Products of Combustion. 525 flame. Bunsen, by a series of ingenious experiments, has arrived at the following results respecting the temperature of flames ('Phil. Mag.' Aug. 1860, p. 92). The temperatures assigned by him are those of the Centigrade thermometer, of which 5° are equal to 9° of Fahrenheit, plus 32° for the difference of the zero :— The heat of the electric current far surpasses all these temperatures, and is at present undeterminable in its degree, by any known process. Products of Combustion. 742. In ordinary combustion the consumption of oxygen is very large, and the vitiation of the air in an enclosed space by the diffusion of the products, very rapid. The writer has found that a common candle, with a thick wick, requires for conduction two gallons of air per minute. This represents 110 cubic inches of oxygen. It will be seen from this how rapidly the air is consumed by the burning of candles or lamps in the deep shafts of unventilated mines. The respiration of an adult consumes the air in like proportion. The consumption of air by the burning of gas, is even larger and more rapid. A cubic foot of coal-gas requires three cubic feet of oxygen for its combustion. This is equivalent to fifteen cubic feet, or ninety-four gallons of air. An ordinary gas-burner will consume this quantity in a quarter of an hour. All the oxygen is not removed under these circumstances, but the air is rendered unfitted for the purposes of breathing or for further combustion. Heating or warming by gas is, therefore, highly injurious, unless there is a free supply of air and provision is made for carrying off the resulting products. The products of ordinary combustion in oxygen are chiefly carbonic acid and water. These are quite unfitted to sustain combustion, and unless removed as they are produced, they speedily arrest the process. In the burning of coal or coal-gas, the production of sulphurous and nitric acids is an additional source of noxious impurity. A burning candle or a jet of coal-gas is immediately extinguished in carbonic acid, but carbonic acid will serve as a medium for the combustion of certain substances when heated to a high temperature. Thus the metal sodium burns with great 526 Removal of Products of Combustion. brilliancy in a current of pure carbonic acid, the gas becoming in this case as much a supporter of combustion as oxygen itself. These exceptional conditions, of course, admit of explanation on chemical principles. They show that there is no real distinction between a product and a supporter of combustion. 743. In combustion in air, the oxygen alone is consumed, the nitrogen is set free, and mixes with the carbonic acid produced at the expense of the oxygen. Air is therefore rapidly contaminated by this process; and in a confined space, the nitrogen and carbonic acid, as a result of the heat evolved, accumulate in the upper part of the vessel or apartment. Neither of these gases is respirable, and neither will support ordinary combustion. The following experiments will illustrate the deterioration of air under these circumstances. Fix three wax tapers to a stout wire placed upright, and about three feet in height, so that one is at the upper part, one at the lower, and the third in the middle. Light the tapers, and invert over them a tall stoppered shade, leaving a slight space for the entrance of air below. The rapid accumulation of deoxidized air (nitrogen) and carbonic acid in the upper part of the shade, will be indicated by the early extinction of the upper and middle tapers, while the lower one will continue to burn. If, when the lower taper is burning dimly from impurity of the air, the stopper is removed from the shade, a current of air is immediately set up, the gaseous products of combustion are carried off, and the lower taper will burn with a still brighter flame. This experiment establishes the necessity for a rapid removal of the products of combustion, and the results are equally applicable to the contamination of air by the respiration of animals. 744. When combustion takes place in rarefied air, as when a candle is placed under a receiver from which the air has been partially removed by the air-pump, the flame is elongated, becomes less luminous, and is soon extinguished. According to observations made by Dr. Frankland, in 1859, on the summit of Mont Blanc, it appears that at this elevation the amount of combustible consumed is as great as at the level of the sea, although the light emitted by a burning candle is considerably less. same room. 745. But combustion may go on at one part of a closed room while the burning substance will be extinguished in another part of the Fix in the stoppered aperture of a bell jar, by means of a closely-fitting cork, a glass tube, about an inch in diameter. The tube should rise several inches above the level of the jar, and Principles of Ventilation. 527 should reach on the inside to within two inches of its base. Mount in a plate two pieces of wax taper, one sufficiently tall to reach nearly to the top of the jar when placed over it, the other so short, that when ignited, the point of the flame only, will be enclosed by the open end of the glass tube fixed in the jar. Light the tapers and invert the jar over them, not pressing it down closely at the base. The tube should be so adjusted to the short taper, as to act like a chimney to it, care being taken that it is not touched by the flame. In a short time, if the cork is well fitted, the tall taper will be extinguished, but the short taper will continue to burn. In the one case, the products of combustion are not carried off, in the other they are, and the supply of air is continually renewed. As a proof of this, if we hold over the chimney-tube a small gas-jar, the deposition of water on the glass will be apparent, and after a time, the presence of carbonic acid may be proved by pouring lime-water into the jar. (The production of carbonate of lime will be indicated by a milky appearance of the lime-water.) The proper ventilation of apartments, implies a strict adjustment of the supply of pure air for combustion and respiration, and a provision for the complete removal of the products as they are formed. 746. The subjoined engraving (fig. 179) will serve as an illustration of these principles. Two tall chimney-glasses, A B, are fitted closely to holes in the top of a small box, which is made quite airtight, so that no air can enter or escape except by the chimney-glasses. A short piece of wax taper is lighted and placed in the glass, B. It burns readily by reason of the air derived from the box, and continuously supplied by a descending current through the glass, A. The consumed air escapes by B, as indicated by the arrow. If A is closed with a glass plate, the taper B Fig. 179. in B will be soon extinguished owing to the non-supply of air. If B is closed in a similar manner,—the taper is also extinguished by reason of the non-escape of the products of combustion, carbonic acid, and aqueous vapour,-the carbonic acid collected in B rapidly 528 Combustion and Deflagration. arresting combustion. Nitre-paper in smoky combustion, brought near the top of A, indicates the current downwards by the course of the smoke, while when placed over B the smoke is carried upwards. By breathing gently into the glass, A, or bringing near to it a jar containing carbonic acid, the taper will be extinguished. On the other hand, a jar of oxygen brought near to it will cause the taper to burn more brightly. The figure serves, also, to illustrate the method by which mines are ventilated. B represents what is called the upcast shaft, in which a fire is kept constantly burning, while A corresponds to the downcast shaft, which serves to supply fresh air to all the workings. Combustion by Oxygen salts.-Deflagration. 747. It is not, in all cases, necessary that oxygen should be free, or in the gascous state, in order that combustion should take place. Saline compounds which contain oxygen, such as the nitrates and chlorates, when mixed with combustible substances, such as sulphur and charcoal, give rise to combustion of the most intense kind, often amounting by its suddenness to explosion. Nitre, charcoal, and sulphur are the constituents of gunpowder. When mixed in certain proportions and heated to about 500°, the solid compound is suddenly converted into gaseous and vaporous matter, expanding into two thousand times its volume and rending and destroying all substances with which it is in contact. Gun-cotton presents another illustration. The fibre of cotton is here incorporated with the elements of nitrous acid, and the oxygen, which forms a large proportion of them, is sufficient to cause the sudden conversion of the whole of the carbon and hydrogen in the cotton, into water and carbonic acid. Finely powdered charcoal, mixed with an equal portion of powdered nitrate or chlorate of potash, burns, when heated, with great violence, giving rise to the phenomena of deflagration. A mixture of twenty-eight parts of ferrocyanide of potassium, twenty-three parts of white sugar, and forty-nine parts of chlorate of potash, is known under the name of "white gunpowder." In combustion it produces a large amount of gaseous matter, consisting of nitrogen, carbonic acid, carbonic oxide, and aqueous vapour. It is a dangerous compound to prepare or even to preserve. This composition, when dry, is exploded by friction or percussion, by heat, or by the contact of concentrated sulphuric acid. 748. Combustion of vapours.-Slow combustion.—When atmo. : spheric air, mixed with certain inflammable vapours, as ether, is allowed to be in contact with a moderately heated body, a very slow union may take place without flame. This phenomenon has been called invisible combustion. It is remarkably exemplified on placing a small coil of platinum wire, moderately heated, in such a mixture the combination of the vapour with air then goes on in the immediate vicinity of the hot wire, with sufficient disengagement of heat to maintain the wire in a feebly luminous state, at a low red heat. Thus the vapour always arising at a common temperature from the open mouth of a glass containing ether, if made to pass through a red-hot coil of platinum wire, will, while combining with the oxygen of the air, give out heat enough to keep the wire red-hot so long as the vapour lasts. A ball of finely divided platinum (spongy platinum), mixed with clay and dried, has a similar property. If heated to redness and then placed on the wick of a spirit lamp, it will remain glowing, so long as any vapour of spirit is evolved. This has been called Sir H. Davy's aphlogistic lamp. All liquids which evolve vapour and do not block up the pores of the platinum, operate in a similar manner. Even solids which emit vapour, undergo this kind of combustion. Thus a platinum ball heated and placed on a block of camphor continues to glow, and causes a slow combustion and volatilization of the camphor. "Fuel." 749. HEAT being, in the sense already explained, the life of the universe, and man having command over nature chiefly by his power of controlling heat, as a result of combustion, it is of interest to inquire what substances can be employed economically as fuel for producing and maintaining artificial heat. The all-important substance, oxygen, forms part of our atmosphere, and therefore penetrates and is present wherever man can breathe, ready at once for his service. Then for the purpose of combining with the oxygen, there are chiefly two other substances also very abundant in naturenamely, carbon and hydrogen, which are the great materials of all things of vegetable origin, as trees, bushes, grasses, &c., and therefore of coal-beds, many of which are evidently the condensed remains of forests which existed in remote past time. Carbon is found nearly alone in the hard coal called anthracite, or stone-coal, but it is united with a large proportion of hydrogen in caking or bituminous coal, and in wood, resins, tallow, and oils, including the rock oils of Canada and Pennsylvania. The mixture of inflammable gases |