The dry-house is a large building of masonry, quite high, and pierced with many openings, provided with narrow blinds, to prevent the direct light of the sun from penetrating into the interior. Care is taken also to surround the dry-house with large shade-trees. The cakes take from three to four days to dry, after which they are packed in small boxes and carried to Calcutta, the great market of Bengal. The details above given apply to the factories managed by European planters. The natives operate in nearly the same manner, but with less care, and consequently their products are inferior. The average product of indigo in Lower Bengal is stated at 4,000,000 kilograms, or 8,840,000 pounds per year. The most remarkable fact to be noticed in these operations is, that the blue principle is developed by chemical action from certain absolutely colorless principles existing in the plant. The theory of the change effected is still somewhat in doubt, because no chemist has studied the fresh plant, and observed upon the spot the phases of the operation of the production of indigo on a large scale. But the most accepted theory is that derived from the researches of Dr. Schunck, upon the _isatis_ or woad-plant, which produces indigotine in a much less degree than the true indigo plants; viz., that the indigo exists in the plants combined with sugar, forming a glucoside, to which he gives the name _indican_. This compound, under the influence of fermentation in the manufacturing process, is supposed to be unfolded into indigo and sugar. Without dwelling upon this question, which is beyond our province, we observe that the plants of the genus _indigofera_ are used for the production of commercial indigo, on account of the greater richness in the coloring principle. Other plants, which furnish the same coloring principle, indigotine, are more frequently used directly in dyeing to furnish the blue principle than they are for the production of indigo. The most important of these plants, although there are others, such as the _Polygonum tinctorium_ and the _Nerium tinctorium_, is the _Isatis tinctoria_, which produces pastel, or woad. This, plant belongs to the family of cruciferæ, and is a biennial. It is represented in the accompanying figure. [Illustration: Drawing of the Isatis tinctoria plant]