THE BLOWPIPE.
Originally, the blowpipe was only a simple, conical tube, more or less curved towards its point, and terminated by a very small circular opening. By means of this, a current of air was carried against the flame of a candle, and the inflamed matter was directed upon small objects, of which it was desirable to elevate the temperature. Workers in metal still derive immense advantages from the use of this little instrument: they employ it in the soldering of very small articles, as well as for heating the extremities of delicate tools, in order to temper them. But since the blowpipe has passed into the hands of mineralogical chemists, its form has been subjected to a series of very curious and important modifications. In spite, however, of these ameliorations, which rendered the instrument better adapted for the uses to which it was successively applied, we are far from having drawn from it all the advantages to which we might attain, were its employment not as fatiguing as it is difficult. We require no other proof of this than the small number of those who know well how to make use of the blowpipe.
The most economical blowpipe is a tube of glass, bent near one end, and pointed at its extremity. A bulb is blown near that part of the tube which corresponds with the curvature ([pl. 3], fig. 7.) This bulb serves as a reservoir for moisture deposited by the air blown into the tube from the mouth. If you employ a tube without a bulb, the moisture is projected in drops into the flame, and upon the objects heated by it—an effect which is very inconvenient in practice. To put this instrument into action, accustom yourself to hold the mouth full of air, and to keep the cheeks well inflated, during a pretty long series of alternate inspirations and expirations; then, seizing lightly with the lips the mouth of the blowpipe, suffer the air compressed by the muscles of the cheeks, which act the part of a bellows, to escape by the beak of the blowpipe, which you will be able to do without being put to the least inconvenience with regard to respiration. When the air contained in the mouth is pretty nearly expended, you must take advantage of an inspiration, to inflate the lungs afresh; and thus the operation is continued. You must never blow through the tube by means of the lungs; first, because air which has been in the lungs is less proper for combustion than that which has merely passed through the nose and mouth; secondly, because the effort which it would be necessary to make, to sustain the blast for only a short time, would by its frequent repetition become very injurious to your health.
The jet of flame produced by the mouth-blowpipe can only be used to heat small objects: when instruments of a considerable bulk have to be worked, it is customary to employ the lamp, or glass-blower’s table.