58 THE GENERAL^ CHARACTERS OF THE PROTEINS 



(copper sulphate or silver nitrate), it is possible to solve the problem 

 of chemical equilibrium by a graphical method, and, given the per- 

 centage composition of a complex, to determine into how many 

 phases it will separate, and the composition of each phase. 



Pauli has investigated in some detail the conditions of precipita- 

 tion by zinc sulphate. He found that there were two maxima of pre- 

 cipitation when this salt was added to an egg-protein solution of a 

 certain strength. The precipitating power increased gradually be- 

 tween concentrations corresponding to 'OOI to 0*05 normal. From 

 this point the precipitating power gradually decreased, until the 

 concentration corresponding to that of a normal solution was reached. 

 From this point onwards till the concentration 2 N was attained, 

 no precipitation whatever took place. On increasing the concentra- 

 tion beyond this point, however, precipitation recommenced, and 

 increased with increasing strength of the salt solution until a second 

 maximum was attained. The precipitates obtained with the lower 

 dilution were irreversible, that is to say, they would not redissolve on 

 addition of water ; those obtained at the higher dilution were, on the 

 other hand, reversible and redissolved on dilution of the supernatant 

 fluid. The numbers just given refer only to solutions containing a 

 definite amount of egg-protein. In this particular case the limits 

 between which the maximum formation of the irreversible precipi- 

 tate and the incipient formation of the reversible precipitate took 

 place were 0*05 and 2 N. These limits are wider with a lower 

 protein concentration and narrower with a higher concentration. 



These examples are sufficient to indicate the extremely complex 

 conditions governing the composition of the precipitates produced 

 when solutions of the salts of heavy metals are added to protein 

 solutions. In the majority of cases compounds of definite chemical 

 composition cannot be isolated. This statement does not, however, 

 preclude the possibility that certain proteins of more acidic character, 

 such as caseinogen, should form salts of definite chemical com- 

 position, and further investigation of precipitates with high copper 

 content, such as those derived from plant proteins and caseinogen, 

 seems desirable. 



Various theories have been advanced to account for the nature 

 of the precipitates produced by salt precipitation. These have been 

 summarised in the paper of Pauli referred to. They may be more 

 fittingly discussed in dealing with the general chemical physics of 

 colloidal solutions and the theory of salt precipitation of colloids. In 

 the present state of knowledge the precipitates produced by the salts 

 of the heavy metals cannot be regarded as of sufficiently definite 

 chemical nature as to make them of value in fixing the characters of 

 the protein from which they are derived, and for this reason they 

 have been treated in a somewhat cursory manner. 



SECTION XVIII. THE OXIDATION OF THE PROTEINS. 



In recent years the chemical examination of proteins, with a 

 view to elucidation of the constitution, has been confined chiefly to 

 the study of the hydrolysis products. This method, apart from the 

 value of the results yielded, is a logical one, in view of the fact that 

 all evidence with regard to the biological utilisation points to the 

 fact that the first stage of protein degradation is one of hydrolysis. 



