Some gunsmiths, when such work is finished, heat it red hot, smear it with prussiate of potash or cyanide of potassium, and while hot, plunge it into cold water, letting it chill. This produces a superficially hardened surface that is not “skin deep,” and as soon as this surface becomes abraded will wear away rapidly. If the case-hardening of the expert manufacturer be examined, it will be observed that the surface of such work has a fine grayish appearance, and in many places mottled with colored tints that are pleasing and beautiful to the mechanical eye. It will be further observed that the hardening is of such depth that it will wear for a long time. In fact it will wear better than hardened steel. The condition of the material is that of a hardened steel surface stretched over and shrunk upon the iron body of the work. It is stronger than steel, for it has the tenacity of iron for its interior. It has the advantage of steel, inasmuch as it may be bent when cold to a limited degree, and when so hardened will not break as readily as steel. This property of bending is not confined to all articles, as they may case-harden entirely through, and then they will be very brittle and easily broken, but by drawing them to temper after hardening, in the same manner as a tool is drawn to temper, they may be of any hardness desired. _A good way to Case-Harden._—The easiest and perhaps the best way to case-harden gun work is to have a number of short pieces of common gas pipe, such as will be adapted to the size or quantity of the work, and have one end of these pieces securely plugged or closed. One way will be to heat the pipe and close by flattening the end with a hammer on the anvil, but it is a “slouchy” way of doing it. A neater way is to have a gas fitter cut a thread in the pipe and then screw in a plug, such as are used to close ends of gas pipe; if such cannot be obtained, drive in a cast-iron plug and upset the end of the pipe so that it will not readily come out. In these pieces of pipe place the work, packing it well with good, fine bone-dust, such as is used by farmers for fertilizing land. Be careful to so pack that the different pieces of work will not touch each other. Stop the open end of the pipe with a cover, but in such a manner as to be readily opened; place the pipe and its contents in a good fire, letting it remain at a red heat for fifteen minutes or more, dependent upon the thickness of the articles or the depth they should be hardened. Remove from the fire and quickly empty the contents of the pipe into a pail of cold water. If pieces of gas pipe cannot very well be obtained, thimbles from old carriage hubs may be used instead. Plug up the small end, fit a cover to the large end and use as if it were gas pipe. As these thimbles are made of cast iron they will not bear the rough usage nor the heat that wrought iron will withstand. Common cast malleable iron makes the best receptacles to contain work for case-hardening.