[235] Stannous chloride is usually sufficiently contaminated with stannic salt. Add a few drops of bromine- or chlorine-water to 100 c.c. of a ‹pure› stannous chloride solution.
[236] The action is an exceedingly sensitive qualitative ‹test for gold›. By a modification of the test Donau was able to detect as little as 2E−9 gram of gold (‹Monatshefte f. Chem.›, «25», 545 (1904)).
[237] The nature of the reduction reaction is discussed in Chapters XIV and XV.
[238] Faraday, ‹Proc. Royal Soc.›, «8», 356 (1857), ‹Phil. Mag.› (4), «13», 401 (1857) (Stud.).
[239] Zsigmondy, ‹Liebig's Annalen›, «301», 30 (1898).
[240] Bredig, ‹Z. f. Elektrochem.›, «4», 514, 547 (1898), and ‹Anorganische Fermente›, 1901. Colloidal preparations of platinum, silver and many other metals may be prepared in the same way. In ether, colloidal preparations of the alkali metals may be made (Svedberg, ‹Ber. d. chem. Ges.›, «38», 3616 (1905), «39», 1708 (1906)).
[241] ‹Argentum Credé› may be conveniently used. It contains, with the metallic silver, a small percentage of albumen, which is added for reasons discussed below. Brown solutions are formed at once.
[242] First a thin ‹film of› black ‹carbon› is produced ‹round the metal›, then the latter appears in the form of a filigree of silver.
[243] For details of the preparation, see A. A. Noyes, ‹J. Amer. Chem. Soc.›, «27», 94 (1905).
[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).