JACQUES LOEB 239 



literature of colloid chemistry. By using gelatin in the condition of a 

 fine powder we are able to wash away the excess of electrolyte after it 

 has had time to act on the gelatin. 



When we put 1 gm. of powdered gelatin for 30 minutes at 20 or 15° 

 into 100 cc. of 3 m/1024 HCl or HNO3, and then wash the gelatin on 

 the filter to remove the excess of acid, we obtain a gelatin with pH 

 of about 4.7 ; i.e., of the isoelectric point. When we use a higher con- 

 centration than 3 m/1024 HCl, we get a pH< 4.7, and a gelatin which 

 is on the acid side of the isoelectric point and which is able to act only 

 as cation; when we use an acid less concentrated than 3 m/1024, we get 

 after the excess of acid is washed away a gelatin whose pH > 4.7 

 and which being on the alkaline side of its isoelectric point can act 

 only as an anion and can combine only with a cation. 



In order to get clear results it is well to work with a series of dif- 

 ferent concentrations of an acid, e.g. m/4, m/8, m/16, etc., to m/8192 

 HCl or HNO3, so that we obtain series of gelatin solutions with dif- 

 ferent pH on either side of the isoelectric point. After the excess of 

 acid is washed away by several perfusions with H2O, the gelatin is 

 perfused three times with 25 cc. of the same solution of a salt, e.g. 

 m/8 NaBr or m/8 NaCNS or m/16 AgNOg, to bring about a reaction 

 between salt and gelatin. It is necessary to stir up the gelatin on the 

 filter while the salt solution is poured on to bring about as complete 

 a contact between the powdered particles of gelatin and salt as pos- 

 sible. After the salt solution has drained off, the excess of salt is 

 washed away by six perfusions with 25 cc. of H2O each. The gelatin 

 is then melted and made into a 1 per cent solution whose pH is deter- 

 mined and which is then analyzed for Br or silver, as the case may be. 

 We shall describe some of the results obtained by this method. 



1 gm. of finely powdered gelatin is put for 30 minutes at 15°C. into 

 each of a series of beakers containing 100 cc. of HNO3 varying from 

 m/8 to m/8192 ; and into one beaker containing 100 cc. of distilled water 

 serving as a control. The gelatin is then put on a filter, washed, and 

 is perfused three times with 25 cc. of m/16 AgNOa in the way de- 

 scribed, and subsequently six times with 25 cc. of H2O to wash away 

 the excess of AgNOs. The swelling is measured while the gelatin is 

 still in the cylindrical funnel, and the gelatin is melted and made into a 

 1 per cent solution. This part of the experiment is of course carried 



