67. In addition to these, we may mention that Mr Martin Lister (Plalosoph. Trans. vol. vi. remarks, that .• the common hawthorn caterpillar will strike a purple or carnation, with lye, and stand; the heads of beetles and pismires will, with lye, strike the same carnation co lour, and stand ; and die amher-coloured scolopendra will give, with 1)e. a most beautiful and pleasant amethys tine, and stand." In another part of the same volume, Mr Lister mentions an insect cimes, whose eggs, bruised upon white paper, " stain of themselves without any addition of salt, of a lively vermilion colour." 68. Dr Bancroft is of opinion, that the colouring juices of some of these animals, particularly that of the bucci num, might still be rendered beneficial in staining or printing fine muslins, for which but little colouring mat ter is required. No substance, he remarks, will afford a substantive purple of equal beauty and durability, and capable of being topically applied to linen and cotton, with so much simplicity and expedition. But the dif ficulty of procuring and preserving the shellfish, pre sent great obstacles to their ever being extensively em ployed for the purposes of dyeing; particularly, as we can obtain by other means, colours more beautiful, and equally lasting.
II. Of Vegetable Substantive Colours.
69. The substantive colours obtained from vegetables are more numerous than those which are of animal ori gin, and far more interesting to the dyer. Of these, the most singular, as well as the most important, is in digo, a blue colouring substance extracted from a genus 01 plants known by the name of indigoferr and indigo, t•l ich arc cultivated fur its production in America and the West Indies. Indigo consists of a peculiar colour i..g matter, which may he denominated its basis, and s. rich being combined with a certain portion of oxygen, is, during its union, insoluble by any means unless ueb as exert an agency more or less destructive on the basis itself. This basis is, in its uncombined state, en tirely destitute of colour ; and seems to be formed by et lain peculiar secretoty organs, possessed by a few pal:len:as plants. Di' Ilsncroft states, that the expressed jai •e of the leaves of the indigo plant communicate to calico a greenish tinge, which in dyeing approaches to blue, and ultimately assumes that colour ; and that re peated applications of it to the same tput, increase the deepness of the shade, and at last produce a full blue. Ile suspects, however, that the colouring matter ut the indigo, when thus applied in its native state, does not absorb so much oxygen, as when it undergoes the fey mentive process by which it is usually prepared for the purposes of dyeing.
70. In another part of this work, (Sec INDico,) we shall describe the various processes which are em ployed for preparing indigo, and confine ourselves at present to a description of its nature and propel ties as a dye. In its prepared state, indigo is of a very rich blue
colour, which varies, however, in its shade, in different specimens. This difference seems to be owing not so much to any real difference of quality in the colouring matter, as to the foreign substances with which it is oc casionally united. When indigo is pure, it is light and friable ; tasteless, and almost destitute of smell ; and having a smooth fracture. Some varieties are lighter than water ; and the lightest is generally the purest. Indigo is frequently adulterated by adding to it gummy, resinous, and earthy substances, particularly an extract from the fruit of the embryopteris glutinifera, denomina ted gaub in the East Indies. Muth its weight and its purity are affected by the presence of lime, which hav ing been used in excess, as a precipitate, had subsided with the colouring matter, and carried down other im purities along with it.
71. The finest and most valuable indigo was formerly brought from Guatimala ; but since the manufacture of this commodity engaged the attention of the British inhabitants in the East Indies, indigo superior even to that of Guatimala has been imported, in considerable quantity, from that quarter. Dr Bancroft enumerates three varieties of American indigo: of which the first, called by the Spaniards flora, has a very fine blue colour; the second, which bears the name of sabre salicntc, is vio• let ; and the third, named corti-colur, is copper-colour cd. When the first of these, he adds, sells at 9.s. a pound, the second is commonly thought to be worth 7s. and the third 58. 6c1. The finest blue indigo from the East Indies commonly sells 20 per cent. higher even than the finest glowing, (though the last probably con tains nearly as much colouring matter as the first); and 70 or 80 per cent. higher than the best copper-co loured.
72. Bertlyullet has proposed a method of determining the relative values of dilkrent specimens of indigo, by dissolving equal portions of each in sulphuric acid, and afterwards destroying their colour by means of oxymu riatic acid; that specimen being considered as the most valuable which required the greatest quantity of oxymu riatic acid to destroy its colour. Dr Bancroft has sug gested another method of ascertaining the same thing, which is more simple, and perhaps not less accurate. lie proposes to mix equal portions of different solutions in sulphuric acid, and, after diluting these with a cer tain quantity of water, to compare the shades of colour possessed by the several mixtures.