Care should be taken not to use the logwood too much, as with the sulphate of iron it darkens more than most drugs: therefore the black vat, made either with alder-bark, or the other preparation mentioned in dyeing cotton, is preferable to the sulphate of iron. NUT-GREY. The fustic decoction, archil, and a little logwood are put into water moderately hot, the silk is then returned, and when the liquor is exhausted, the silk is taken out, and to soften the colour the solution of sulphate of iron, or the black vat, is used. The silk is then returned once more, and if the colour does not appear sufficiently even, some red spots still remaining, it may be concluded that it requires a little more sulphate of iron. _Observe_ that, as sulphate of iron is the general base of all greys, if this be deficient in quantity, the colour is apt to change in dyeing, and to become rough and uneven. To know whether the colour be sufficiently softened, it should be examined, and if it wet easily, after having been wrung on the peg, it wants sulphate of iron. On the contrary, if it wets with a little difficulty, the colour is sufficiently softened. Too much sulphate of iron stiffens the silk considerably, making it harsh, and even depriving it of a part of its lustre; to remedy this it must be extra washed and wrung at the peg; this process carries off the sulphate of iron. BLACK-GREYS. These are alumed and welded as for yellow, and, when the liquor is exhausted, part of it is thrown away, and some logwood is added; when the logwood is exhausted, sulphate of iron is added, sufficient to blacken the colour, the silk is then washed, wrung, and finished in the usual way. IRON-GREY. For iron-grey it is necessary to boil the same as for blues: this colour is much more beautiful when laid on a very white ground. By having the drugs made into decoctions before-hand, greys either in woollen, silk, or cotton, may be dyed at a heat not much above what the hand will bear; and in a rotation of shades from light to dark, and varied, blue, red, yellow, brown, &c. with ease and with pleasure; so may, likewise, many stone-drabs, and other light brown drabs, as the mixture of yellow, fawn, and black, produces nut-browns, &c. _On dyeing silk of a_ PRUSSIAN-BLUE COLOUR. The application of colours derived from the mineral kingdom to dyeing is one of the most striking modern improvements in our art. MR. RAYMOND received from the French government in 1801, eight thousand francs, (more than three hundred pounds sterling,) as a reward for communicating to the public his process for dyeing silk of a uniform fast and bright _Prussian-blue colour_ by the application of that well known pigment. His process is as follows.