We obtain the deep shades in this species of vat with greater celerity than in all others, a fact which undoubtedly depends on the greater power which potash has of dissolving indigo than is possessed by lime. Experience proves that the potash vat has the advantage in point of celerity of nearly a third; but this is balanced by the inconvenience resulting from the darker shade, which we must attribute to the large quantity of coloring matter of the madder dissolved by the alkaline lee, and which becomes fixed on the stuff with the indigo. To render this vat in its most favorable state, the indigo should be made to undergo a commencement of hydrogenation before turning it into the mixture; for this purpose we prepare in a small copper a bath analogous to that in the vat, to which the pounded indigo is added. This bath is maintained for twenty-four hours at a moderate heat, taking care to stir it from time to time. The indigo assumes a yellowish color, becomes dissolved, and in this state is turned into the vat; we thus avoid many delays and losses in its preparation, and indeed it would be desirable if a similar plan were adopted with all these compounds. German Vat.—This vat is of nearly similar dimensions to that used for the woad, being three times the size of the potash vat. Its diameter is about 6½ feet, and its depth 8½ feet. Having filled the copper with water, we are to heat it to 200° Fah.; we then add 20 pailsful of bran, 22 lbs. of carbonate of soda, 11 lbs. of indigo, and 54 pounds of lime, thoroughly slaked, in powder. The mixture is to be well stirred, and then set aside for two hours; the workman should continually watch the progress of the fermentation, moderating it more or less by means of lime or carbonate of soda, so as to render the vat in a working state at the end of twelve, fifteen, or, at the most, eighteen hours. The odor is the only criterion by which the workman is enabled to judge of the good state of the vat, he must therefore possess considerable tact and experience. In the process of dipping we introduce 84 lbs., 106 lbs., or even 130 lbs. of wool, in a net bag, similar to that used in the woad vat, taking care that the bag is not allowed to rest against the sides of the copper. When the wool has sufficiently imbibed the color, we remove the bag containing it, and allow it to drain for a short time over the vessel. We operate in this way on two or three quantities in succession; we then remove the vat, and set it aside for two hours; we must be careful, from time to time, to replace the indigo absorbed by the wool, as also to add fresh quantities of bran, lime, and crystallized carbonate of soda, so as constantly to maintain the fermentation at a suitable point.