63 



general nature as acid swelling. The swelling increases with increasing 

 concentration of alkali to a maximum and then falls until such 

 concentration is reached that the gelatin softens and dissolves. 

 Swelling is repressed, here too, by addition of neutral salts. 



While neutral salts are capable of repressing the swelling of gelatin 

 in acids and alkaUs, it must not be overlooked that they are also 

 capable of producing swelling. Procter found that, with increasing 

 concentrations of sodium chloride, gelatin swells to a maximum 

 and then contracts steadily until the solution is saturated. The 

 swelling was not so marked as in the case of acids, however, the 

 gelatin taldng u]) a maximum of only about 17 times its weight of 

 water as compared to three times this amount with HCl. He also 

 noted that the gelatin removes some of the salt from solution, 

 suggesting combination, but that the addition of HCl again Uberates 

 this salt and causes the salt to become more concentrated in the 

 external solution than in the solution absorbed. 



Loeb has done some work on neutral salts that should be mentioned 

 here. In each of a series of experiments he placed two grams of finely 

 powdered gelatin into a cylindrical funnel, the powder being held 

 in the cylinder by a circular piece of filter paper. One sample was 

 perfused six times in succession with 25 c c. of distilled water and 

 the amount of swelling noted, which was taken as the height in 

 miUimetres to which the gelatin rose in the cylinder ; this sample was 

 taken as the standard. Another sample was perfused twice with 

 25 c.c. of M/8 NaCl, and then four times with 25 c.c. of distilled water ; 

 the swelling here was several hundred per cent, greater than that 

 of the standard. Still another sample was perfused six times in 

 succession with 25 c.c. of M/8 NaCl ; it did not swell to any greater 

 extent than the standard. Loeb attributes these results to a chemical 

 combination between the gelatin and salt, a highly ionizable sodium 

 gelatinate being formed. In the third experiment, much swelling 

 was prevented by the excess of salt present; when this was washed 

 away, as in the second experiment, the gelatin swelled to a much 

 greater extent than in pure water. He confirmed this view by showing 

 that, when placed in an electric field, gelatin which has been treated 

 with NaCl migrates to the anode. A sample first perfused with 

 calcium chloride solution and then with distilled water showed very 

 httle more swelling than the standard. He accounts for this by 

 sa3Tng that the calcium gelatinate formed is only very shghtly 

 ionizable. 



CoUagen, fibrin, and other proteins behave much hke gelatin when 

 immersed in solutions of acids, alkafis, or salts, and are probably 

 subject to the same general laws. Other examples of imbibition are 

 the swelling of agar-agar, gums, and cellulose in water and the sweUing 

 of rubber in organic solvents. It would take volumes to mention 

 all of the work done on this subject. 



Numerous attempts have been made to explain the molecular 

 mechanism of imbibition, particular attention bemg paid to the 

 explanation of the pecuUar sweUing curve. Most of these, however, 

 have been guilty of drawing largely upon the imagination for some- 

 thing that would agree qualitatively with the experimental data 



