CRYSTALLOID is a substance which dialyses through a parchment membrane. Dialysis is a process for separating col loidal and crystalline substances. If a salt solution 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 is replaced by one of glue, it will be found that the membrane is impermeable. The salt solution is termed a "crystalloid" and the glue a "colloid." By adding hydrochloric acid to a container holding a solution of an alkaline silicate, no precipitate will fall. and the solution will contain hydrochloric acid, an alkaline chlo ride (e.g., salt) and silicic acid. If the solution be transferred to a "dialyser," the hydrochloric acid and the alkaline chloride will pass through the parchment while the silicic acid will be retained.