The modes of heating works for soldering are extremely varied, and depend jointly upon the magnitude of the objects, the general or local manner in which they are to be soldered, and the fusibility of the solders. It appears to be now desirable to advert to such of the modes of applying heat enumerated in the tabular view, as are of more general application, leaving the modes specifically employed in heating works to their respective sections.

In hard-soldered works, the fires bear a general resemblance to those employed in forging iron and steel, and already described;* in fact, the blacksmith's forge is frequently used for brazing, although the process is injurious to the fuel as regards its ordinary use. Coppersmiths, silversmiths, and others, use a similar hearth, but which stands further away from the upright wall, so as to allow of the central parts of large objects being soldered; the bellows are always worked by the foot, either by a treadle, as in fig. 90, p. 203, or more commonly by a chain from the rocking-staff terminating in a stirrup.

The brazier's hearth for large and long works, is a flat plate of iron, about four feet by three, which stands in the middle of the shop upon four legs: the surface of the plate, serves for the support of long tubes and works over the central aperture in the plate which contains the fuel, and measures about two feet by one, and five or six inches deep. The revolving fan is commonly used for the blast, and the tuyere irons, which have larger apertures than usual, are fitted loosely into grooves at the ends, to admit of easy renewal, as they are destroyed rather quickly. The fire is sometimes used of the full length of the hearth, but is more generally contracted by a loose iron plate; occasionally two separate fires are made, or the two blast pipes are used upon one. The hood is suspended from the ceiling, with counterpoise weights, so as to be raised or depressed according to the magnitude of the works: and it has large sliding tubes for conducting the smoke to the chimney.

Furnaces are occasionally used in soldering, or the common

* Some parts of the remarks on forging iron and steel, p. 195 to 205, and also of those on hardening and tempering steel, 240 and 246, refer to similar applications of heat to those required in soldering.

FUEL; BLOWPIPE. 437 fire is temporarily converted into the condition of a furnace from being built hollow, or by the insertion of iron tubes or muffles, amidst the ignited fuel, as already explained in reference to forging and hardening. For want of any of these means, the amateur may use the ordinary grate, or it is better to employ a brazier or chafing-dish containing charcoal, and urged with hand-bellows blown by an assistant, as then both hands arc at liberty to mauage the work and fuel.

Fresh coals are highly improper for soldering, on account of the sulphur they always contain; the best fuel is charcoal, but in general coke or cinders are used. Lead is equally as prejudicial to the fire in soldering, as it is in welding iron and steel, or in forging gold, silver, or copper; as the lead readily oxidizes and attaches itself to the metals that arc being soldered or welded, preventing the union of the parts, and in almost all cases rendering the metals brittle and unserviceable.

There are many purposes in the arts which require the application of heat having the intensity of the forge fire or of the furnace, but with the power of observation, guidance, and definition of the artist's pencil. These conditions are most efficiently obtained by the blowpipe, an instrument by which a stream of air is driven forcibly through a flame, so as to direct it either as a well-defined cone, or as a broad jet of flame, against the object to be heated, which is in many cases supported upon charcoal, by way of concentrating the heat.

The blowpipe is largely used - namely, in soldering, in hardening and tempering small tools, in glass blowing for philosophical instruments and toys, in glass pinching with metal moulds made like pliers, in enamelling, and by the chemist and mineralogist, as an important means of analysis: the instrument has consequently received very great attention both from artizans and distinguished philosophers.

Most of the blowpipes are supplied with common air, and generally by the respiratory organs of the operator; sometimes by bellows moved with the foot, by vessels in which the air is condensed by a syringe, or by pneumatic apparatus with water pressure.* In some few cases oxygen or hydrogen, or the same gases when mixed, arc employed; they arc little used in the arts.

• Tilley's Blowpipe. Trans. Soc of Arta, vol. 31, p. 106; and Toft's Blowpipe. See also Griffin on the Blowpipe.

The ordinary blowpipe is a light conical brass tube, about 10 or 12 inches long, from one-half to one-fourth of an inch diameter at the end for the mouth, and from one-sixteenth to one-fiftieth at the aperture or jet; the end is bent as a quadrant, that the flame may be immediately under observation.

Fig. 304, represents the same instrument when fitted with a ball for collecting the condensed vapour from the lungs; it is seen by the enlarged section, fig. 305, that the tube is discontinuous, and any moisture within it, proceeding in the direction of the arrow, is arrested in the ball. There are several other blowpipes for the mouth, with various contrivances,* such as series of apertures of different diameters, joints for portability, and for placing the jet at different angles, and projecting parts to support the instrument upon the table, but none of these are in common use.

The lungs may be used for the blowpipe with much more effect than might be expected, and with a little practice a constant stream may be maintained for many minutes, if the cheeks are kept fully distended with wind, so that their elasticity alone shall serve to impel a part of the air, whilst the ordinary breathing is carried on through the nostrils for a fresh supply.