Now, the indigo of commerce does not contain more than say from forty or fifty per cent of pure indigotine, and of course will require a smaller quantity of bleaching powder to decolor it; or the quantity of bleaching powder to decolor a given weight of pure indigo may be weighed out, and the sample to be compared having been dissolved in strong sulphuric acid, and diluted with water, is to be poured in and stirred or shaken well until the point of decoloration is ascertained. In this case it is best to weigh out at least twice as much of the sample to be tested as was used of pure indigo, and to measure the solution in a graduated glass vessel,—an alkalimeter, for example,—so that by measure we may know exactly how much of the sample we add to the solution of bleaching powder. Thus the relative coloring values of the samples may be readily ascertained. If you have no purified indigo on hand, you can make a good comparative trial of your samples against a perfectly good sample of Bengal indigo, which may be kept for a standard of comparison. Very useful practical results may thus be obtained. It is well, however, to keep on hand a standard sample of pure indigo, prepared from reduced or white indigo, as directed by Berzelius (vol. vi. page 3, French ed., 1832), and in Muspratt’s Chemistry applied to the Arts (Dyeing, Indigo). In the analysis by reduction of indigo, the process is simply as follows: Reduce the indigo to fine powder, and weigh it; weigh out an equal quantity of pure quicklime (made from pure white marble). Measure in a graduated vessel a certain volume of water. Slack the lime with a portion of this water. The rest of this water is to be used in rubbing up the indigo in a mortar. Then the slacked lime is to be mixed with the indigo, rubbing the substances well together. Introduce the whole into a large flask; 1½ to 2 litres (about 3 to 4½ pints) of water is required for 1 gramme (or about 15 grains of indigo). The flask and contents are then to be exposed to a heat of from 176° to 190° F. for some hours. This is best effected in a water bath. By this digestion the lime is made to combine with the indigo brown, and the coloring matter is set at liberty. Dissolve in the liquor a little protosulphate of iron, exempt from copper, and reduced to a fine powder. The flask is to be corked and well shaken, and allowed to cool. When the sediment is settled, decant the clear solution by means of a syphon into a graduated glass. The coloring matter oxidizes by exposure to the air; and to favor this oxidation and to keep the lime in solution, add muriatic acid to the liquor. When the liquor has become clear, filter and collect the precipitate on a weighed filter, which wash with hot water, and dry at a temperature of 212° F. Thus we can learn, by weighing the filter again, how much indigotine is contained in the sample.