1166 PKOTEIDS. 



G-eneral reactions of the proteids. 



1. Heated with strong nitric acid, they or their solutions 

 turn yellow, and this colour is, on the addition of ammonia, 

 or caustic soda or potash, changed to a deep orange hue. 

 (Xanthoproteic reaction.) 



2. With Millon's reagent 1 they give, when present in 

 sufficient quantity, a precipitate, which turns red on boiling. 

 If they are only present in traces, no precipitate is obtained, 

 but merely a red colouration of the solution when boiled. 



3. If mixed with an excess of concentrated solution of 

 sodium hydrate, and one or two drops of a dilute solution of 

 sulphate of copper, a violet colour is obtained, which deepens 

 in tint on boiling. (Piotrowski's reaction.) 



The above serve to detect the smallest traces of all proteids. 



4. Render the fluid strongly acid with acetic acid, and add 

 a few drops of a solution of ferrocyanide of potassium ; a pre- 

 cipitate shews the presence of proteids, except true peptones 

 and some forms of albumose. 



No general method can be given for the quantitative estima- 

 tion of the various proteids. For this some special manuals 

 should be consulted and use made of the reactions which are 

 specifically characteristic of each proteid as given below. 



CLASSIFICATION OF THE PROTEIDS. 

 The following classification is both convenient and concise. 



CLASS I. Native albumins. 



Soluble in distilled water. Solutions coagulated on heat- 

 ing, especially in presence of a dilute (acetic) acid. Not 

 precipitated by carbonates of the alkalis or by sodium chloride, 

 or generally by solutions of neutral salts. 



1. Egg-albumin. 2. Serum-albumins. 



CLASS II. Derived albumins 2 (Albuminates*). 



Insoluble in distilled water and in dilute neutral saline 

 solutions ; soluble in acids and alkalis. Solutions not coagu- 

 lated by boiling. 



1. Acid-albumin. 2. Syntonin. 3. Alkali-albumin. 



1 A solution of mixed mercurous and mercuric nitrates prepared by dissolving 

 mercury in nitric acid. 



2 Casein was at one time placed in this group with reference chiefly to its 

 solubilities and precipitability. It is now known to be a nucleo-albumin, and 

 is classed as such. (See p. 1207.) 



