It is remarkable, that although an indigo ground for wool enriches the black, yet for silk and cotton it is not generally considered necessary. Latterly, however, we believe the dyers of black on cotton do first give it an indigo ground before the black is given. This is, nevertheless, not a new method, for D'Apligny describes the process in his Art of Dyeing, _for linen and cotton yarns_; these are first dyed sky-blue in the vat, then wrung out and set to dry. They are galled in the proportion of one part galls to four of yarn, being left twenty-four hours in the gall liquor, wrung out anew, and set to dry: about ten pints of iron liquor to every pound of yarn are then poured into a tub, in this the yarn is turned on sticks, and worked with the hand for a quarter of an hour, it is then wrung out and aired. This operation is twice repeated, adding each time a new dose of the iron liquor; the yarn is aired once more, then wrung out, well washed, and dried. To complete the dyeing of the yarn a weight of alder-bark equal to that of the yarn is boiled with a sufficient quantity of water for an hour; to this is added one half of the bath which has served for the galling and sumach. The whole is boiled for two hours. When cold the yarn is put in and worked, aired occasionally, and then left in the bath for twenty-four hours, when it is wrung out and dried. To complete it, it is steeped and worked in the residue of a bath of weld, to which a little logwood is added; it is then taken out, wrung, and immediately passed through a tub of warm water, into which one part of olive oil to sixteen of yarn has been poured. It is finally wrung out and dried. See URE'S _Berthollet_, vol. ii. page 18. _Another iron liquor; pyrolignite or acetate of iron._ Although we have described an _iron liquor_ in a preceding section, it may be useful to give the following process for another here. Fill a cast-iron boiler with pyrolignous acid, add to it old iron well oxidized, and boil. The solution of the oxide will take place rapidly. When the iron grows clean, and the solution black as ink, throw the whole into a cask, to be employed as occasion shall require. _To dye cotton_ BLACK, _by using the preceding solution._ Prepare the cotton as usual, by giving it a blue ground; gall it, and pass it through a bath of the solution of pyrolignite of iron diluted with lukewarm water. Renew the gallings and the passings through the bath of pyrolignite of iron till a deep and brilliant black is obtained. Finish by passing the cotton through olive oil thus: throw on some lukewarm water a little olive oil, pass the cotton through it; the cotton absorbs the oil, but it must be worked a long time in the bath to diffuse the oil equally. Dry in the shade. The cotton is now a perfect and very durable black. Every time the bath of pyrolignite is used, what remains must be thrown away; the old baths are never added to the cask.