JACQUES LOEB 55 



times with 25 cc. of a HCl solution which varied for the different filters 

 from m/8 to m/8192, and after this each filter was perfused four times 

 with H2O to remove the HCl not bound by the gelatin. 



The result is given in the curves of Fig. 5 for the conductivity, 

 osmotic pressure, swelling, and alcohol number. The curves show a 

 marked minimum at about pH = 4.8; i.e., the isoelectric point. 

 The appearance of this low minimum of the values for conductivity, 

 osmotic pressure, swelling, etc., characteristic of the isoelectric point, 

 was at first a surprise and a puzzle to the writer. On the right side 

 the curve of osmotic pressure rises to a value of 125 to 150 mm. which 

 corresponds to the value for gelatin treated with m/32 NaOH and 

 washed ten times with H2O (without any subsequent treatment with 

 HCl). On the acid side of the isoelectric point the curve of osmotic 

 pressure rises to that point which corresponds to the effect the same 

 concentration of HCl has on gelatin not previously treated with alkali. 

 Hence the result of the acid treatment is on the left, more acid side 

 from the isoelectric point, exactly the same as when we use gelatin not 

 previously treated with m/32 NaOH and apply to it the same treat- 

 ment with HCl as in this experiment; while on the right, less acid side 

 of the isoelectric point, it is the same as if the gelatin had been treated 

 with NaOH of increasing concentrations. 



This permits of only one explanation, which in the light of the 

 experiments published in the preceding parts of this paper is as fol- 

 lows. Sodium gelatinate, or perhaps more correctly a negative 

 gelatin ion, cannot continue to exist except at hydrogen ion concen- 

 trations lower than that defining the isoelectric point. This means 

 that in the acid solutions all or most of the sodium gelatinate must have 

 given off its sodium ions and must have been retransformed into the 

 weak gelatin acid; or some other constitutional (tautomeric?) change 

 must have occurred in the gelatin molecule on the acid side of the 

 isoelectric point. On the alkali side of the isoelectric point the sodium 

 gelatinate could and did continue to exist. 



This experiment was repeated with different concentrations of acids 

 and alkalies, . all confirming our conclusions. The experiment was 

 also reversed, i.e. the gelatin was first treated with acid and then with 

 alkali, and again our theory was confirmed. 



It is of interest to see our view confirmed also by another set of 

 facts. It has long been known that proteins and probably ampho- 



