252 AMPHOTERIC COLLOIDS. II 



Experiments with Dyes. 



Gelatin is readily stained with the basic dye neutral red at the point 

 of neutrality. Our theory demands that gelatin thus stained with 

 neutral red should give off all its stain at the isoelectric point and on 

 the acid side from the isoelectric point; while the neutral red should 

 be retained when pH > 4.7. This turns out as the theory demands. 

 1 gm. of finely powdered gelatin was put for 20 minutes at about 15°C. 

 into a series of beakers each containing 1 cc. of m/100 neutral red. 

 The gelatin was then filtered and washed twice with 25 cc. of distilled 

 water. All the gelatin was stained a beautiful deep red. Each fun- 

 nel was then perfused three times with a definite concentration of 

 HCl, the concentration for the various funnels varying from m/8 to 

 m/8192, and this was followed by eight perfusions with 25 cc. of H2O 

 each. In the funnels treated with a concentration of HCl of m/512 or 

 above, the gelatin became decolorized (the neutral red being washed 

 away); while all the funnels treated with acid of a lower concentra- 

 tion retained their deep red color and did not give off their stain. 

 The decolorized gelatin was then melted, made into a 1 per cent solu- 

 tion whose pH was determined. It was found that the gelatin pre- 

 viously treated with m/512 HCl and decolorized had a pH =4.7, 

 that treated with 3 m/1024 having a pH = 4.6, and so on. Hence the 

 gelatin was not able to retain its neutral red when brought to the 

 isoelectric point or to a pH ^ 4.7, as our theory demands. 



Michaelis and Davidsohn^ working with large blocks of gelatin, 

 gained the impression that gelatin is slightly stainable with both basic 

 and acid stains at the isoelectric point. We are inclined to believe 

 that our method of staining gelatin in a finely divided condition and 

 then ascertaining the pH where the dye is given off again by the 

 gelatin, gives more rehable results than their method of working with 

 large blocks of gelatin. We intend to return to the problem of stain- 

 ing in a future communication. 



SUMMARY. 



1. It is shown by volumetric analysis that on the alkaline side 

 from its isoelectric point gelatin combines with cations only, but not 



^ Michaelis. L., and Davidsohn, H., Biochem. Z., 1913, liv, 323. 



