A simpler process is to warm the dyed goods for five or ten minutes in a weak solution of the orange-colored salt, bichromate of potash, acidified with a little acetic acid—or of the not uncommon chemical, sulphate of copper, long known to chemists as blue vitriol. When the best dyes are used, like those in the preceding list, it is not often necessary to use either of these reagents. But when, as sometimes happens, one is obliged to use dyes of this general class, bought at the country store without a chance of knowing how fast they are, it is well to know about it. For a piece of goods the size of an ordinary linen skirt, the after-treating bath would be made as follows: In two and a half gallons of hot water, dissolve two tablespoonfuls of sulphate of copper, one tablespoonful of bichromate of potash, and two teaspoonfuls of ordinary acetic acid (equivalent, say, to three or four teaspoonfuls of strong vinegar). The goods, after dyeing and rinsing, but before drying, should be soaked in this bath and heated for ten minutes until not far from the boiling point. They should then be taken out, rinsed carefully, and dried. This after-treatment does not benefit every single color of this class, but it helps greatly the fastness to light and to washing of almost all of them. The chief objection to it, besides the time and expense, is that the shade of the finished goods is often considerably changed by the process. _Properties and Uses of the Salt Colors._—Generally speaking, the shades produced by the individual members of this group cover all the colors of the rainbow and include several good greys. It is hard, however, to get a full deep black on cotton or linen with these dyes, without using the “diazotizing and developing” process of after-treatment. The dyes go on the fibre in a soluble form, and unless a developing process like this is used they combine directly with the fibre, and do not form a coating or layer upon it, as do some of the “developed” dyestuffs. Accordingly, no matter how fully or how deeply we dye a piece of yarn or cloth with a black dye of this class, the finished goods will show _grey_, a very dark grey, to be sure, but still grey, and not a flat, heavy, true black. The color of most of the salt blacks is greatly improved, however—as well as their fastness to light and washing—by soaking the dyed goods, after rinsing, in a solution containing four or five spoonfuls of formaldehyde to the gallon.