JACQUES LOEB . 249 



because the anion is divalent, H2SO4 combining with gelatin in equiva- 

 lent and not in molecular proportions as do the weak dibasic or 

 tribasic acids; e.g., tartaric or phosphoric. 



A few words are necessary concerning the method of these experi- 

 ments. We can measure the amount of swelling by determining the 

 increase in weight of a given mass of gelatin or by determining its 

 increase in volume. We have adopted the following simple and quick 

 volumetric method (although we intend to supplement these experi- 

 ments later with gravimetric experiments) . 



Dry powdered gelatin, of pH= 7.0, was sifted and the grains no 

 longer going through Sieve 50 but going through Sieve 40 or 30 

 were selected for the experiment. We had therefore fairly uniform 

 grains of not too small a diameter. Doses of 1 gm. each of such 

 powder were weighed out, each dose was put for an hour into 100 cc. 

 of m/128 acetic acid at 10°C. to bring the gelatin to the isoelectric 

 point. The powdered mass was then put on a filter and washed five 

 times with 25 cc. of distilled water at 5°C. In the acetic acid solu- 

 tion and during the washing on the filter the powdered gelatin is 

 stirred constantly. 



Each dose of originally 1 gm. of dry powder which had meanwhile 

 absorbed a certain quantity of liquid (which was about the same for 

 each gram of the isoelectric powder) was then put for 1 hour at 

 about 20° into 100 cc. of different concentrations of the acid or base 

 whose influence on swelling was to be tested, and the suspension was 

 constantly agitated. It was found that in an hour the granules of 

 gelatin had reached the maximal swelling in each solution. To 

 measure the relative amount of swelling in different acids or alkalies 

 and at different pH the suspension was poured into graduate cylinders 

 of 100 cc. each (and all of the same diameter) in which the granules 

 fell very rapidly to the bottom. The cylinders were kept in a water 

 bath at 20°C. for about 10 to 15 minutes and the volume occupied 

 by the gelatin granules was then read. This volume included a cer- 

 tain amount of solution between the granules and therefore the real 

 volume of the gelatin was smaller than that read. While therefore 

 the method cannot be used to measure the absolute amount of swelling 

 it allowed us to determine the relative influence of different acids or 

 bases on the swelling for the same pH. 



