282 EFFECT OF VARYING PRESSURE ON COMBUSTION. burner, under the ordinary pressure of 30 inches of mercury, at 100, the same volume of gas for each diminution of 1 inch in the pressure gave 51 less light; the diminution of light being directly as the diminution of pressure: so that at 14 inches, the light emitted instead of amounting to 100 was reduced to 18'4; and this rate of diminution was found to hold good with the light of hydro-carbon flames generally. On the other hand, the luminosity of a flame may be proportionately increased by augmenting the atmospheric pressure. So rapidly does this effect increase, that an ordinary spirit-lamp which burns in the open air with scarcely any measurable amount of light, and without smoke, becomes powerfully luminous in air at a pressure of 4 atmospheres, and when supplied with air compressed still more, even burns with a smoky flame. The result of these and other carefully devised experiments led to the unexpected conclusion that the combustion of gaseous matter is rendered less perfect in proportion as the density of the atmosphere is increased; and on the other hand, within certain limits, the more rarefied the atmosphere in which flame burns, the more complete is its combustion. No reduction in the temperature of the flame, within certain limits (as low as 14 inches of mercury), is produced by a reduction of the pressure of the surrounding air. The decrease in illuminating power in a rarefied atmosphere is attributed by Frankland, and with strong probability, to the greater readiness with which the oxygen of the atmosphere finds access to the interior of the flame, owing to the greater mobility of the particles of the air under diminished pressure. (494) Theory of the Blowpipe.-The temperature of a flame may be very materially increased by augmenting the activity of the combustion, and concentrating its effect by diminishing the extent of surface over which it would otherwise take place. It is upon this principle that all blowpipes act: a jet of air or of oxygen is thrown into the interior of a flame; the combustion is thus rendered more rapid, it is limited to a much smaller space, and is entirely changed in character. The mouth blowpipe is one of the most valuable and portable instruments of research which the chemist possesses: he is enabled by its means, in a few minutes, to arrive with certainty and economy at results which without its aid would require much expenditure both of fuel and of time; and it often affords information which could be obtained in no other way. The mouth blowpipe consists essentially of a bent tube, THEORY OF THE BLOWPIPE. 283 FIG. 318. terminating in a fine uniform jet, with a chamber for the condensation of moisture from the breath. A very convenient form of the instrument is shown at fig. 319. It consists of a conical tube of tin plate, about eight inches long, open at the narrow end, a, which is rounded off so as to adapt itself to the lips, and closed at its lower end, from the side of which projects a brass tube, b, about an inch in length, upon which is fitted a small brass jet. This jet is inserted to a short depth into the flame of a candle, about an eighth of an inch above the wick; when a current of air from the blowpipe is directed horizontally along the surface of the wick, the flame loses its luminosity, and is projected laterally in the form of a beautiful pointed cone, in which three parts are distinctly discernible (see fig. 319). In the centre is a well-defined blue cone; outside that is the brilliant part of the FIG. 319. flame, terminating at a, and exterior to that is a pale yellow flame, c. The different parts of this flame possess very B different properties. The blue cone is formed by the admixture of air with the combustible gases rising from the wick, and it corresponds to the blue portion, a, of an ordinary flame, fig. 317. In this part of the flame combustion is complete, and the oxygen introduced by the jet is in excess: the points where the excess of oxygen is absorbed by combination with fresh portions of the combustible vapours which are constantly rising from other parts of the wick, are clearly defined by the surface which seems to limit the blue cone. In front of this blue cone is the luminous portion, containing unburnt combustible gases at a high temperature, which of course have a powerful tendency to combine with oxygen. If a fragment of some metallic oxide, such as oxide of copper, sufficiently small to be completely enveloped by the luminous portion, be introduced into this part of the flame, the oxide will be deprived of oxygen, in consequence of the superior attraction of the hot gases for this element, and the oxide will be reduced to the metallic state : hence this portion is termed the reducing flame of the blowpipe. At the apex, c, of the flame, the effects are reversed. Here, atmospheric oxygen at a high temperature is mechanically carried forward along with the completely formed products of combustion, and a fragment of any readily oxidizable 284 USE OF THE MOUTH BLOWPIPE. metal, such as lead, copper, or tin, if placed at this point will quickly become coated with oxide; and hence this spot is termed the oxidating flame of the blowpipe. A good illustration of the opposite actions of these contiguous portions of the flame is afforded by the effects which they respectively produce on a piece of flint-glass tubing. The silicate of lead contained in the glass is partially decomposed in the reducing flame, and the glass at this point becomes black and opaque from the reduction of the oxide of lead to the metallic state; but by placing the blackened part for a few seconds in the oxidating flame, oxygen is again absorbed by the metal, and the transparency of the glass is restored. (495) Use of the Mouth Blowpipe.-The art of maintaining a continual blast by the mouth blowpipe is not easily described, but it can be acquired by practice without much difficulty. When a substance is to be examined by the blowpipe, it may be first heated alone in a small glass tube, in order to observe whether it melts or decrepitates, or is volatilized wholly or partially. It may next be heated in a narrow tube open at both ends, to ascertain whether it burns, or changes colour, or emits any odour. It should then be ascertained whether it is reduced to the metallic state, and if it be reduced, what is the colour of the metal; whether it fuses easily, or whether it is brittle, crystalline, or malleable. These observations upon reduction may be best made when the globule is exposed to the flame upon a disk of charcoal, which may be conveniently supported, as proposed by Mr. Griffin, in the manner shown at 1, fig. 320, which represents an edge view of a slip of tin plate, about 8 inches long and half an inch wide, bent at one end so as to hold the charcoal disk. No. 2 shows it in front. The charcoal should be sawn into slices about the third of an inch in thickness, so as to present a surface across the grain. A small cavity should be formed upon the upper surface of each disk for the reception of the fragment of material under examina FIG. 320. 2 FIG. 321. tion, which should be about the size of a pin's head, or a grain of mustard-seed. Sometimes when the substance is not easily reducible, a platinum wire bent into a hook at one extremity forms a more convenient support, as shown at fig. 321. It may by this means be ascertained whether the substance imparts any colour to the flame; whether the body, if it be fusible, yields a transparent, an opaque, or a coloured bead; whether any change be produced in the substance, according as it is heated in the reducing or in the oxidating flame. The employment of certain fluxes often aids the judgment of the operator by the colour or appearance thus produced. The most important of these fluxes are borax (592), microcosmic salt (621), and carbonate of sodium. When either borax or microcosmic salt is used, a platinum wire forms the best support; but when carbonate of sodium is employed, especially for the purpose of reducing the metals, a support of charcoal is required. Different forms of the blowpipe have been proposed, according to the purposes for which the instrument is destined. The glass-worker usually requires a large supply of air to be main tained uninterruptedly for long periods, and he commonly employs a pair of double bellows, worked by the foot. A portable blowpipe for glass-working may be made as follows:-A rectangular box of zinc, fig. 322, about 14 inches high and 6 wide, is divided into two chambers, c and d, by a diaphragm which passes obliquely nearly to the bottom of the box; these chambers communicate with each other below; one of them, d, is open above, and is covered with a loose lid ; the other chamber, c, is closed at the top: a blowpipe jet, e, passes just FIG. 322. a through the covering of this chamber, which is further supplied with a longer pipe, a b, passing down to within a short distance of the bottom, covered with a flap of silk to prevent the return of the water in case the operator should suddenly cease to blow through a. If the box be now partially filled with water, the pressure of the column of liquid will expel the air through the jet, e, in any desired direction. By blowing down the long pipe, the operator can renew the supply of air as often as may be necessary; it bubbles up into the closed chamber, c, driving the water 286 WATER BLOWPIPE-OIL-GAS. back into the open one, when the column of liquid, by its pressure, renews the blast as before. The gas-burner, f, can be raised or lowered as may be necessary, and by means of a sliding joint, g, can be made to approach towards or recede from the jet, e, as may be required. An oil-lamp may be used when gas is not at hand; it has, indeed, the advantage of giving a more intense heat than gas, and it is less likely to reduce the oxide of lead contained in flint glass. Where a very intense heat is required, a spirit-lamp, or gasflame, through which a current of oxygen from a gas-holder is directed, may be employed; and occasionally, in cases where a still stronger heat is requisite, recourse may be had to the oxyhydrogen blowpipe, in which, owing to the complete intermixture of the two gases, the flame is solid, and therefore of small dimensions. 8 8 8 = 56): (496) OIL-GAS, Tetrylene or Butylene (H, or C.H Sp.Gr. of Gas, 1854; of Liquid at 54°, 0·627; Mol. Vol. This compound was discovered by Faraday to be one of the constituents of the gases obtained by the destructive distillation of oil. It is almost insoluble in water, but is taken up freely by alcohol, and still more abundantly by oil of vitriol. Oil-gas is condensed at o° F. to a colourless liquid; the gas itself is colourless, and burns with a white, powerfully luminous flame. It contains the same proportions of carbon and hydrogen as olefiant gas, but the two elements are condensed in oil-gas into half the bulk which they occupy in olefiant gas. One volume of this gas requires 6 times its bulk of oxygen for its complete combustion, 4 volumes of steam and 4 volumes of carbonic anhydride being the products. Consequently its composition may be thus represented :— § III. COMPOUNDS OF CARBON WITH OXYGEN. (497) Besides carbonic oxide and carbonic anhydride, carbon forms several other oxides, which are not known in the separate form, but which, when united with the elements of water, possess acid characters: viz., |