He first converts, by a gentle calcination, sulphate of iron into a red sulphate of iron: this he dissolves in sixteen times its weight of warm water and filters. The silk, prepared as for indigo dye, is put into the solution of iron, and left there for a shorter or longer time, according to the shade of blue that is wanted; it is then taken out and wrung very dry over a pole placed above the vat. It is then thoroughly cleansed by being twice beetled, plunging and agitating it each time in running water. Dissolve in pure water heated to 167°, and put into a deal vat, one ounce of _ferroprussiate_ of _potash_, for every twelve ounces of silk to be dyed. When the prussiate is dissolved add one part, or even rather more, of muriatic acid, stirring the mixture well. When the liquor has acquired a greenish colour, and about 144° of heat, the silk must be immediately plunged into it and stirred about for some minutes. The silk having received the dye in an equal manner, it is taken out of the vat, well wrung on a pole above the vat, and then taken to the stream to receive two or three beetlings, and be plunged and agitated in the water, in order that it may be entirely freed from any portion of the prussiate of iron not truly combined with it. Lastly, the silk being well washed in the stream, and thoroughly wrung, is to be placed loosely on the poles, as in the preceding operations; after which it must be well agitated in a large vessel three-fourths filled with cold water, to which must be added, for a hundred pounds of silk, two pounds of water of ammonia. The blue colour immediately becomes many shades deeper, of a much richer and brighter tint, and at the same time is fixed more perfectly in the silks. This change is effected in a few minutes. The silk must then be wrung by the hand and rinsed in the running water without beating. After this, it is dried on the poles in the same manner as other dyed silks. It need not be left on the poles more than twenty-four hours: but, nevertheless, this colour so far from fading in the drying, as is the case with many colours, is improved by it. The solution of a little soap added cold to the ammonia bath, improves it, giving also softness to the silk, and rendering it more easy to separate. The soap should be uniformly dissolved. For the substance of the above process, we are indebted to Dr. URE'S _notes on Berthollet_, vol. ii. p. 422. The _prussiate of potash_ is now to be obtained as a regular article of trade from the dry-salters in this country. _Woollen cloth_ takes also the above dye, but it must be left longer than silk in the iron mordant. _Chromate of lead for_ YELLOW _on_ SILK _and_ COTTON. _Chromate of lead_, as a pigment has been for some time in use; _M. Lassaigne_, in 1820, made public a process for dyeing cloth with this article, which has since become pretty common in this country.