76 SOAPS AND PROTEINS 



of acid makes the gelatin " liquefy" or " go into solution." Using 

 Figs. 48 and 49 to illustrate what has been said, the addition of 

 acid to a previously solid gelatin moves the whole system from 

 some such lower region as Z, 7, X, or W into one of the upper 

 zones like V or U. 



3. Throughout the experiments described in the previous 

 sections we have used the formation of a dry gel as the measure 

 of the " gelation capacity " of a colloid. We may now attempt 

 to say just what this means. It obviously includes more than 

 the term salvation capacity. The latter measures the solubility 

 of the solvent in the colloid material and is synonymous with the 

 swelling capacity. Gelation, however, includes not only this 

 value but more namely, everything embraced within the region 

 of the emulsification or enmeshing of a " solution " of the colloid 

 material in the solvent, within the solvated colloid as an external 

 " dry " phase. It embraces everything in Figs. 48 and 49 up 

 to and including the zone V. 



4. Just above this region it is apparent that the more solid 

 phase may no longer be adequate to enclose all the " solution " 

 of colloid in solvent. When this upper region is reached the 

 colloid system tends to " sweat " or to use the term of THOMAS 

 GRAHAM the gel shows " syneresis." We may still have before 

 us a gel, but it is now no longer " dry." 



When the dispersion of a liquid in an enveloping phase which 

 is also a liquid is compared with the dispersion of a liquid in a 

 more solid (crystalline) phase, (as indicated in the zones V of 

 Figs. 48 and 49) it is clear that the tendency to " leak " the 

 liquid phase is greater and is more likely to occur early in the 

 case of the latter system than in the former. It is for this 

 reason that the more " solid " gels regularly show earlier and 

 greater syneresis than the more " elastic " or " liquid " gels. 



To go sufficiently above the region U is to be in the regions 

 E and D. We now no longer say that there is syneresis or that 

 this has become excessive but we say that the gel has gone into 

 or persists in the " sol " state. 



5. As a final word we should like to emphasize the fact that the 

 concept of the lyophilic colloid as outlined here sets no limitations upon 

 the nature of the materials that may make up such a system and 

 makes no specifications as to the nature of the forces which guarantee 

 the stability of the colloid system. They are in general any or all 



