39. TO GIVE STEEL A TEMPER TO CUT MARBLE.--No temper can be given to steel, in which hardness is combined with tenacity, more than in that given to files, at the file manufactories, which is accomplished by the following process.--To boiling water, add about twice as much finely ground muriate of soda, as the water will dissolve, and as much rye flour as will, with the other, make a thick paste; lay a coat of this paste over the steel, (which must be ground, or filed previous to tempering,) and subject it to a full red heat, in a fire of charcoal, mixed with about a third part of animal coal, (coal of bones, horns, leather, &c.) and then suddenly plunge it three or four feet deep, in exceeding cold water. By thus immersing the steel rather deep in the water there is a double advantage; for the water which becomes heated, by contact with the steel, will rise and its place be supplied continually by fresh cold water; and at the same time, the pressure of the water on the coating of paste, will make it adhere more closely to the steel while it is cooling. The paste may then be shelled off, and the steel will be found as bright as before, or at least, will not have been essentially oxydized by the operation. 40. TO WASH IRON OR STEEL WITH COPPER.--Dissolve sulphate of copper in water, in the proportion of one to three; wash the iron or steel with it, and it will instantly be covered with reduced copper. This is best performed by applying the solution with a brush, which must be followed directly with a sponge of clear water. In this manner any letters or figures may be drawn with a camel-hair pencil, or a pen, and if it be on polished steel, the letters or flowers will assume the brilliancy of the steel and appear like highly polished copper. It may sometimes be requisite to cleanse the metal by washing it with diluted muriatic acid, that the copper may adhere the more readily. If the steel thus ornamented, be held over a charcoal fire, the copper figures become blue first; and when the steel becomes blue, the copper takes a gold colour; but is restored again to its original colour, by diluted muriatic acid. 41. TO GIVE IRON THE WHITENESS OF SILVER.--To nitric acid, diluted with an equal quantity of water, add as much mercury as the acid will dissolve; then add to the solution, three or four times as much water, and having given the iron a coat of copper, as directed in the last experiment, brush it over in the same manner with the diluted nitrate of mercury; its appearance will be equal, if not superior to that of real silver. In this manner any common, or rough iron work, may be apparently silvered at a most insignificant expense.