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.