Indigo, a drug for dyeing blue, prepared from the indigo plant, or Indigofera tinctoria, L a native of the warmer parts of Asia, Africa, and America.

Instead of describing the particular culture of this exotic, which cannot be raised with success, even in the hot-houses of our climate, we shall briefly point out the characters of the best indigo that is imported, and which has frequently undergone the various processes of adulteration with earth, ashes, and pounded slate.—The genuine drug ought to be of a rich dark blue colour, approaching to black, and, when broken, to display the lustre of copper: nor should it sink in water, or leave behind any sediment, after being dissolved. The purest indigo is brought from Guatimala, in South America.

Although large quantities of this dyeing material are annually produced in the British colonies, and thence imported, yet we might easily dispense with this costly drug, which, to the detriment of native productions, and especially that of woad, was first brought to Europe by the trafficking Dutch, about the middle of the 16th century. Indeed,there is every reason to believe, that many plants of English growth would yield excellent substitutes for indigo.—We have, in the progress of this work, already hinted at several vegetables of this description; and, as a repetition of their names and properties would be here superfluous (though incomplete, at the present letter of the alphabet) we shall purposely delay that useful task till the conclusion of our labours, when it will appear in the General Index of Reference.

Indigo is much used in washing, to impart a blueish cast to linen; painters also employ it as a water-colour; and dyers consume large quantities in the various tints of blue cloths.

In March 1797, a patent was granted to Mr. Joseph Barton, chemist, for an improved method of preparing indigo for dyeing wool, etc. in a more perfect manner than has hitherto been discovered. As, however, this patent is not expired, and the process is too expensive to be attempted for the gratification of the experimenter, we refer the inquisitive reader to the 9th volume of the Repertory of Arts and Manufactures, where it is minutely described.

By the 36th Geo. III. c. 40, indigo and cochineal may be imported from any place, in British ships, or such as belong to friendly nations, free from duty, except that of convoy, until the 29th of September 1802, and to the end of the then next session of parliament.