Still, in making large purchases, as a measure of wise precaution the chemical test should be added. This is used to ascertain the proportion per cent of indigotine which a given indigo has. The determination of the quality of indigotine contained is not alone sufficient to fix the value of an indigo. With an equal yield of indigotine, the indigoes are always to be preferred which have a light and soft paste; and for the preparation of the indigo vat the preference should be always given to the violet red rather than to the clear blue indigoes. The chemical works which treat of this subject give elaborate details of a great number of processes for determining by chemical tests the amount of indigotine, or the coloring material in indigoes. To give these numerous processes would only confuse the reader. In our own confusion upon this subject we submitted the descriptions of these various processes to one of the most eminent and practical of American chemists, Dr. Charles T. Jackson, an official State Assayer for the State of Massachusetts, who has had much experience in testing indigo, with a request that he would describe the process which he approves and practises. He has obliged us by the following communication:— Boston, Nov. 21, 1872. No. 47 Court Street, Room 4. John L. Hayes, Esq. Dear Sir,—In reply to your inquiry as to the simplest method of analyzing indigo, I would say that I first ascertain the amount per cent of earthy matters and metallic oxides, in the samples brought to me, by burning a weighed quantity in a counterpoised platinum crucible, until all organic matters are removed or consumed, and then weighing the ashes obtained. The ash is then subjected to analysis in the usual way, and lime, alumina, peroxide of iron, and some other earthy impurities are separated. Then, to determine the amount of coloring matter, or indigotine, I make use of a standard sample of pure reduced indigo, which is dissolved in the most concentrated sulphuric acid, and diluted with water after solution. Then I ascertain how much bleaching powder (chloride of lime) is required to dissolve the solution. This is the quantity required for absolutely pure indigo.