DIALYSIS (from the Gr. &0., through, Anew, to loosen)  , in chemistry, a process invented by Thomas Graham for separating colloidal and crystalline substances . He found that solutions could be divided into two classes according to their action upon a porous diaphragm such as parchment . If a solution, say of salt, be placed in a drum provided with a parchment bottom, termed a " dialyser," and the drum and its contents placed in a larger vessel of water, the salt will pass through the membrane . 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." This method is particularly effective in the preparation of silicic acid . 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 parchment, while the silicic acid will be retained .