The colouring principle of logwood exists iu the timber in the form of a glucoside, from which it is liberated as haematoxylin or haematin by fermentation. Haematoxylin was first isolated by Chevreul in 1810, and its constitution was subsequently investigated by Erdmann, who found it to have the formula C 16 H 14 O 6 . It forms two crystalline hydrates, the principal combination being with three molecules of water. This hydrate, C 16 H u Oθ +3H 2 O, is a colourless body very sparingly soluble in cold water, but dissolving freely in hot water and in alcohol. By exposure to the air, especially in alkaline solutions, haematoxylin is rapidly oxidized into haematein, with the development of a fine purple colour. The reaction consists in the elimination of two atoms of hydrogen, thus haematoxylin hydrate (C 16 H u O β + 3H 2 O) + O = haematein (C 15 H 12 0ς + 3H 2 O) + H s O. This reaction of haematoxylin is exceedingly rapid and delicate, rendering that body a valuable laboratory test for alkalies. It is remarkable that the difference of two atoms of hydrogen between colourless haematoxylin and coloured haematein is precisely that which exists between the white and the blue forms of indigo. By the action of hydrogen and sulphurous acid, haematein is easily reduced to haematoxylin.

Logwood is prepared for use by dyers, &c., in the form of chips and raspings, and as a solid brittle black extract. Chipped logwood is moistened with water and spread in thin layers till a gentle fermentation sets up, whereby, under the influence of liberated ammonia, haematoxylin is formed from the glucoside. By exposure to the air, through repeated turnings of the mass, haematein is developed from the haematoxylin, and the chips gradually become coated with the brilliant metallic green crystals of haematein. Logwood extract, largely used in calico printing, is obtained from oxidized chips and raspings by lixiviation, the solution being concentrated at as low a temperature as possible.

The principal use of logwood is for dyeing wool and woollen goods, on which it produces, with various mordants, shades of blue from a light lavender to a dense blue-black, according to the amount of logwood used. It is more employed in combination with other dye-stuff's than as the sole tinctorial agent, the best and most permanent blacks on wool, known as woaded blacks, being first dyed blue in the indigo vat, and finished black with logwood and bichromate of potash. In calico-printing logwood is used to produce steam purples, for the production of which the calico is mordanted with stannate of soda, and printed with a strong solution of logwood extract thickened with starch. By steaming, the haematein of the logwood combines with binoxide of tin precipitated in the fibre, and thus develops a bright purple colour. Logwood blacks, which are a standard product of print works, are produced by mordanting with iron liquor, passing the calico through a logwood solution, and developing and fixing the colour by treatment with a weak solution of bichromate of potash.* Logwood blacks assume a bright red tint by the action of dilute acids, a test by which they can readily be distinguished from aniline and other fast blacks. Logwood is also largely used in the preparation of Ink (⅛.v.), and to a small extent in medicine. The imports of logwood into the United Kingdom during the year 1880 amounted to 69,280 tons, the estimated value of which was <£192,392.