COLLOID CHEMISTRY had its begin ning in the discovery by Thomas Graham in 1860-61 that aqueous solutions of various sub stances did not act in the same way when sub jected to diffusion through parchment mem branes such as used in osmosis experiments. He noted that in the case of substances which formed crystals when the water was evaporated from their solutions, the aqueous solution passed through the membrane unchanged; while in the case of substances which did not so crystallize, but formed amorphous solids, the water alone passed through the membrane, leav ing the substance on the farther side, proving that these latter substances had not been in true solution, but in suspension. This discovery.led Graham to divide all soluble substances into two classes: the crystalloids, like salt, sugar, alum, etc., and the colloids, like gelatine, gum arabic, raw albumen, etc. The conclusion reached by Graham was that the colloid molecules were constituted by grouping together the smaller crystalloid molecules, and that the difference in diffusibility was explainable on the relative size of the molecules.
For some years the newly opened field of research was not cultivated, but interest in Graham's discoveries was revived in 1900 by Hardy, who attempted a classification of the colloids into reversible and irreversible groups.
Later, investigators made experimental in quiries as to the electrical constitution of the colloids, and the ultramicroscope confirmed the supposition that the colloids did not form true solutions, but mixtures. Since then researches have been many, particularly by the German scientists, and the development of the chemistry of the colloids has been rapid. The literature of this branch of chemistry now numbers hun dreds of professional papers and several sub stantial volumes.
The practical applications of colloid chemis try have been many, .particularly in the dyeing and tanning industnes, the manufacture of soaps, casein and albumen products and gelatine photographic films; the study of enzymes, fer ments and soils; the purification of drinlcing waters and of sewage; and in the investigation of many important bacterial and biological prob lems, such as the reactions of toxins and anti toxins, and the applications of immuno-chemis try. Consult Burton, E F.,