DIALYSIS, in chemistry, a process invented by Thomas Graham for separating colloidal and crystalline substances. If a salt solution be placed in a drum provided with a parchment bot tom, termed a "dialyser," and the drum and its contents placed in a larger vessel of water, the salt will pass through the mem brane; hence the derivation (from the Gr. 8i2, through, X betv, to loosen) . If the salt solution be replaced by one of glue, gelatin or gum, it will be found that the membrane is impermeable to these solutes. To the first class Graham gave the name "crystalloids," and to the second "colloids." By adding hydrochloric acid to a dilute solution of an alkaline silicate, no precipitate will fall, and the solution will contain hydrochloric acid, an alkaline chloride, and silicic acid. If the solution be transferred to a dialyser, the hydrochloric acid and alkaline chloride will pass through the parch ment, while the silicic acid will be retained.