94 PHYSIOLOGY 



(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 produced at the junction of the two fluids. A similar 

 coagulative effect is given 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 is 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. 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 precipitation 

 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 difficult, 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 t In- 

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

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

 are conventionally divided, and leave the closer study of the individual 

 proteins to be dealt with in connection with the organs or tissues in which 

 they are found. 



(1) THE PROT AMINES. These occur in the body only in combination 

 with other groups. They are obtained from the ripe spermatozoa of certain 



