[Illustration: Fig. 67.--Bench Gas Blowpipe]
The air is conveyed through an 8-in. brass tube J 1/4 in. in diameter, which should be smooth inside. This latter point is of some importance, and, if preferred, a glass tube may be used instead of brass, the current of cold air having a sufficient cooling effect to prevent undue heating. The end should be cut off sharp with a file in the ordinary way and left in that condition. Smoothing the edge by fusion in a flame will not improve matters, but rather the reverse. Of course, the other end, which comes outside, must be smoothed to prevent injury to the indiarubber tube used for making connection with the bellows. The air tube must be held firmly in the centre of the gas tube, while capable of being moved in or out for the purpose of adjusting the flame. This can be done quite satisfactorily by means of a short brass tube or nipple K, threaded to screw into the [T] (_see_ Fig. 67). A sound cork should be driven into this short tube so as to entirely fill it, a hole being made with a cork-borer to admit the air tube. This hole must be exactly central, and the cork must grip rather tightly.
A foot-bellows is generally used for supplying the air, the bellows being connected with the air jet J by means of an indiarubber tube. The tube D, which should extend an inch or so below the bench, is to be connected with the gas supply.
=A Simple Gas Blowpipe.=--A simple form of gas blowpipe is shown in Fig. 68, the rubber tube connecting it with the gas supply being fixed on the pipe at the point of connection with the cock. To construct the appliance, one end of a piece of brass gas pipe of the required length with, say, a 3/8-in. bore, is bent as shown, whilst at the back of the curve thus made a hole is drilled to admit a tube A 5/16 in. in diameter. This should have one end (_see_ dotted lines) bent to correspond with the angle previously formed in the larger tube, whilst its other extremity should be bent upwards. Make these pipes red hot where they are to be bent, and, if they are afterwards plunged in cold water, the material will to some extent be softened, and its tendency to split will be obviated. The smaller tube is passed through the hole in the bend of the larger one, the ends being almost flush and quite concentric. Solder the parallel portions of the tubes together, and then fix a gas-regulating cock to the larger one, as in Fig. 68. The end is then connected to an ordinary bracket or burner by means of an indiarubber tube G, and a short piece of tubing is fitted with a bone or other mouthpiece, and attached to the projecting end of the air tube. This instrument will do any soldering, and will be suitable for melting gold, silver, and brass, or brazing odd jobs in iron or steel. Of course, when used for the last-named purpose it would be in conjunction with asbestos tubes or other supports.