BLOW PIPE. An instrument for exciting intense combustion upon a small scale; it is extensively used in many branches of the arts, and also in philoso phical 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 pro duced. 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 wholeprocess is under the eye of the observer. In the blow pipes used by enamellers, glass-blowers, and others, the current of air is maintained by a small pair of double bellows.
Early in the present century, Dr. Hare, of Philadelphia, made a most im portant 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 dia meter (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 the air must enter through the lower cavity to restore the equilibrium. When the depressed, the leather being bulged in an opposite direction, the cavity it is diminished, and the air being thus compressed, forces its way through the lateral valve into the lower compartment of the cask, which com partment being previously fill 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 m 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 pro duced 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 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 in strument was unsafe to use ' • that the wire gauze would not prevent the Implosion of the gases; that in several cases, when used by the most experienced and cautious operators, the instruments were burst. The explosions were tre mendous, 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. In this state the instrument remained, until Mr. Goldsworthy Gurney applied himself to its improvement, and after numerous experiments, which are highly interesting, and are fully detailed in his published lectures, he succeeded in producing an instrument unattended with the slightest danger in its use, and admirably adapted both for scientific inves tigation, and for various operations in the arts. The annexed engraving is a representation of the instrument. A is the safety chamber; B a water trough, through which the gas is made to pass from the gasometer D by the cock C, through a tube which reaches to the bottom of the water trough ; E is a cock fitted into the neck of the same, from which it is thrown out should an explosion take place on the surface of the water. F is a gauge, to indicate the necessary height of the column of water in the trough. G is a transferring bladder, which is made to screw and unscrew to and from the stop-cock H, for the purpose of supplying the gasometer with gases, which may be charged and recharged at pleasure, by an assistant, during its action, so as to keep up the most intense flame for any length of time. A. valve is placed between the gasometer and the transferring bladder, which prevents the return of the gas. I I is a light wooden or stiff pasteboard cap, which combines sufficient strength with great lightness, so that in case an explosion of the gasometer should happen, it is merely thrown a short height into the air, by the force breaking the strings which connect the cap to the press board. To these strings are attached small wires, which pass through the table of the instrument, as at L, into the press board below, where they are secured ; this press-board is kept in a horizontal position by the stand, so that when the requisite pressure is given to it, the cap I I is brought to bear equally on the gasometer D. The gasometer bladder (or silk bag) is tied to a piece of bladder, which screws into a long tube laid into and across the table, which permits it to be unscrewed at pleasure from the body of the instrument, and immersed in water when it requires softening, affording also the means of fixing on another bladder, if any accident should render it necessary. The stop-cock of the charging bladder G is fixed to one end of the tube just described, and the stop-cock of the water trough on the other end. To operate with this instrument, pressure by the hand is applied to the press-board, which draws down the cap I I on the gasometer D, and forces the gas which it contains through the stop-cock C, and through the water tube and safety chamber A, to the jet at the end, where it is burned. When the pressure on the press board is too slight, or when the band is taken off, the flame returns into the safety chamber, and is extinguished. When it is required to suspend the ope ration, the hand need only be taken off' the pressing board, the water in the trough acts as a self-acting valve in preventing the escape of gas from the instrument, and saves the necessity of turning the stop-cock. A silk tube is attached to the end of the tube before described, in the water trough, which prevents the splashing of the water, sometimes occasioned by unskilful manage ment. We omitted to state that the safety chamber A is filled with numerous discs of very fine wire gauze closely packed, and should the flame be driven in, which will sometimes happen, it will not enter the bag or reservoir D, but will explode above the surface of the water in the chamber B, merely driving out the cork. An improvement has, however, been since introduced in the con struction of the safety chamber, by Mr. Wilkinson, of Ludgate-hill, by which the retrogade motion of the flame appears to be effectually prevented, and a much larger jet may be employed than heretofore with perfect safety. This im provement consists in filling the chamber A with alternate layers of wire gauze and of asbestos, previously beaten with a mallet, and pulled out to resemble floss silk. Mr. Wilkinson received from the Society of Arts a silver medal, for his communication on the subject, and we understand that Mr. Hemming has recently made some further improvements in the construction of the instrument. We must here advert to the wonderful effects produced by the oxy-hydrogen blow pipe, which almost instantaneously reduces the hardest and most refractory sub stances. Gun flints are instantly fused by it, and formed into a transparent glass; china melts into a perfect crystal. All kinds of porcelain are readily fused, pre viously assuming a beautiful crystallized appearance. Rock crystal is quickly melted, giving out a beautiful light. Emerald, sapphire, topaz, and all the other precious stones, melt before it into transparent glassy substances. Barytes, strontian, lime, and alumina, exhibit very striking and beautiful phenomena. Magnesia fuses into hard granular particles, which will scratch glass. The metals, even plating, are all quickly fused by it; and all descriptions of stones, elates, and minerals, are melted, sublimed, or volatilized, by its all-subduing power.