36 THEORY OF COLLOIDAL BEHAVIOR 



Miss Field 1 has shown that by carrying the washing process a 

 step further the last traces of ash can be removed from the 

 powdered gelatin. In bringing powdered gelatin to the iso- 

 electric point and washing with water of the pH of the isoelectric 

 point we can quickly make the gelatin completely ash-free. If 

 the protein is soluble at this point (as is the case with crystalline 

 egg albumin) it is only necessary to carry out the dialysis at the 

 pH of the isoelectric point to obtain the protein free from iono- 

 genic impurities. 2 



This fact is a further support of our contention that at the 

 isoelectric point proteins can combine with neither anion nor 

 cation. 



We may call attention to one interesting fact which is in 

 harmony with these results. It has always been known that 

 pepsin digestion occurs in nature in an acid medium. The 

 reason for this connection of an acid reaction with pepsin digestion 

 was cleared up by Northrop 3 who found that the hydrogen ion 

 concentration at which pepsin commences to act on a protein 

 varies with the isoelectric point of the protein and that the action 

 always occurs on the acid side of the isoelectric point. It 

 seems to follow from the experiments of Pekelharing and Ringer 4 

 that pepsin is an anion like Cl which can only combine with a 

 positive protein ion. This combination between pepsin and 

 positive protein ion seems to be the prerequisite for the falling 

 apart (or digestion) of the protein ion. 



1 FIELD, A. M., J. Am. Chem. Soc., vol. 43, p. 667, 1921. 



2 Miss Field's paper as well as the writer's paper referred to were over- 

 looked by C. R. SMITH (J. Am. Chem. Soc., vol. 43, p. 1350, 1921) who 

 also describes a method of preparing ash-free gelatin. 



3 NORTHROP, J. H., J. Gen. Physiol., vol. 3, p. 211, 1920-21. 



4 PEKELHARING, C. A. and RINGER, W. E., Z. physiol Chem., vol. 75, 

 p. 282, 1911. 



