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Combustion has already been defined in a restricted sense as oxidation proceeding with such vigour and the liberation of so much energy, that heat and light are emitted. The term combustion is now applied broadly to any reactions in which heat and light are evolved, irrespective of whether or not oxygen is present. Thus it is correct to speak of the combustion of hydrogen in chlorine, of phosphorus in bromine, or of copper in sulphur vapour. But owing to the fact that our atmosphere contains large supplies of oxygen, it is evident that by far the greatest number of examples of combustion in ordinary occurrence are due to oxidation. Combustion implies chemical change; it does not include such purely physical phenomena as occur, for example, when electric discharges are passed through Geissler tubes.

If oxidation is accompanied by the evolution of a small amount of heat only, and no light, it is frequently termed slow combustion. The term is not altogether an appropriate one, for although in most cases the oxidations referred to may be really slow, this is not always the case. A familiar example is afforded by nitric oxide, which readily combines with oxygen to yield the peroxide. The reaction is rapid and exothermic, a marked rise in temperature being observable.

2(NO) + (O2) = (N2O4) + 40,500 calories.

A pretty lecture experiment consists in admitting oxygen to a large glass bell jar filled with nitric oxide, and containing the bulb of an air thermoscope. As the brown fumes are formed, the thermoscope registers a sharp rise in temperature.

Accepting, however, the extended use of the word "slow" in this connection, the reaction may be described as a good example of slow combustion. The term slow combustion is usually limited to cases of oxidation. Thus, for example, the slaking of lime is accompanied by considerable heat evolution:

[CaO] + Aq. = CaO . Aq. + 18,330 calories.

Such a reaction, however, is not generally regarded as an example of slow combustion. Slow combustion is usually facilitated by the presence of a solid phase which may be the combustible substance itself - as in the case of phosphorus - or even an inert substance, such as porcelain, when the combustibles are gaseous only. This is well illustrated in the slow combustion of hydrogen, which is dealt with in the sequel. When slow combustion is accompanied by decided luminosity it is termed phosphorescence. The term luminescence includes all kinds of light emission whether purely physical, as in Geissler tubes, or chemical. But the term phosphorescence is conveniently restricted to chemical luminosity. It is not an exceptional phenomenon, but an intermediate stage between typical slow and rapid combustion. The best known example, of course, is that of phosphorus; but under suitable conditions sulphur, arsenic, and many other substances may be observed to phosphoresce. The reason why it is so obvious in the case of phosphorus lies in the fact that its phosphorescent temperature interval ranges from 7° to 60° C., and thus includes ordinary atmospheric temperatures south of the Arctic Circle. At 60° C. phosphorus catches fire or ignites - in other words, phosphorescence has culminated in rapid combustion.

Had Europe possessed an Arctic climate with a maximum temperature below 7° C., it is possible that the discovery of the phosphorescence of phosphorus might have been long delayed. Upon ignition with a lighted taper the phosphorescent temperature interval would have been so rapidly passed that the phenomenon would not ordinarily be observed.

Phosphorescence, therefore, is a frequent accompaniment of slow oxidation. It does not necessarily imply incomplete combustion. In the case of sulphur, for example, sulphur dioxide is produced just as in rapid combustion, but ozone is produced if the temperature is of the order of 200° C.

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