Page:The American Cyclopædia (1879) Volume V.djvu/141

 COMBUSTION 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 con- nected with metallurgical operations in sum- mer and winter, which is no doubt correctly attributed to the volatile products of combus- tion 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 cir- cumstances 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 di- minishes 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 pur- pose 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 oxy- gen, 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 ex- periments of Bunsen and Fyfe that a consider- able increase of heat was thus gained over that consumed in the decomposition of the vapor. Its use, however, in practical operations, de- mands 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 avail- able heat generated by their combustion. An opinion has long been current, and not among the unlearned alone, that combustion was re- tarded 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 Grnelin in his "Handbook of Chemistry," without question- ing their soundness. In these experiments ta- pers and candles were burned alternately in a dark room and in the sunshine in the open air, the result always being a more rapid combus- tion in the former. The chemical rays of the solar beam were supposed to interfere with the oxidation of the fuel, and this was con- firmed by the apparent greater rapidity with which a taper was made to burn in the red COMBUSTION (SPONTANEOUS) 137 than in the violet extremity of the solar spec- trum. In 1857 a paper was read before the American association for the promotion of sci- ence, 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 inten- sity 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 por- tion 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 baro- metric pressure and temperature. COMBUSTION, Spontaneous, the ignition of in- flammable bodies without the application of fire, and without obvious cause of increase of temperature. Occurrences of this sort, for- merly very mysterious, are now explained by the well understood liability of certain bodies to undergo chemical changes which develop sufficient heat to set them on fire. Recently expressed fixed oils are particularly disposed to oxidize when exposed to light and air. They then absorb oxygen, and give out carbonic acid and hydrogen. If the process goes on rapidly, as it usually does when the oil is diffused through light inflammable substances, as cot- ton, tow, the waste used for lubricating ma- chinery, oatmeal, &c., the heat may be sufficient to set these on fire. This is the most frequent cause of spontaneous combustion. Cloths sat- urated with oil, or covered with varnish of oil and turpentine, have thus been inflamed. By being piled together in quantity, the danger is increased by the accumulation of heat. In consequence of the frequent occurrence of cases of spontaneous combustion in " charged silks," or silks that have been treated with grease or oil for the purpose of increasing their weight, the German railways in 1872 refused to receive them for transportation. Bitumi- nous coal lying in large heaps is liable to be ignited by the heat evolved in the decomposi- tion of the sulphuret of iron which it commonly contains. At the mouths of the pits the slates and refuse coal, which contain the most of this mineral, and in which the process of de-