Some of the more simple and less difficult processes of dyeing both _cotton_ and _silk_, are described in the preceding chapters; we shall now describe those, not only for black, but for some other colours, which require more care and attention. _For ungumming and boiling silk, &c. see Chap. VI._ Silk has a strong affinity for galls, and advantage is sometimes taken of this: for silk, being a valuable article, is often galled to excess, merely to increase its weight. Cotton has a strong affinity for iron, and iron has the same for _gallic acid_, wherever it may be found; therefore, in sumach, alder-bark, &c., iron unites with the acid, whenever both are connected by the medium of water. _Tannin_, doubtless, has also some share in such dyeing processes, although what does not even now appear to be well understood. Black, MACQUER observes, is rather difficult to be dyed upon silk; or, at least, there is reason to think so, from the numberless experiments which have been found necessary to the attainment of a good black, as well as from the multitude of heterogeneous ingredients which Macquer admitted into the composition of his various processes for this dye, some of which consisted of arsenic, corrosive sublimate, litharge, antimony, plumbago, and about ten other ingredients! we shall not, therefore, detail such preposterous mixtures; one, however, we may just put down by way of showing what the art was in Macquer's time. Take twenty quarts of strong vinegar, one pound of black nut galls pounded, and five pounds of iron filings; these ingredients are to be mixed in one vessel. _To dye skein silk_ BLACK _for velvets, Genoa process, (from Macquer.)_ The silk should be ungummed by boiling it four hours with a quarter of its weight of white soap, and afterwards to be well cleared from the soap. Take, for every hundred pounds of silk, twenty pounds of galls in powder, and boiled one hour; two pounds of sulphate of iron; twelve pounds of iron filings; and twenty pounds of gum arabic or senegal. This process is very simple: here are the gallic acid and tannin of the galls, and the iron of the sulphate and the filings. But we must proceed to a more modern process. _To dye silk_ BLACK, _the London process._