THE COLLOIDAL CONDITION 215 



Surface adsorption is a common phenomenon. It was exten- 

 sively studied by the physicist, Willard Gibbs, who showed 

 that adsorption will take place whenever the surface tension 

 of the adsorbing body will be lowered by the concentration in 

 its surface layer of the material which is available in the solution 

 or other surrounding medium. 



As applied to colloidal phenomena, adsorption may be exhib- 

 ited in either one of four different ways, as follows: (1) A crystal- 

 loidal substance which is in solution may be adsorbed on the col- 

 loidal particles of a hydrosol, so that if the mixture be dialyzed, or 

 filtered through a so-called " ultrafilter " (i.e., a filter with pores so 

 small that it will retain colloidal particles) the dissolved crystalloid 

 will remain with the separated colloidal particles, or the dis- 

 solved crystalloid will not react chemically as it would in a free 

 solution. For example, if to a solution of methylene blue, which 

 dyes wool readily, there be added a small quantity of albumin (a 

 colloidal substance), the dye is adsorbed by the albumin and will 

 no longer color wool with anything like the same readiness. (2) 

 During gel-formation, electrolytes and other soluble substances 

 which may be present in solution in the liquid may be adsorbed 

 out of the solution and appear in the gel. For example, a pre- 

 cipitate of aluminium hydroxide, or of silicic acid, is nearly always 

 contaminated with the soluble salts which are present in the 

 solution, and can be prepared in pure form only by repeated filter- 

 ing, redissolving, and reprecipitating. (3) Colloidal substances 

 may be removed from sols by being adsorbed upon porous mate- 

 rials like charcoal, fuller's earth, hydrated silicates, etc. For 

 example, animal charcoal (or bone black) is used commercially 

 for the clarification of sugar solutions, because it adsorbs out of 

 these solutions the colloidal proteins, coloring matters, etc., with 

 which they are contaminated. (4) Finally, colloids mutually 

 adsorb each other, as in the case of the " protective colloids " 

 previously referred to. 



Certain characteristics of adsorption phenomena are of interest 

 and importance from both the physiological and the industrial 

 point of view. The following may be mentioned: (a) Amount of 

 adsorption. Relatively more material is adsorbed out of dilute 

 solutions than out of more concentrated ones. An increase of ten 

 times in the concentration of the dissolved material results in only 

 four times as much adsorption by the colloidal substance which 



