410 SCIENCE PROGRESS 



brought order out of chaos in recent years. For progress in 

 this direction we are chiefly indebted to the work of Procter 

 in England, Pauli in Austria, and Loeb in America. 



Gelatin is a stronger acid than base, so that acid has to be 

 added to its solution in water to bring the gelatin to the 

 isoelectric condition. At the isoelectric point the hydrion 

 concentration (Cjj-) of the solution is approximately 2-5 x io~^ 

 that is, the pH (= - log Ch) ^ is about 47, which is on the 

 acid side of the neutral point of water (pH = 7-0). The 

 theory of amphoteric electrolytes shows that at the isoelectric 

 point their solutions should contain a maximum number of 

 neutral particles and should therefore possess peculiar pro- 

 perties ; in accordance with this it has been found that the 

 properties of swelling, viscosity, osmotic pressure, etc., show 

 a minimum at that point, whilst the precipitation by alcohol 

 is most pronounced. A i per cent, solution of isoelectric 

 gelatin is at first transparent. After some time, which is 

 the shorter the lower the temperature, the solution becomes 

 opaque and in the course of time a precipitate may deposit ; 

 raising the temperature again gives a clear solution. The 

 setting of the solution to a gel is a different process from this 

 precipitation, since cloudiness or opacity is not necessarily 

 connected therewith. 



On the acid side of the isoelectric point (pH < 4*7) gelatin 

 should behave as a base and form gelatin acid salts, whilst 

 on the alkaline side (pH > 4-7) it should act as an acid and 

 form metal gelatinates. Loeb has endeavoured to show that 

 this is true in several ways, of which the following may be 

 quoted. Separate quantities of i gram each of powdered 

 gelatin (going through sieve 60, and not through sieve 80) 

 are brought to different hydrion concentrations by putting 

 them for i hour at about 15° C. into 100 c.c. of nitric acid 

 solutions varying in concentration from M/8, M/16 . . . down to 

 M/8192. After filtering and allowing the acid to drain off, 

 they are washed once or twice with 25 c.c. of water at 5°C. 

 to remove acid remaining between the granules. The different 

 portions, which now possess different pH's, are then put for 

 an hour into beakers containing M/64 silver nitrate at a tem- 

 perature of 15° C, after which they are separately collected 

 on a filter and washed 6 to 8 times, each with 25 c.c. of ice- 

 cold water. This washing removes the silver nitrate held in 

 solution between the granules, since the silver in combination 

 with the gelatin is not thus removed (or at least only very 

 slowly by altering the pH). The separate quantities are then 



^ For the meaning of pH see the article on " Soil Reaction," by Fisher, 

 Science Progress, 1922, No. 63, pp. 408-25. 



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