This method of lead-burning is considerably troublesome, and is rarely used, except when the lead is too thick to be melted conveniently by means of the blowpipe, or the oxyhydrogen flame. The latter is, however, always used when possible by those who can accomplish the operation, which requires a much greater degree of skill than the process described above.

Similar processes are applicable in the case of the other metals. Thus brass may be burned together by placing the part3 to be joined in a sand mould, and pouring a quantity of molten brass on them, afterwards reducing the parts by means of the file, etc, to proper dimensions. The sine qua non is plenty of molten metal, made a trifle hotter than usual. Pewter is generally "burned" by the blowpipe or a very hot copper-bit. In angles, where bent over sharp corners, and in seams, one edge is allowed to stand over the surface of the other, and a strip of the same metal is then laid along the intended junction. This joint is burned, as mentioned, by melting the surfaces and edges by means of a blowpipe or the hot soldering-iron, and the superfluous metal is filed off, leaving the joint, if at an angle, looking as if it had been made out of the solid. The principle of the process is the same whatever be the mode in which it is performed; and when hot metal is used as the sole agent of heat, it is necessary to have plenty of it, and to see that the parts to be joined are clean.

It is scarcely necessary to say that the autogenous method is the only proper method of remedying the defects in castings, and notwithstanding the trouble attached to it, should always be attempted with all metals for which it is applicable, and all articles in which it is possible. It is not to be supposed that trifling defects in iron castings will be remedied by this means, though there is no very great difficulty in accomplishing it, as flanges are often burned on to pipes and wheels, but with the more costly or easily worked metals, the practice of this process would be attended with advantage.

Dr. Hoffman suggests endeavours being made to employ the oxyhydrogen flame for effecting autogenous joints in all metals. The operation is already conducted with complete success in the case of 2 essentially different metals, lead and platinum, and offers the advantages of being cleaner, stronger, and more economical of time and materials.

For all leaden vessels and chambers to be used in contact with acid vapours or liquids autogenous soldering is the only admissible way of making a joint. The apparatus employed consists of a hydrogen gas generator, or "burning machine," as it is commonly called, an "air vessel" or portable bellows, some indiarubber tubing, and a set of gas-cocks and jets. The hydrogen generator is shown in Fig. 148 : a is an airtight leaden cistern, having a perforated shelf b, and an opening c in the top; d is another leaden cistern with a perforated shelf e. A pipe f connects the cisterns a d, passing through a as far as the shelf b, which it just perforates. The hinged cover g being turned back, the cistern a is filled with sheet zinc cuttings, and the cover is closed. Diluted oil of vitriol, say 1 qt. of the acid to 1 gal. water, is poured into the cistern d, and finds its way through the pipe f into the bottom of the cistern a, rising through the strainer b, and surrounding the zinc. The acid acts upon the zinc, forming zinc sulphate, with consequent liberation of hydrogen.

As the hydrogen gas is Bet free, it passes through the cock and pipe h into the leaden vessel i, partially rilled with water, and, passing through the water, it becomes purified, and escapes at the pipe k; m is the pipe through which the generator is emptied of acid when the gas is no longer required. The vessel i may be removed from its place by unscrewing the nut close to the cock on the pipe h, and may be filled with water or emptied through the pipe n. The pipes m and n are plugged with corks; o are short pieces of pipe supporting the shelf b, to which they are attached.

Burning Or Autogenous Soldering Continued 149Burning Or Autogenous Soldering Continued 150Burning Or Autogenous Soldering Continued 151Burning Or Autogenous Soldering Continued 152Burning Or Autogenous Soldering Continued 153Burning Or Autogenous Soldering Continued 154

The air vessel consists simply of a wooden cask open at the top, containing a cylinder of zinc, with a closed top, having a hole and cover in the centre, as shown in Fig. 149, which is drawn on a scale of 1/2 in. = 1 ft. The cask a is partially filled with water, the cover b (which is coated underneath with sheet indiarubber to make it shut close) is opened, the cylinder c is raised, and the cover is closed again, preventing the escape of air from the cylinder except through the small pipe d. A weight e is placed on the top of the cylinder, to keep the cover b firmly closed, and give force to the current of air issuing from d, the weight being conveniently represented by a 1/4-, 1/2-, or 1-cwt., according to the pressure of air required.

A small bellows, Fig. 150, is sometimes used by plumbers for obtaining a supply of air. It is more portable than the air vessel, but cannot be used without an assistant to work it.

Indiarubber tubes a b (Fig. 151) connect the gas generator and air vessel or bellows with a pan of brass cocks and breeches-pipe c. The gas and air, being admitted through these cocks, unite in the tube d, and, passing through the brass pipe e and jet f, may be ignited, and produce an intensely hot flame, by which leaden sheets may be joined without the aid of any flux.

The lead to be burned must first be scraped bright, and where a strong seam is required as for instance in the bottoms of chambers, strips of clean lead are run on in the manner of solder. But it is essential to success that all the surfaces to be subjected to the flame be bright and dry, and that no moisture be sufficiently near the seam to be drawn into it by the heat. Several jets are in use, with holes of various sizes, for procuring a large or small flame, according to the special requirements of the work in hand; and the intensity of the heat is also regulated by the proportions and quantities of gas and air admitted through the cocks. As it is imperative that the flame should not be subject to sudden variation, little brass tubes are fitted to the nozzle to guard the flame from air currents when working out of doors or in draughty places. (Lock's ' Sulphuric Acid.')