How oxygen supports combustion no Elementary Treatise explains; but leaves it to be imagined, that oxygen is somewhat of a burnable nature, or that it generates heat when blown into a fire. The fact is, it supports combustion only mechanically. The centrifugal pressure, by the medium of space, decomposes the fuel; electric matter, entering the ignited fuel, displaces medium of space, and the fire goes out; oxygen prevents the entrance of electric matter, and permits the medium of space to enter the fuel freely, the pressure from without gives centrifugal force. In this twofold manner of service oxygen promotes the continuance of the kind of decomposition known as combustion. A live coal is greatly _deficient_ of electric matter; when just fallen from the fire it is said to be red and hot, after a few minutes black and cold; all of which are but mental effects. On the hearth the coal acquires electric matter from the air, which displaces medium of space, and becomes extinguished; so would the fire were there no oxygen in the surrounding air. Hence it would seem, that the interstices of oxygen are too diminutive for electric matter to enter, but are sufficiently large for those of the medium of space to pass through, thence into the fuel. Should the utility of the nitrogen of oxygen in combustion be questioned, because nitrogen alone puts an end to the combustion of a candle; it may be answered, that, as the imponderable element of oxygen air, from being highly evanescent, is not obtainable without the nitrogen, and as by the service of both together combustion is increased, so may both be considered supporters of combustion; the grosser element serving as a carrier to the minor, and, as it were, giving it momentum sufficient to penetrate beyond the surface of the half-decomposed, or previously ignited fuel. COMBUSTION. A piece of wood, like everything else when placed within the medium of fire, suffers de-electrisation and acquires medium of space: this twofold procedure continuing, the wood becomes split or burst asunder, and its elements gradually forced out by the centrifugal pressure; some of which are precipitated, some contribute to the medium of fire, others are recombined differently and exist for a short space of time as flame, and others, with matter from the air, form soot. Such is the most rational theory of combustion, consistent with the _inertia_ of matter and the absence of heat. Friction rubs away electric matter, percussion forces it out, combustion and ignition follow, and without being promoted by either heat or fire. The kindling matter of a coal-laid fire requires the de-electrising spark at first, and the de-electrised kindling de-electrises the coal; the wood fire, effected by means of friction, is independent of even the spark of fire for its commencement, from having been otherwise de-electrised at first. Within the fire, one part de-electrises another, and the centrifugal pressure decomposes the whole.