DYEING BY THE INDIGO VAT. _The Copperas Vat_.—For dyeing cotton, the method of reduction found by experience to be the most convenient and practical is founded upon the action of the hydrate of the protoxide of iron in the presence of lime. The hydrated protoxide of iron is obtained from sulphate of iron (green vitriol, or copperas) with freshly burned lime. Certain precautions should be observed in the use of these materials. The copperas used for the preparation of these vats should be free from sulphate of copper, because the oxide of copper which would be formed in the vats rapidly oxidizes the reduced indigo, and causes its precipitation in the bath. The copperas ought not to contain red oxide of iron, nor sulphate of alumina. The coppery or oxidized vitriol may be purified by boiling the solution with pieces of iron, which precipitates the iron and neutralizes the oxide. The lime ought to be pure, containing no magnesia; when slacked lime has been exposed to the air, even for a short time, it absorbs carbonic acid, and becomes converted into chalk. The lime, therefore, should always be newly slacked. The ingredients, then, of a copperas vat are water, pure or purified green vitriol, indigo ground into a homogeneous impalpable paste, and pure and freshly slacked lime. The proportions used in different establishments are exceedingly variable. Those which answer for a laboratory vat, or a small vat used for precipitating the white indigo immediately for printing, are: indigo, one part; sulphate of iron, two parts; slacked lime, three parts. These proportions are not enough for the large vats used in dyeing pieces. In them it is necessary to make the quantities of lime and sulphate of iron larger than the theory of the vat requires. The excess of lime and hydrate of iron serve the purpose, whenever the vat is stirred, to repair the losses of indigo caused by its oxidation from contact with the air. Schutzenberger gives the proportions generally used by the dyers of France, as follows:— Indigo 1 part. Crystallized sulphate of iron 3 „ Freshly slacked lime 3 „ Others, he says, use more lime than copperas, as in the following proportion:— Indigo 2 parts. Sulphate of iron 5.5 „ Quicklime 6.5 „