An instrument for exciting intense combustion upon a small scale; it is extensively used in many branches of the arts, and also in philosophical experiments upon metallic substances. In its simplest form it is merely a conical brass tube, curved at the small end, in which is a very minute aperture; and a stream of air being urged through it by the mouth against the flame of a lamp or candle, a heat equal to that of the most violent furnaces may be produced. The body intended to be operated upon should not exceed the size of a peppercorn, and should be supported upon a piece of well-burned close-grained charcoal, unless it be of such nature as to sink into the pores of the charcoal, or to have its properties affected by its inflammable quality. Such bodies may be placed in a small spoon made of pure gold, silver, or platinum. Many advantages may be derived from the use of this simple and valuable instrument. It is portable; the most expensive materials, and the minutest specimens of bodies, may be used in the experiments; and the whole process is under the eye of the observer.
Early in the present century, Dr. Hare, of Philadelphia, made a most important improvement in the blow pipe, by substituting for the flame of a lamp that arising from a mingled current of oxygen and hydrogen, by which means he succeeded in producing a more intense heat than had ever been obtained before, except by the concentration of the sun's rays in very large and powerful lenses. As these gases, however, can only be procured by chemical means, a more perfect method of supplying the currents of gas than by means of the common bellows became desirable, on account of the great leakage of the latter, and the Doctor, turning his mind to the subject, devised a machine equally applicable for supplying either oxygen gas or common air. This machine, which he denominated the Hydrostatic Blow Pipe, is represented in the annexed engraving, of which the following is an explanation.
The Hydrostatic Blow Pipe consists of a cask, divided by a horizontal diaphragm into two parts D D. From the upper apartment, a pipe of about 3 inches diameter (its axis coincident with that of the cask) descends, until within about 6 inches of the bottom. On this is fastened by screws, a hollow cylinder of wood B B, externally 12 inches in diameter, and internally 8 inches. Around the rim of this cylinder a piece of leather is nailed, so as to be air-tight On one side a small groove is made in the upper surface of the block, so that a lateral passage may be left when nailed on each side of the groove. This lateral passage communicates with a hole bored vertically into the wood by a centre bit; and a small strip of leather being extended so as to cover this hole, is made, with the addition of some disks of metal, to constitute a valve opening upwards. In the bottom of the cask there is another valve opening upwards. A piston rod, passing perpendicularly through the pipe from the handle h, is fastened near its lower extremity to a hemispherical mass of lead L.
The portion of the rod beyond this proceeds through the centre of the leather which covers the cavity of the wooden cylinder, also through another mass of lead like the first, which, being forced up by a screw and nut, subjects the leather between it and the upper leaden hemisphere to a pressure sufficient to render the juncture air-tight. From the partition, an eduction pipe E is carried under the table, where it is fastened by means of a screw to a cock which carries a blow-pipe, so attached by a small swivel joint, as to be adjusted in any required direction. A suction pipe passes from the opening covered by the lower valve, under the bottom of the cask, and rises vertically close to it on the outside terminating in a union joint for the attachment of any flexible tube which may be necessary. The apparatus being thus arranged and the cask supplied with water until the partition is covered to about the depth of 2 inches, if the piston be lifted, the leather will be bulged up, and will remove in some degree the atmospheric pressure from the cavity beneath it, consequently he air must enter through the lower cavity to restore the equilibrium.
When the piston is depressed, the leather being bulged in an opposite direction, the cavity beneath it is diminished, and the air being thus compressed, forces its way through the lateral valve into the lower compartment of the cask, which compartment being previously full of water, a portion of the fluid is pressed up through the pipe into the upper apartment. The same result ensues each time that the stroke is repeated, so that the lower compartment soon becomes filled with air, which is retained by the cock until its discharge by the blow pipe is necessary. Dr. Hare, in his oxy-hydrogen blow pipe did not mix the gases in his gas reservoir, but supported the flame of the hydrogen by a current of oxygen issuing from different jets. Subsequently, it was found that the heat produced was materially affected by the proportions in which the gases were mixed, and that the greatest intensity of heat was obtained by two volumes of hydrogen united with one of oxygen; and various attempts were made to mix and burn the gases in their due proportion, but with little success, until the important im provement effected in the instrument by Dr. Clarke, Professor at Cambridge.
This improvement consisted in first mixing the gases in a bladder, in the exact proportions to form water, and afterwards condensing them in a strong iron chest, by means of a condensing syringe. To an opening at the end of this chest he attached a great number of layers of fine wire gauze, through which the mixed gases were driven by their elastic force into a small tube, at the end of which they were inflamed. By this arrangement he obtained a much greater heat than had been effected by Dr. Hare's invention, and was enabled to make a great number of experiments highly interesting to science. Unfortunately, however, for the general adoption of his plan, it was soon found that his instrument was unsafe to use; that the wire gauze would not prevent the explosion of the gases; that in several cases, when used by the most experienced and cautious operators, the instruments were burst. The explosions were tremendous, and resembled the bursting of a bomb, the fragments of the iron chest being scattered with great force in all directions. After trying various plans to render the invention safe, the Doctor, as a protection, had the iron chest placed behind a brick wall at the back of the operator, the gases being conveyed through a tube passing through the wall.