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Aluminium                                          Aluminium Chloride
and have therefore the common formula of M2'S04, M2'"(S04)31 24H20, but occasionally written as half, namely, M' M'"(SO,)2, (H20)12. The principal alums are ammonia alum, chrome alum, and potash alum.
Ammonia Alum (Ger., Afnmoniakalaun; Fr., Alun Ammoni-acal; Ital., Allume di A?nmoniaca). (NH4)2S04A12(S04), 24H20 = 906. Solubility: 1 in 7 of cold water, 1 ino'2 of boiling water, 1 in 2 of alcohol.
Chrome Alum (Ger., Chro?nalatm ; Fr. Alun de Chrome; Ital., Allume di Cromo). K2S04, Cr2(S04)3, 24H20 = 998. A deep purple crystalline salt, soluble 1 part in 10 of cold water, in-soluble in alcohol. Its solution is purple by reflected, and reddish by transmitted, light. It is used for hardening gelatine as, for example, in the preparation of emulsions for dry plates to prevent frilling.
Potash Alum (Ger., Alaun; Fr., Alun; Ital., Allume comune). K2S04A12(S04)3, 24H20 = 948. Is found native in some places, but is usually made from aluminous clay. It is used for render-ing the films of gelatine less liable to mechanical injury, by hardening them, and also clears them from stains. (See Clearing Bath.) It has also been recommended as a Hypo-eliminator (q.v.), but its action and benefit are doubtful. Solubility : 95 in 100 of cold water, 10 in 8 of boiling water ; insoluble in alcohol and ether.
Aluminium (Ger. and Fr., Aluminium; Ital. Alluminio). Al = 27. A silvery white metal, formerly obtained principally by reducing the chloride with sodium, but now by the reduction of the oxide, alumina, in an electric furnace. It has been used to replace brass for lens mounts and camera fittings on account of its lightness ; its weight, bulk for bulk, being less than half that of brass. It has also been used as a substitute for magnesium, for flash light, and has been suggested by Lainer as a means of precipitating silver from residues. (For the use of aluminium in connection with the Flash Light for instantaneous work in the evening, see Flash Light.)
Aluminium Chloride (Ger., Almniniumchlorid; Fr., Chlorure d'aluminium ; Ital., Clorure di aluminid). A12C16 is formed by heating alumina and charcoal and passing over it a stream of chlorine gas. It occurs in white tabular crystals which are