Combustion, a chemical process, in which bodies combine to form a new compound, with the evolution of heat, and usually, light. In ordinary cases of combustion, oxygen is one of the combining bodies, and the substance with which it unites disappears with it in a gaseous form. It was formerly regarded as an essential element in combustion, but the phenomena of light and heat, characteristic of rapid combustion, are observed when chlorine combines with phosphorus and with some metals when these are in a powdered state; also in the action of cyanogen and potassium, and of sulphur upon iron filings and copper leaf. Some bodies also burn in the vapors of iodine, bromine, and fluorine. As commonly witnessed, combustion is a process taking place in the presence of atmospheric air, which furnishes the oxygen to support it; and it is conducted, not, as most other chemical operations are, for the sake of its products, but for the phenomena which attend it. The development of these depends upon the rapidity with which it goes on, and this distinguishes it from the other cases of oxidation, which are sometimes called slow combustion, as when metals rust, spirituous liquors turn to acetic acid, and the blood by respiration is oxidized.
The views which prevailed respecting the nature of combustion before the discovery of oxygen gas by Priestley in 1774, and the development of its properties by Lavoisier in the succeeding year, were necessarily incorrect. With the ancients, fire was an element that devoured other bodies and converted them into itself. Dr. Hooke in 1665, and Mayow soon after, advanced the opinion that there existed in the air a "nitrous spirit," which dissolved bodies susceptible to its influence when their temperature was suitably raised, and that the light and heat were the sensible effects of the rapid motions taking place. This theory, though close to the truth, was for a long time lost sight of in the general acceptance of the celebrated phlogistic theory, which was advanced soon afterward by Becher, professor at Mentz, and ably sustained by Stahl, professor at Halle. They considered that in combustion a certain element, which Stahl named phlogiston (from Gr. to burn), left the burning body, and the product was this body deprived of its phlogiston, by regaining which it was restored to its original character; as sulphur or phosphorus when consumed became sulphuric or phosphoric acid, and on regaining their phlogiston were again sulphur or phosphorus. It was known that the earthy "calx," into which some of the metals were transformed by fire, gained rather than lost weight; but this was explained by attributing to phlogiston a principle of levity. Notwithstanding the defects of the theory, it was still an important step in the progress of chemical science, serving first to group correctly together the phenomena of combustion, acidification, and respiration. Its nomenclature was incorporated with the science, and when Priestley made his great discovery of the new kinds of air, he gave to nitrogen, which he supposed to be a combination of air with the phlogiston of the combustible, the name of phlogisticated air, and to oxygen or pure air that of dephlogisticated air.
Lavoisier, by subjecting the products of combustion to the test of weighing, showed that the combustible gained weight by the process, and he proved, on restoring it to its former condition (as in the case of a metallic oxide), that the substance taken up and given out again was the pure air of Priestley, to which he gave the name of oxygen, from its acidifying property l and ). Thus was established the antiphlogistic theory, that in every case of combustion oxygen combines with the burning body. Dr. Black's theory of latent heat was adopted to account for the production of light and heat; the latter being evolved or rendered sensible when substances without change of form pass from a rarer into a denser state, also when a gas becomes liquid or solid, or a liquid solidifies. The oxygen of the air was supposed to contain heat and light in a latent state, which were evolved with its change into a more condensed form, and the products of combustion were supposed to have less combined or specific heat than the original substances. But this application failed in the case of combustion of solid bodies by explosion, the gaseous compounds expanding in some instances to 2,000 times their original bulk, and yet producing intense heat instead of cold, as the theory would require; and the specific heat of the new compounds, in this as in the combustion of charcoal, it was shown by Dulong and Petit, was often quite equal to, and sometimes exceeded, that of the combining bodies, and this, moreover, bore no relation to that evolved in combustion.
Davy considered that the burning body and the supporter of combustion were in opposite electrical conditions, and that the heat and light were evolved in the discharge of these electricities; which view was also held by Berzelius, though unsustained by any positive proof. Despretz ascertained the number of pounds of water which the burning of 1 lb. of different combustibles would heat from the temperature of 32° to 212° F. The following are some of his results:
1 lb. of
Lbs. of water.
Charcoal from wood.....................
Wood containing 20 per cent, of water___
25 to 30
Olive oil, wax, etc........................
90 to 95
Carbon and hydrogen are the two common elements, which by uniting with oxygen produce combustion. They are furnished in a variety of forms suitable to this application, the source of all which is traced to vegetable growth; and this ever continues to gather up the products of combustion, and, separating them by decomposition, places them again in condition to renew the process. In combination they assume a volatile form, and float upward with the air rarefied by the heat, thus allowing the admission of fresh supplies of oxygen to constantly reach the ignited body. Though the combustible bodies are enveloped in the atmospheric air, and are ever disposed to unite with its oxygen, the process cannot commence until the temperature of the combustible has been raised to a certain point, when it is said to catch fire; the process thus begins, and afterward evolves the heat necessary for its continuance. - The condition of the air as to temperature, density, and the presence of aqueous vapor, variously affects the process of combustion. Increase of density adds to the quantity of oxygen in a given volume, and consequently may be expected to increase the rate of combustion. The effect of temperature is less understood, but so far as it diminishes the density of air it must retard combustion.
A sensible difference is perceived in the rate of combustion of large fires connected with metallurgical operations in summer and winter, which is no doubt correctly attributed to the volatile products of combustion not so freely quitting the burning bodies to rise up in the rarefied air of summer as in the denser winter atmosphere, and thus retarding the operations. Aqueous vapor in some circumstances is found to retard combustion, in others to accelerate it. Unless subjected to the degree of heat necessary to decompose it, it takes the place of atmospheric air, and diminishes the proportion of effective oxygen present. Steam is employed as an active agent for extinguishing fires, and also in small quantity to increase their effect. For this purpose a jet of steam is discharged under the grate bars of a furnace, or the ash pit is made a reservoir for water, which is evaporated by the heat radiated downward, and the vapor carried up by the draught is decomposed in passing through the incandescent coals. Its oxygen takes up a portion of carbon, forming carbonic oxide, which, as it meets more oxygen, is converted into carbonic acid gas with the production of much heat.
So its hydrogen seizes a portion of the highly heated carbon, and is converted into carburetted hydrogen, or in part escapes, till meeting an equivalent of oxygen it is burned with the reproduction of water. It was shown by the numerous experiments of Bunsen and Fyfe that a considerable increase of heat was thus gained over that consumed in the decomposition of the vapor. Its use,' however, in practical operations, demands the exercise of some judgment; for in excess, or with insufficient supplies of air, its effect would be the reverse of that intended. So also the vapor should be made to come up through the bars, and not be raised from among the coals at the cost of a portion of the available heat generated by their combustion. - An opinion has long been current, and not among the unlearned alone, that combustion was retarded by the light of the sun shining upon the fire. This apparent effect is accounted for by some on the principle that all flames are less visible in a strong light. On the other hand, a series of experiments made by Dr. Thomas McKeever of England, and published in the "Annals of Philosophy" in 1825, support by their results the popular impression, and these conclusions are referred to by Gmelin in his "Handbook of Chemistry," without questioning their soundness.
In these experiments tapers and candles were burned alternately in a dark room and in the sunshine in the open air, the result always being a more rapid combustion in the former. The chemical rays of the solar beam were supposed to interfere with the oxidation of the fuel, and this was confirmed by the apparent greater rapidity with which a taper was made to burn in the red than in the violet extremity of the solar spectrum. In 1857 a paper was read before the American association for the promotion of science, by Prof. J. L. Le Conte, describing a series of experiments recently made by him with the object of further testing this question. In these he adopted the precaution of securing absolute calmness in the atmosphere around the burning body, and of depriving the beam of light of its sensible heat, which might, by rarefying the air, retard combustion. He also by concentrating the rays increased the intensity of the solar light nearly tenfold, with the view of thus exaggerating and rendering more apparent their supposed influence. The cone of sunlight was made to strike upon the flame of a wax candle, counterpoised in a balance, its lower margin illuminating the charred portion of the wick, while the upper boundary of the pencil traversed the flame near its apex.
In each experiment the candle was allowed to burn for 10 or 15 minutes, till a steady flame was obtained; and then, as soon as its weight was reduced to that in the opposite scale, a certain quantity (60 or 100 grs.) was removed from this, and the combustion was continued till the equilibrium was again established. Whether in the dark or in the sunlight, no sensible difference was found in the rate of combustion; but this decidedly varied with the conditions of the atmosphere as to barometric pressure and temperature.