THE COLLOID-CHEMISTRY OF SOAPS 75 



a lowering of temperature from a higher level. The gel first 

 shows signs of liquefaction where the soaped-solvent particles 

 begin to touch and thus to form the external phase, as in the 

 regions V or U. It is for these reasons that the region of greatest 

 ambiguity and of greatest hysteresis is found in the broken middle 

 portions of the diagram (D, E, F and W, V, U). Just as long 

 periods of time are required to make solution phenomena attain 

 thru final values, just so must mutually soluble systems subjected 

 to changes in their environment be expected to come only slowly 

 into a state of their final equilibrium. 



2. Some years ago we showed, in the case of gelatin, 1 that 

 the " swelling " of this substance and its liquefaction are not 

 identical processes and that the latter is not a mere continuation 

 of the former. When ordinary gelatin is thrown into water, 

 it swells up somewhat, but the amount of this swelling is enor- 

 mously increased if a little acid is added to the water. If lique- 

 faction were a mere continuation of this swelling, then the addition 

 of a little acid to a gelatin near its gelation point ought to make 

 it set. As a matter of fact, not only does this not happen, but 

 the addition of such acid to a previously solid gelatin makes it 

 liquefy. As maintained at that time, an increased " swelling " 

 was declared to be an increased capacity for taking up the solvent; 

 an increased tendency to liquefy, the expression of an increase 

 in the degree of dispersion of the colloid material. 



The concept of the lyophilic colloid as here developed now 

 permits us to state more clearly just what each of these views 

 embraces. Increased swelling due to increase in hydration or 

 sol vat ion capacity means increased solubility of the solvent < 

 the dispersed substance. When acid is added to gelatin (thus 

 forming an acid gelatinate) water is more soluble in the newly 

 formed material than in the neutral gelatin. Acid is therefore 

 said to increase the swelling of protein. But an acid proteinate is 

 also more soluble in water than is the neutral protein. If the 

 cMiM-rnt ration of the system is properly chosen, the addition of 

 M ' i'l will therefore make a gelatin/water system, solid by itself, 

 tend to remain " in solution " or, as more generally stated, the 

 gelatin fails to "set." Expressed the other way about, the presence 



* MARTIN H PUCHBB: 8cienc, 42, 223 (1015); MARTIN H. FUCBBR 

 and M HOOKKR: ibid , 46, 189 1 -1 ' \ Chem. Soc , 40, 



272 (1918); ibid , 40, 202 (1018); ibid., 40, 308 (1018). 



