CONH,. 

 Amino-acids are derived from organic acids by the replacement 

 of one or more hydrogen atoms in the organic radical by -NH,, 

 thus, amino-acetic acid, or glycocoll, is CH2(ISrH2).COOH. 



Amides occur widely distributed, especially in immature 

 plants, and since they apparently are incapable of forming flesh 

 when fed to animals it is important in food analysis to dis- 

 tinguish between them and the more valuable albuminoids. 

 Many amides have been found in various plants, but asparayine 



