THE PROTEINS 107 



salt necessary for the precipitation of any particular protein is charac- 

 teristic 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 disintegra- 

 tion 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 constitution. At the present time it is obviously 

 impossible to admit any classification on such a basis, since the neces- 

 sary knowledge is wanting, and we have therefore to make use of 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 

 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 occur. 



(1) THE PROTAMINES. These occur in the body only in com- 

 bination with other groups. They are obtained from the ripe sperma- 

 tozoa of certain fishes, where they occur in combination with nucleic 

 acid. They are characterised by the very large amount of bases 

 contained in their molecule, amounting to 85 per cent, of the total 

 substance. It was formerly thought by Kossel that the protamines 

 contained only diamino-acids and bases, but it has been shown later 

 that a small proportion of mono -amino -acids may also be obtained 

 from their disintegration (v. Table, p. 100). On account of their 

 constitution they possess strongly basic characters and form well- 

 marked salts, e.g. sulphates and chlorides, as well as double salts with 

 platinum chloride. They contain no sulphur and do not coagulate 

 on heating. 



(2) HISTONES. This class of proteins, like the protamines, only 

 occurs in combination with other groups, such, for instance, as nuclein 

 and hsematin. They may be obtained from red blood corpuscles, 



