THE COLLOID-CHEMISTRY OF SOAP MANUFACTURE 185 



hydrated, whence the late jellying of the lower of the two phases 

 in the vat. After such jellying has occurred, the lower phase is 

 qualitatively similar to the upper phase but different in quan- 

 titative composition. The lower phase, too, is an emulsion of 

 salt water in hydrated soap, but viewed as a whole this system 

 is poorer in soap, and richer in water and sodium chlorid. 



To produce half-settled soap, still less sodium chlorid is used in 

 the finishing of an originally curded soap or, as commonly prac- 

 ticed, not enough salt is added to the hot contents of the soap 

 kettle to get any salting-out. On cooling, the kettle contents sim- 

 ply become fairly solid. Separation into two distinct phases 

 does not occur, the contents of the kettle or soap vat simply 

 changing to a soap which in essence is an emulsion of salt water in 

 a relatively highly hydrated soap. 



If the attempt is made to increase still further the water- 

 holding capacity of a soap, a mixture is finally obtained which is 

 no longer definitely solid. We then have a so-called soft soap. 



The above paragraphs have described the finishing process in 

 soap manufacture as carried out with sodium chlorid. In prac- 

 tice, however, other materials having an action similar to that of 

 sodium chlorid may be, and are, used to accomplish similar 

 ends. Instead of being " filled " with sodium chlorid solution, 

 soaps may be filled with ingredients more useful in washing, like 

 sodium carbonate, borax or water-glass. 1 Under such circum- 

 stances there are also obtained as final products, emulsions or 

 " solutions " of these various materials in hydrated soap. 



2 



The inequality in distribution of such a dissolved substance 

 as sodium chlorid. alkali or sodium carbonate between I he 

 two phases commonly produced in a soap vat (the upper soap- 

 rich phase and the lower soap-poor phase) has often been inter- 

 preted in the terms of " adsorption." There is still much debate 

 about the nature of adsorption, though it is generally assumed 

 that the union between adsorbent and adsorlxnl material is of a 

 physical character and that it follows the adsorption law. which 

 states, in lu-ief. lhat an adsorbent will take up ivlativrly nmre 

 of a dissolved substance from a dilute solution than from a 

 1 See the succeeding section, page 192. 



