COMBUSTION is the term applied to the process of burning, which usually consists in the oxygen of the air uniting with the constituents of the combustible substance. Thus, the C. of coal is due to the oxygen of the air passing into a state of chemical union with the carbon and the hydrogen of the coal, forming carbonic acid and water-vapor (HO). Such chemical combinations are always accompanied by the production of more or less heat, as in the case of decaying wood and other vegetable matter; but it is only when the action is so rapid as to evolve intense heat accompanied by light, that the process is called burning or combustion. Though the gaseous oxygen lies as much to do with the process as the more solid material, coal, wood, paper, or cloth, yet the latter is alone styled the combushble or burning body, whilst the oxygen is invariably named the supporter of combustion. A few substances burn at ordinary temperatures, such as phosphorus, which glows when exposed to the air; but the generality of sub stances, such as wood, coal, etc., require to be raised in temperature or be set fire to before they possess the power of combining with the oxygen of the air. The amount of heat given out by the various combustibles wheal burned, is capable of being measured, and is definite. The same weight of the same combustible invariably evolves the same amount of heat during its complete C.; but different combustible substances give off different amounts of heat. The mode in which the heat evolved may be measured, is either (1.) To observe the quantity of ice which a given weight of the combustible will melt when burning; (2.) To noticcithe weight of water which the combustible will con vert into steam; or (3.) To estimate the number of pounds of water which the burning body will raise from 32° to 212° F. The last plan is the more easily managed and accu rate, and serves as the index in the following table, which gives the number of pounds of water raised from 32° to 212° F. during the C. of one pound of each of the burning bodies: Charcoal, pure 78 lbs. of water.
" from wood 75 dried... 36 41 " undried .. 27 Coal, bituminous 60 14 Turf and peat 25 to 30 41 Aaohol 674 Olive oil, wax, etc 90 to 93 Ether • 80 Hydrogen ... ....... 2361 The amount of heat evolved appears, however, to be proportional to the quantity of oxygen required to burn the various combustibles. Thus, when a similar volume of oxygen gas, or even ordinary air, is allowed to flow against the various combustible sub stances, the following results are obtained: One lb. oxygen combining with Raises from 82 to 212 degrees F.
hydrogen 29i lbs. of water. Charcoal ... 29 Ether Alcohol 28i While the absolute amount of heat evOlved (hiring the C. of 'any burning body is the same, yet the sensible heat may vary according to the rapidity of the process. Thus, when phosphorus is exposed to the air at ordinary temperatures, it very slowly combines with oxygen, and gives out little heat at any one moment, but it is diffused over a great length of time; whilst if the phosphorus is set fire to in the air, it burns vividly, and gives out much heat and light for a short time; and still further, if the burning phos phorus be placed in pure oxygen, it enters into most vivid C., and evolves a most intense heat and brilliant light for a still shorter time. In the latter instances, the heat evolved at any one moment is greater, because more rapid, than that given off at the same time during the slower process of C. ; but when allowed to proceed to a termination, there is as much heat produced during the whole time occupied in its development The same remark applies to the coal placed in a furnace. So long as the door of a furnace is open, and there is little draft of air through the fuel, a moderate amount of heat is evolved, which may last for several hours; but when the door is shut, and much air is drawn through the coal, the latter is more quickly burned, and more heat is evolved during a shorter period of time than before, but in the long-run there is the same amour of heat evolved.