or fed with copperas and lime, upon which occasion the sediment must first be stirred up, and then allowed to settle again, so as to leave the liquor clear. The indigo-blue, however, is in course of time gradually removed, and by degrees the vat becomes capable of dyeing only pale shades of blue. When the color produced by it is only very faint, it is no longer worth while using it, and the contents are then thrown away. In dyeing cotton with indigo, it seems to be essential that the reduced indigo-blue should be in contact with lime. If potash or soda are used in its place, it is impossible to obtain dark shades of blue.” FERMENTING VATS FOR WOOL DYEING. The application of indigo-blue to wool and woollen tissues is always made by means of vats, which have special names; as, the pastel or woad vat, the urine vat, German vat, molasses vat, &c. The reduction or _hydrogenation_ of the indigo-blue is the result of a peculiar fermentation, which is developed within an alkaline liquor by means of nitrogenized substances and bodies rich in sugar or hydrocarbonized substances. It is known that in these conditions, especially where the temperature is slightly raised, the sugar is converted into butyric acid, and at the same time carbonic acid and hydrogen are set free. We find here the source of the nascent hydrogen which fixes itself upon the indigo-blue, and transforms it into white indigo, which is soluble in the alkalies of the vat. It has recently been observed that the butyric fermentation proceeds from the development of minute infusoria. These animalculæ live without any supply of oxygen, and, in fact, are killed in its presence. They therefore live at their ease in the vat of reduced indigo, where no oxygen is permitted to enter.