96 PHYSIOLOGY 



(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 concentra- 

 tion 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. Class II. Class III. 



Sodium chloride. Potassium acetate. Ammonium sulphate. 



Sodium sulphate. Calcium chloride. Zinc sulphate. 



Sodium acetate. Calcium nitrate. 

 Sodium nitrate. 

 Magnesium sulphate. 



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

 render the precipitated protein insoluble. 



The salts of the first class require much higher concentration for the pre- 

 cipitation of the proteins than those of the second, and these than those of 

 the third. Since the degree of concentration of any salt necessary for the 

 precipitation of any particular protein is characteristic for this body, it is 

 possible to employ a fractional process of salt precipitation in order to 

 separate mixtures of proteins into their components. Owing, however, to 

 the tenacity with which different colloids adhere to one another it is diffi- 

 cult, even after many repetitions of the process of fractional salting out, to 

 obtain products which can be regarded as free from admixture. For the 

 purpose of fractional precipitation the salts most frequently employed 

 are those of the third class, namely, ammonium sulphate and zinc sulphate. 

 We shall have to deal with results obtained by this method when treating 

 of* the separation of albumoses and peptones. The precipitability of 

 different proteins with neutral salts serves also as the basis of the ordinary 

 classification of these bodies. 



THE CLASSIFICATION OF PROTEINS 



It is possible that in the future, when we know all the disintegration 

 products of the various proteins and the manner in which they are arranged 

 in the molecule, the classification of these bodies will be based on their con- 

 stitution. At the present time it is obviously impossible to make any classi- 

 fication on such a basis, since the necessary knowledge is wanting, and 

 we have therefore to use a purely artificial classification, such as that 

 adopted by the Chemical and Physiological Societies in 1907, based chiefly 

 on the solubilities of the various proteins in water and salt solutions. We 

 shall here only indicate the characters of the main groups into which proteins 



