CH. II.] GENERAL REACTIONS. 35 



10. Hydrolysed Proteins. Formed by the action of acids, 

 alkalies and certain enzymes on the native proteins. 



(i.) Metaproteins. Soluble only in acids and alkalies, 

 (ii.) Proteoses or albumoses. Soluble in water, not coagulated 



by heat, precipitated by ammonium sulphate, 

 (iii.) Peptones. Like the albumoses, but not precipitated by 



ammonium sulphate. 



(iv.) Polypeptides. Simple peptones, formed of non-amino 

 acids only. 



C. General Reactions. 



For the following reactions use egg-white that has been well beaten with 

 6 times its volume of water or serum that has been diluted ten times with water. 



(i.) The proteins give certain colour reactions (see 

 pages 38 to 41). 



( 2 . ) They are precipitated by the salts of the heavy metals. 

 The salts that are most used are lead acetate, mercuric 

 chloride and nitrate, ferric chloride, copper sulphate, and zinc 

 sulphate. The mechanism of the precipitation is somewhat 

 complex, and probably varies for different salts and 

 different concentrations. In a good many cases it seems 

 to be due to the adsorption of the metallic kation by the 

 negatively charged colloidal protein. For this reason the 

 precipitation is best obtained when the reaction of the 

 medium is somewhat alkaline, the protein then being 

 negatively charged (see p. n). Also the precipitate is 

 often soluble in acid. It is often soluble in an excess of the 

 -metallic salt, probably because the charge on the protein 

 becomes positive owing to the adsorption of the excess of 

 positive ions. 



9. Treat 3 cc. of the solution with a few drops of mercuric 

 nitrate. A white precipitate is obtained. This will partially or 

 completely dissolve in a saturated solution of sodium chloride, pro- 

 vided that the solution does not contain free acid. The solubility 

 in sodium chloride is due to the fact that mercuric chloride is 

 formed. This salt differs from the nitrate in that it is only feebly 

 dissociated. 



