Mr. _M'Kernan_ gives another form for _pencil blue_ with indigo: the principal differences between which and the above, consist in adding equal parts of brown sugar and _gum senegal_ to it, which, in regard to the addition of the gum, is, we presume, a great improvement. Dr. URE (_Notes to Berthollet_, vol. ii. page 437.) gives a similar form from _Vitalis_, for _topical_ or _pencil blue_; but he adds, it was much used formerly. Another blue, of less permanence but more brilliance, is now preferred; it is made thus:— Into an earthen pot four ounces of finely ground and sifted Prussian blue are to be put. Over this must be slowly poured, stirring all the while, sufficient muriatic acid to bring it to the consistence of syrup. The mixture is to be stirred every hour for a day, and afterwards thickened with from four to eight pots (of two _litres_ each; a litre _French_ contains about two wine pints;) of gum-water, according to the shade wanted. _To dye cotton a_ FAST BUFF. Take a brown stone pan or pipkin, glazed. It must not be the common glazed wares, because these are glazed with _lead_, and the acids will dissolve the lead; if, therefore, such are used, the lead being dissolved, will be mixed with the dyeing materials, and sometimes totally spoil the dye. Stone-ware, if used with care, will bear the fire: such ware is usually glazed with muriate of soda or common salt. Having a proper vessel capable of holding from two quarts to a gallon, fill it half-full of strong _nitric acid_, to which add, in small quantities at a time, either _old horse-shoe nails_ from the farrier, they being the purest iron, or the _cuttings of tin-plate_ from the tin-man's, for this is also very pure iron, although covered with tin; but the small portion of tin in the iron is not inimical to the dye. Be careful not to put too much in at a time, nor to stoop near to it while the solution is going on, as the red fumes arising are very noxious; and if the iron be added in large quantity, so much effervescence would be produced, that a considerable part of the liquor would be thrown over the top of the vessel. When this solution is prepared in haste the air is greatly contaminated, and therefore it is best to prepare it long before it is wanted, and slowly, by dropping hourly small quantities of the iron into the acid, and then little if any perceptible fumes will arise. Continue this process till, by stirring with a glass rod or tobacco-pipe, you find the iron dissolves more slowly: by keeping a little iron at the bottom, and occasionally adding the acid, you may always have this preparation at hand.