_Bleaching Powder._—The value of bleaching powder {206} depends upon the amount of available chlorine that it contains. This is determined by means of a standard solution of arsenious acid. The standard solution is prepared by dissolving pure arsenious oxide (As_{2}O_{3}) in sodium carbonate solution. To test the powder, a quantity, say about 5 grms., is taken, and is carefully ground up in a mortar with a small quantity of water, more water is then added, and the coarser particles allowed to settle for a few seconds. The milky liquid is then run off into a graduated flask; the residue is again ground up with water, and the operations repeated until the whole of the powder is transferred to the flask. The flask is now filled up to the proper mark, carefully shaken, and an aliquot portion withdrawn as quickly as possible. It is necessary to take both the soluble and insoluble portions in order to obtain uniform results. Standard arsenious acid solution is now run in until the solution ceases to produce a blue colour with iodide of potassium and starch papers. The method of estimating the amount of available chlorine in bleaching powder by determining the quantity of ferrous sulphate it is capable of oxidising is fallacious, as by this means calcium chlorate is included in the result. Bleaching powder should contain about 35 per cent. of available chlorine, but the percentage frequently falls below this amount, especially in warm weather. Great care should be exercised in the sampling of bleaching powder, as indeed of all chemicals, in order to insure an average result. Small portions should be taken from different parts of the bulk; the whole should then be carefully mixed, and, if necessary, reduced to powder. Portions of the mixture should be taken, pounded, and again thoroughly mixed. If this process be repeated once or twice, a perfectly uniform sample will be obtained. _Alum, Sulphate of Alumina, Alum Cake, &c._—The points to be considered in an examination of these products are (1) the percentage of alumina contained in them; (2) the {207} percentage of iron, and (3) the amount of free sulphuric acid present. (1) The alumina is estimated as such by precipitation with ammonia. Ferric oxide is also thrown down with alumina, and must therefore be deducted from the amount of the mixed oxides obtained.