We ought also to state here (from _Berthollet_, vol. ii. p. 4.) that commonly more simple processes than any of those above described are employed for black. Thus the blue cloth is simply turned through a bath of gallnuts, when it is boiled for two hours. It is next passed through a bath of logwood and sulphate of iron for two hours without boiling, after which it is washed and fulled. A black may also be dyed _without a blue ground_ with walnut rinds or the roots of the walnut tree; in this case the cloth receives a dun ground from the walnut husks or roots, and is afterwards made black in the manner above described, with logwood and sulphate of iron. The blacks, however, _without_ the blue ground are only given in general to inferior cloths. The _colouring principle of logwood_ is called _hematin_; it is crystalline, of a rosy-white, and, viewed through a lens, very brilliant; its taste is slightly astringent, bitter and acrid; exposed to the action of fire in a retort it affords all the products of animal substances, and also a small quantity of ammonia, which proves that it contains nitrogen. It dissolves easily in boiling water; on adding some acid very gradually, it changes to yellow and then red. Potash and ammonia give the solution of hematin a purple red; if a great excess of these alkalies be added, the colour becomes violet-blue, then brown-red, and finally yellow-brown. In this state it is decomposed and cannot be recovered by any acids. Protoxide of lead, protoxide of tin, hydrate of tritoxide of iron, hydrate of copper, oxide of zinc and its hydrate, flowers of antimony and oxide of bismuth combine with hematin and give it a blue colour, with the loss of the violet shade. See _notes_ to _Ure's Berthollet_, vol. ii. p. 420. See the explanation of _protoxide_, &c. under OXIDE in Chapter I. The above facts concerning logwood may, by the ingenious dyer, be applied on many occasions with great success. _To dye wool_ GREY. All greys, from the darkest to the lightest, are composed of black in varying proportions. They are of great use in dyeing, not only for their own colours, but also when applied to other colours, which operation is called _saddening_ or _darkening_. Some greys have a woad ground of blue, then of logwood, sumach or sulphate of iron, of which decoctions of the three last, for expedition, should be in readiness when wanted. When a succession of light shades, in particular, is required, in some instances the chemic blue is used: when we treat of the mixture of black, or rather grey with red and blue, the utility of grey will be seen. _Mixture of_ BLACK _or_ GREY _with_ RED _and_ BLUE. These produce an infinite number of all shades of grey as _sage_ grey, _slate_ and _lead colour_, and others still darker. _On_ BROWNS, FAWNS, OLIVES, _&c._