Dialysis
a phenomenon observed by Thomas Graham in the middle of last century, consisting in the passage through an animal membrane or parchment of solutions of crystalloids, such as salt or sugar. He carried out his experiments by placing the dialyzer (i. e., a vessel having a bottom made of parchment) in a larger vessel filled with water. He then found that when a crystalloidal solution was poured into the dialyzer, the substance in solution would pass through the diaphragm into the water in the outer vessel. Colloidal solutions, however, would not pass through such diaphragms and so Graham was able to separate colloidal from crystalloidal substances by this means.