88 PHYSIOLOGICAL CHEMISTRY 



precipitate on boiling. Neutral solutions, especially if salts 

 have been removed by dialysis, will precipitate only imperfect- 

 ly. Solutions of some proteins, i.e., casein, gelatine and sec- 

 ondary derived proteins such as proteoses and peptones do not 

 precipitate on boiling. In general for each protein there is a 

 specific precipitation temperature, but experimental conditions 

 such as the amount of salts present, the rapidity of heating and 

 other factors cause rather wide variations in the precipitation 

 temperature. A protein precipitated by boiling in weak acid 

 solution cannot readily be redissolved. Some as yet unknown 

 change has taken place in the protein molecule, and the sub- 

 stance is said to be coagulated. Proteins may be coagulated 

 also in other ways. 



Mineral Acids. Proteins are precipitated by the addition 

 of small amounts of the strong mineral acids, hydrochloric, 

 sulphuric and nitric. The precipitate dissolves in excess of the 

 acid, particularly if the solution is heated. Glacial acetic acid 

 does not precipitate proteins. There has been much discussion 

 of the nature of this precipitation. Probably the proteins are 

 thrown down in the form of salts. Precipitation with concen- 

 trated nitric acid is often used as a test for proteins. To the 

 solution to be tested, concentrated nitric acid is added carefully 

 so that it will form a layer at the bottom of the test tube. In 

 the presence of protein a cloudy ring appears at the juncture of 

 the two liquids. 



Salts. Proteins are precipitated by salts. The salts of 

 heavy metals, such as copper, iron, mercury, lead, etc., will 

 throw down the proteins from their solutions. The precipi- 

 tates formed are in many cases true salts of the protein and 

 the metal. Where this is the case, the precipitation takes place 

 best usually in a weakly alkaline solution, for in this condition 

 the proteins are negatively charged, and will combine with the 

 positive metal ions. Some proteins, such as protamines and his- 

 tones, which have large amounts of diamino acds, form alkaline 

 solutions and the protein caries positive charges. More alkali 

 must be added to give these proteins negative charges than in 



