Page:The American Cyclopædia (1879) Volume I.djvu/390

 366 ALUM ALUMINA or, Sulphate of potash per cent If Sulphate of alumina " 86-21 Water " 45-48 100-00 Its specific gravity is T724. It is soluble in 18 parts cold water and in equal weight of boiling water. It consequently rapidly crystal- lizes out of a hot saturated solution. Alum has a sweet astringent taste, an acid reaction, and, like sulphuric acid, dissolves many metals, for example iron and zinc, with evolution of hydrogen gas. Burnt alum, or dried alum, is made by gently heating alum till the water is driven off. Ammonia alum readily loses all its ammonia when heated, and the sulphuric acid may be driven off the remaining sulphate, so that the pure earth alumina will remain. The employment of alum in medicine and the arts is very extensive. It precipitates albuminous liquids and combines with gelatine. It causes dryness of the mouth and throat, and checks the secretions of the alimentary canal, produc- ing constipation, and in large quantities nausea, vomiting, and purging. Its principal use is in dyeing. The goods are mordanted with it and put in the dye, when the colors are precipitated and fixed in the texture of the cloth by the alumina. Alum is added to the size in the manufacture of paper to prevent decomposition, and also to bookbinders' paste for a similar pur- pose. Baths of alum are used in the tanning of leather, and it is applied in the printing baths of photographers. It has also been em- ployed in refining sugar and in the manufac- ture of pigments called lakes. The leather of Hungary is made by impregnating strong hides with alum, common salt, and suet ; and in the coloring of morocco the puce tint is communi- cated- by logwood with a little alum. When alum is added to tallow, it makes it harder. Printers' cushions and the blocks used in the calico manufactory are rubbed with burnt alum to remove any greasiness which might prevent the ink or color from adhering. Water can be purified by means of alum ; the mud that water holds in suspension collects On the addition of O'OOl part of alum (this is equal to seven grains per gallon) in long thick streaks, coagulates as it were, and is immediately precipitated. This process, the principle of which is inexplicable, was first introduced by the Chinese, and has been imitated in various parts of the world. The operation was well known from a very early period in the highlands of Scotland, ac- cording to Dr. Clark, where it is practised with peat water. The Parisian laundresses use it, but it has not been introduced into any of the establishments for the purification of drinking water, partly because alum is a substance never naturally combined in water, and may be re- ceived as a real impurity, and partly on ac- count of public prejudice. In bottling fruits for preservation, alum water is used. A novel application of alum is seen in the lining of some iron safes with a mixture of alum, and sulphate of lime; as the alum contains 24 equivalents of water, when the safe is heated it keeps the sides cool from the evaporation of the water, the contents of the safe remaining uninjured. It is used in the manufacture of bread to increas'e the whiteness of the flour. According to Liebig, this is very injurious, as he supposes the soluble phosphates to combine with the alumina, forming insoluble salts, and the beneficial action of the phosphorus is lost to the system. In the manufacture of lard alum is used as an adulterant. Dr. Hassall says that alum is generally put into the vat in breweries to give the beer a smack of age ; it also imparts a heading to porter, which land- lords are so anxious to raise to gratify their customers. Alum dissolved in water 5s used in the adulteration of gin ; and it is added to artificial port wine, to increase the brilliancy of the color. ALUMINA, the only known oxide of aluminum. It occurs colorless as corundum, and colored by traces of oxide of chromium and cobalt in the ruby and sapphire. It is found in a few places in larger quantities in the form of emery, which is nearly pure alumina. It is very widely disseminated in nature in combination with other bases in the form of double silicates, constituting feldspars, micas, and a large series of important minerals, from the decomposition and disintegration of which clays are composed. It forms the greater portion of the crust of the earth, and, in the form of clay, affects the fertility of every soil. It is not taken up by plants, except in rare cases, nor is it found in the animal kingdom. There has recently been discovered a mineral in the vicinity of Baux, France, to which is applied the name bauxite. It differs materially from clay in being simply a hydrated oxide of alumina and iron without any silica. It is entirely infusible, and crucibles and fire brick made of it remain unchanged when ordinary fire-clay material loses shape and partially fuses. It is extensively employed in the manufacture of sulphate and other salts of alumina, and of the metal aluminum. When perfectly pure, bauxite is composed of sesqui- oxide of alumina 52 '00, sesquioxide of iron 2T60, and water 20*40; but its composition varies considerably, and some varieties contain small quantities of silica and lime. It differs especially from kaolin in not being a silicate but an oxide of alumina. Well known minerals analogous to it are gibbsite and diaspore. Alu- mina may be prepared by adding ammonia to any of its soluble salts (alum for instance), when a gelatinous precipitate of the hydrate of alu- mina is thrown down, having the formula, according to the present chemical theories, of Al a HO 6 =Al 2 Os, 3H a O, after being dried in the air. To obtain it more dense and free of iron, it is now customary to pass carbonic acid gas through a dilute and cold solution of alumi- nate of soda, prepared in the United States from the mineral cryolite, which is brought from Greenland to be used in the manufacture of