The ingredients most usually employed for furnishing the hydrocarbonaceous substances for fermentation are bran and ground madder, although molasses is sometimes used. The nitrogenized material is found in the woad or pastel, which is often added in very large proportions to the fermenting vats. It is observed by the chemists who have studied this subject most carefully, that the preparation of vats, founded upon the principle of fermentation, does not repose upon principles so sure and constant as those of the copperas vat, and that many unforeseen accidents interpose to disturb the work of an inexperienced dyer. The phenomena in fermentations are often complex. It is admitted that in these phenomena theory has not said its last word, and that empiricism is often more fortunate than science. In conducting the operations of the warm fermenting vat, the conceit of the practical dyer, so often remarked upon, is not without foundation. By practical experience and the traditions of his art he has acquired a knowledge of the almost insensible modification in conditions which can change or arrest the chemical reaction. It is the knowledge of the workman, a knowledge almost instinctive, which can never be communicated to the books, and which is most respected by those most profoundly informed in theory. _The Woad or Pastel Vat_.—In former times woad, already referred to, was the only material known to the dyers of Europe for producing the blue color of indigo. For dyeing wool, the use of woad, now abandoned wholly in cotton dyeing, has been retained to the present day, generally for the purpose of exciting fermentation, and without regard to its effect in imparting color to the material to be dyed; for the woad grown in England, and used in the dye-houses of that country, contains no trace of coloring matter. The woad, or pastel, grown in the warmer districts of France contains about two per cent of indigotine, which is regarded in that country as an important addition to the coloring material, especially for improving the tone of the color. Various substitutes, such as rhubarb leaves, turnip and carrot tops, and weld, have been tried, but without advantage, with the exception, perhaps, of weld, which is still used by some dyers. Some chemists regard the use of woad as the remnant of a prejudice; but the better opinion is, that this material possesses peculiar fermentiscible qualities, whose exact action science has yet to resolve.