106 PHYSIOLOGY 



is to boil the protein in slightly alkaline or neutral solution after the 

 addition of 2-5 per cent, of sodium chloride or sodium sulphate. 

 While the solution is in active ebullition 1 per cent, acetic acid is 

 added drop by drop until the reaction is just acid to litmus. By this 

 means a nearly perfect separation of all the coagulable proteins may 

 be effected. 



(2) HELLER'S TEST. On pouring a solution of protein carefully 

 down the side of a test-tube containing strong nitric acid so as to 

 form a layer on the top, a white layer of coagulated protein is pro- 

 duced at the junction of the two fluids. A similar coagulative effect is 

 produced by other strong mineral acids. 



(3) PRECIPITATION BY NEUTRAL SALTS. On addition of a 

 neutral salt in excess to a colloidal solution the relation between 

 the solvent and the particles which are in suspension or pseudo-solution 

 are altered. It is therefore possible in many cases by the addition of 

 neutral salts to separate out the dissolved colloid without otherwise 

 altering its characters in any way, so that, on collecting the precipitate 

 and separating the salt carried down with it, it can be dissolved again 

 by adding water. Some classes of proteins can be salted out very 

 readily, while others require a much higher concentration of salt 

 before they are precipitated. 



The salts which are generally employed for salting out proteins 

 have been divided by Schryver into three classes : 



Class I. 



Sodium chloride. 

 Sodium sulphate. 

 Sodium acetate. 

 Sodium nitrate. 

 Magnesium sulphate. 



Class II. 



Potassium acetate. 

 Calcium chloride. 

 Calcium nitrate. 



The two calcium salts are, however, rarely employed, as they tend 

 to render the precipitated protein insoluble. 



Class III. 



Ammonium sulphate. 

 Zinc sulphate. 



The salts of the first class require much higher concentration 

 for the precipitation of the albumins than those of the second, and 

 these than those of the third. Since the degree of concentration of any 



