Gun barrels are browned by a process of oxidizement. There are several processes recommended. One of which is, to rub the barrel over with diluted nitric or muriatic acid, and then, to lay it by for a week or two, until a complete coat of rust is formed. A brush, made of iron wire, is then applied; afterwards, oil and wax, and the barrel is finished by rubbing it with a cloth. The gunsmiths in Philadelphia use a mixed solution of sulphate of copper, tincture of the muriate of iron, and sweet spirit of nitre. This they apply by means of a cloth. The object is to form a rust, and to render it permanent on the barrel by hard friction along with wax. When sulphate of copper is employed, metallic copper is precipitated on the barrel. A coat of rust, put on in this manner, prevents effectually the oxidizement of the iron; and in point of utility, and the saving of labour in polishing and keeping muskets in order, the browning of barrels is certainly advantageous in the land service. At sea, in particular, where iron is more readily oxidized, this plan ought always to be adopted. With regard to the use of dragon's blood, it is entirely too temporary in its effect to be depended on. I was informed by an intelligent gunsmith, who followed the practice of browning barrels in Europe, that he has known the _browning_ to remain very perfect for years, and that the best mode of insuring its durability is to use the _steel brush_, which _carries in_, as he expressed it, the rust. The oxides, which are formed by the union of oxygen with iron, are two; namely, the black and the red; the first being the protoxide, and the last the peroxide. The black oxide, which is formed by the combustion of iron, and by other processes, contains 56 iron + 16 oxygen. The common rust of iron is the peroxide of this metal, combined with carbonic acid. It may be formed by exposing the protosulphate of iron, or green vitriol, in solution, to the atmosphere, and then adding an alkali. This oxide contains more oxygen than the preceding; it consisting of 56 iron + 24 oxygen. The tempering of cutting instruments, an operation which requires great delicacy and exactness, after that of hardening, is intended to obtain a fine and durable edge; and as this subject may be interesting in a military point of view, we deem the following remarks of use.