[244] This process of separation of substances, which do not pass through membranes, from such as do, is called ‹dialysis›. It was first used by Graham, ‹Trans. Royal Soc.›, London, «151», 183–224 (1861) («Stud.»). [245] Graham made the first extended investigations in this field: ‹Trans. Royal Soc.›, «151», 183 (1861); ‹J. Chem. Soc.› (London), «17», 318 (1864) («Stud.»). He found that amorphous, gelatinous bodies like ferric hydroxide, aluminium hydroxide, silicic acid, gelatine, glue, dextrin, caramel, albumen and similar bodies do not pass through membranes and may be obtained by dialysis in the ‹colloidal› condition. Such substances were called "colloids" by Graham, the name referring to the Latin for gelatine. Substances which pass through membranes readily were found by Graham to resemble in behavior such bodies as are crystallizable when solid; such compounds were classified by him as "crystalloids." That liquids containing substances in the colloidal condition (‹e.g.› arsenious sulphide, gold, silver and many other substances) may be prepared by methods other than dialysis, was found later by many investigators and, in a few cases, previous to Graham, ‹e.g.› by Faraday, ‹loc. cit.› A brief history of the chemistry of colloids is found as an introduction to Wo. Ostwald's ‹Kolloidchemie›, pp. 1–63 (1909). [246] ‹Cf.› Wo. Ostwald, ‹loc. cit.›, p. 193. [247] Before Graham's time, and for the few colloidal liquids then known, this view was held by such men as J. B. Richter, Berzelius and Faraday (‹loc. cit.›) (‹cf.› Wo. Ostwald, ‹loc. cit.›, p. 19). The first extended experimental investigation in support of it was made by Barus and Schneider, ‹Z. phys. Chem.›, «8», 278 (1891). Bredig was also an early and consistent champion of this view (‹vide› his ‹Anorganische Fermente›, 1901). [248] ‹Cf.› Wo. Ostwald, ‹loc. cit.›, pp. 102–114. Graham considered "colloidal silicic acid a liquid miscible with water in all proportions." According to modern ideas, no true miscibility exists, but a suspension or emulsion is formed (see Ostwald, p. 237). [249] Siedentopf and Zsigmondy, ‹Drude's Annalen›, «10», 1 (1903). Zsigmondy, ‹Colloids and the Ultramicroscope› (1909), Chapter V. [250] Zsigmondy, ‹Z. für Elektrochem.›, «8», 684 (1902); Siedentopf and Zsigmondy, ‹loc. cit.› [251] Zsigmondy, ‹loc. cit.›, p. 161. A µµ is 1E−6 mm. The hydrogen molecule is considered to have a diameter of 0.1 µµ (O. E. Meyer), the alcohol molecule one of 0.5 µµ (Zsigmondy, ‹loc. cit.›, plate IV, p. 157). [252] Weimarn, ‹Chem. Zentralblatt›, 1907, II, p. 1293. [253] Paal, ‹Ber. d. chem. Ges.›, «39», 1436, 2859 (1906). [254] Other varieties of heterogeneous colloidal mixtures are tabulated by Wo. Ostwald, ‹loc. cit.›, p. 96.