78 PHYSICAL CHEMISTRY 



less concentration of the precipitant is necessary if it be added 

 suddenly than if added gradually. A less amount of antitoxin 

 is necessary to render a toxin solution inert when added at once, 

 than when added gradually. On the other hand, time is favor- 

 able to the increase of viscosity. Gels become firmer with age 

 and haptogen membranes likewise, even though perfectly pro- 

 tected from evaporation. This phenomenon is known as hys- 

 teresis or secondary change. 



The gels may be considered very viscous sols but the cause 

 of the great viscosity probably lies in the supposed structure. 

 The swelling of gels or gelatine plates may be made to produce 

 a pressure which is thought, by some authors, to be analogous 

 to the osmotic pressure of sols. The pressure is increased by 

 acids and alkalis. The emulsoid gel may be converted into a sus- 

 pensoid gel in the same way that an emulsoid sol is converted 

 into a suspensoid sol by coagulative agents. Some sulphates and 

 tartrates cause gelatine gels to shrink, bromides and nitrates 

 causing them to swell (Hofmeister, 1888). According to Lenke 

 (1916) these two groups differ in that with the first there is an 

 optimum concentration for swelling and with the second the 

 greater the concentration the greater the swelling. Their effect 

 on gelation is the inverse of their effect on swelling, the series 

 being: SCN <Br, NO a <C1 <CH 3 COO <Cit.<Tart. <S0 4 . 

 Glycerine, dextrose and saccharose increase the viscosity of gela- 

 tine and starch gels, and urea decreases it. The reverse is true 

 of their effect on swelling. 



The ionic series which holds for negative emulsoids and is 

 reversed for positive, becomes irregular as we near the isoelec- 

 tric point. This explains the variations in the ionic series found 

 in various physiological experiments since the emulsoids in living 

 cells are near the isoelectric point. 



According to Pauli, anions increase swelling of gelatine gels 

 and cations decrease it, the greater the atomic weight of the 

 cation, the greater its effect. 



Traube (1915) claims that the more a substance hastens sola- 

 tion (solution of a gel), the more it favors the rate and amount 

 of swelling, the more it hastens gelation, the more it disfavors 

 the rate and amount of swelling. He finds that the stronger 

 acids hasten gelation when very dilute (PH = isoelectric point) 



