204 SCIENCE PROGRESS 



With this point in the behaviour of an amphoteric protein 

 cleared up it was possible to reinvestigate the behaviour of 

 various salts with gelatin, bearing always in mind the pH of 

 the reacting solution. It was now possible to show volumetric- 

 ally that when gelatin combined with any cation or of a salt, 

 the combination took place according to the ordinary mass 

 action laws for reacting substances. 



Thus the amount of combination depends upon the amount 

 of acid or basic protein present ; this depends in the first place 

 upon the original concentration of the protein, and then upon 

 the pH at which the reaction is proceeding. 



The amount of cation or anion entering into combination 

 will simply depend upon its concentration, dissociation, and 

 valency ; ions of equivalent valency will therefore produce the 

 same effect upon the same protein solution if they are reacting 

 at the same pH. The " Hofmeister series " therefore now dis- 

 appears, and Loeb has shown that it may be replaced by a 

 series of generalisations that can be briefly stated as follows : 



(i) An amphoteric protein has a definite iso-electric point at 

 which it is practically without reaction either with cations or 

 anions. At hydrion concentrations of pH greater than this 

 (more basic), it behaves as an acid and combines with cations ; 

 when pH is less (more acid), it behaves as a base and combines 

 with anions. 



(2) The amount of combination at any definite pH depends 

 upon the laws of mass action and the valency of the combining 

 ions, exactly as in the case of reactions between crystalloids in 

 solution. 



(3) The influence of different ions upon the physical pro- 

 perties of the proteins can be predicted therefore from the 

 general combining ratios of these ions. 



That the last statement is true has been established ex- 

 perimentally by Loeb in a series of experiments upon the 

 swelling, viscosity, and osmotic pressure of gelatin in presence 

 of different salts at known pH values. Loeb has shown that the 

 physical explanation of this behaviour of different ions upon 

 the physical properties of the protein may be traced to the 

 Donnan Equilibrium obtained when two salt solutions are 

 separated by a membrane impermeable to one ion but per- 

 meable to all the rest. In this case the membrane is the 

 surface of the colloid particles in the heterogeneous protein 

 solution, the ion refused passage is the gelatin ion of the gelatin 

 salt, cation, or anion respectively according to the reaction of 

 the medium. Loeb's original papers need careful study for 

 a full appreciation of his conclusions and their experimental 

 basis. Perhaps the strongest inducement to the botanist to 

 undertake this task will be some tentative indications of the 



