a common phenomenon. It was extensively 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 exhibited in either
one of four different ways, as follows: (1) A crystalloidal substance which
is in solution may be adsorbed on the colloidal 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 dissolved 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 adsorbed out of the solution and
appear in the gel. For example, a precipitate 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 filtering, redissolving, and reprecipitating. (3) Colloidal
substances may be removed from sols by being adsorbed upon porous materials
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
colloida