When the indigo is thus prepared, boil it in the copper with the grounds of the madder and the potash, which fell to the bottom; it is all, then, to be put into the vat at the same time with the indigo; the whole is to be stirred, the vat covered, and heat applied to make it more than blood warm, and to keep it so. The vat should be stirred twice, slightly, both the second and third day, the heat remaining the same; when a brassy scum, divided and interrupted in many places, begins to appear on the surface. On the fourth day, the heat being continued, the scum becomes more perfect and less broken, the froth which rises, upon stirring, is more blue, and the vat a deep green. When it becomes green in this manner, it is an indication that it must be filled; to do which, boil half an ounce of madder, and one pound of potash, in five gallons of water; put in this liquor, and stir it; if it produce much froth, stir it again, and the next day it will be fit for working; which, however, will be sufficiently known by the quantity of froth, and by the brassy and scaly crust on the surface of the liquor, on blowing or stirring which, the liquor beneath is green, although the surface appears brown or blue. When the vat has worked about forty or fifty pounds of serge or worsted, it may be necessary to replenish it with one pound of potash, half an ounce of madder, and a handful of bran; these being boiled a quarter of an hour, are added to the vat. When this vat wants replenishing with indigo, which may be known by the liquor being no longer green, but brown, blue, or almost black, two-thirds of it must be put into a copper; when ready to boil, the scum on the top must be taken off by a sieve, after which it should be suffered to boil, with the addition of two handfuls of bran, a quarter of a pound of madder, and two pounds of potash; soon after it has boiled, it is to be put into the vat with one pound of indigo, prepared as before; the vat being again stirred, and covered, the heat always remaining between blood and fever heat. When an indigo vat has been several times re-heated, it should be emptied out entirely, and set anew, because the colour becomes dull. _The preceding process is from Hellot._ [8] It is necessary that the student should not confound the terms _primitive_ colours here with the _prismatic_ or _primary_ colours, for the discovery of which we are indebted to Sir Isaac Newton. See _the Introductory Chapter_. CHAPTER V. ON DYEING SILK AND COTTON BLACK, &c. _To dye silk black for velvets—To dye silk black, London process—On dyeing cotton black at Rouen—To dye cotton black, London process—For dyeing black, particularly cotton velvets, at Manchester—On dyeing silk and cotton black, with a blue ground—Another iron liquor—To dye cotton black, by using the preceding solution—To dye cotton violet—To dye cotton red—To dye cotton an Adrianople or Turkey red—Miscellaneous observations relative to Adrianople red._