The apparatus consists of a tube through which air is blown from the mouth, a valve through which the air passes into an expanding reservoir, and a blowpipe jet in communication with the reservoir.
In making the valve, several essentials have to be remembered; it must allow a free passage of air into the reservoir, it must open easily, and must close quickly. A satisfactory form of valve is that shown by b, Fig. 16. The moving part consists of a light glass bulb of about three-eights of an inch diameter and having a glass stem of rather under one-eighth diameter and about an inch and a half long. This stem rests in a guide at the end of a brass tube, the bulb contacting against the other end which is approximately shaped. The bulb and its seating are ground air-tight. A very light spring holds the bulb in position.
This valve is fitted into a metal or glass T piece, one limb of which leads to the air reservoir and the other limb leads to the blowpipe jet; the limb containing the valve leads to the tube through which the air is blown in.
A convenient reservoir may be made from a fairly large football bladder. A network of string should be fitted over the outside of the bladder and the strings should terminate in a hook on which a weight can be hung, in order to provide a means of adjusting the pressure at which the air is delivered to the jet. This bladder should be washed out and allowed to drain after use.
The air tube which passes from the valve to the mouth may conveniently be made of brass, but, in order to avoid the continued contact of metal with the lips of the operator, it should be fitted with a non-metallic mouthpiece. It is an advantage from the point of view of portability to have the air tube easily detachable from the T piece containing the valve.
The blowpipe jets, of which there may be several with advantage, may be made of glass tubing, bent to the most convenient angle and having an enlargement or bulb at some point in the tube. This bulb serves as a final condensing place for any traces of moisture that may escape from the larger reservoir.
The whole device, blowing tube, reservoir, and T piece may be fastened to a clamp, so that it can be secured on the edge of any table where blowpipe work is to be carried out. If the blowpipe is to be used with gas, the form of burner described under. "A Simple Form of Blowpipe" will be found quite satisfactory.
The Use of Oil, or Other Non-Gaseous Fuels.—Although gas, when available, is usually preferred on account of its convenience, there are several other fuels which give a hotter flame. They have, also, the additional advantage of not requiring any connecting pipes; but each has its own disadvantage.
One liquid fuel deserves special mention as being rather less desirable than the others; this is alcohol. Although very convenient in use, it has the disadvantage of being rather too highly inflammable and capable of burning without a wick, thus involving a certain fire risk; the flame is scarcely visible in a bright light, and the heat given by either the ordinary flame or the blowpipe flame produced from alcohol is considerably less than that from a similar flame in which coal gas is used. For small work, however, the facility with which a spirit lamp may be lighted may more than counterbalance these disadvantages at times.
Paraffin Wax.—Where there is no coal gas available and the blowpipe is only required at intervals, and especially where high portability is required, there are few fuels so convenient as paraffin wax. This may be obtained in pieces of a satisfactory size by cutting paraffin candles, from which the wick has been withdrawn, into lengths of about half an inch. These cut pieces have the advantage over any oily fuel, such as colza oil, that they can be wrapped in paper or carried in a cardboard box; further they will keep indefinitely, even in the presence of air, without undergoing any perceptible change.