Now, while the heat of combustion depends only on the chemical nature of the material burned, the rise of temperature caused by it depends to a very great extent on the manner in which the combustion takes place. If other gases, such as the nitrogen of the air, are present without themselves adding to the amount of heat produced, part of that amount goes to heat such gases, and as a result, the temperature is considerably lower than if the same substance were burned in precisely the amount of oxygen gas required for its combustion. The rapidity with which a combustion takes place is another factor on which the temperature depends; for heat may he gradually dissipated by conduction even while it is being produced, and so the actual amount remaining at any moment during a slow process of combustion may be very small. Thus, when phosphorus is exposed to the air at ordinary temperatures, a slow process of oxidation (com bustion) takes place, very little heat being given out at any given moment. If ignited in the air, phosphorus burns vividly, giving out much heat and light for a short time. Finally. if ignited in an atmosphere of pure oxygen, it enters into most vivid combustion. evolving, for a very short time, a most intense heat and a brilliant light. An analogous phenomenon may he observed when coal is burned in a furnace. So long as the door of the furnace is open and there is but little draught through the fuel, the evolution of heat is moderate and may last several hours. But when
the door is shut. and much air is drawn through the coal, the latter is more quickly burned; the temperature is higher because more heat is evolved during a shorter period of time, but in the long run the amount of heat produced is the same.
Since the process of combustion is a form of chemical transformation more striking and more commonly met with than any other process, it early attracted the attention of scientific ob servers. But since, on the other hand, it involves the consumption and formation of gases, it baffled their ingenuity for many centuries. Those light, colorless, aeriform substances had, in the first place, to be discovered; and for a long time they escaped the attention of observers, in spite of their being present everywhere and con stantly interfering with experimental work. Further, the peculiar properties of gases, to gether with the striking appearance of flames and fires, gave birth to the erroneous idea that material bodies are capable of losing weight by combining with certain 'principles' and of gain ing weight by having such 'principles' taken away from them. Thus the phenomena of com bustion long hampered the progress of science. But it was the same phenomena that also finally led to truer conceptions; for their correct inter pretation by Lavoisier formed the cornerstone upon which rests the gigantic structure of the chemistry of to-day. See FOOD; FUEL; THERMO CHEMISTRY ; CHEMISTRY.