RAREFACTION

, in Physics, the rendering a body rarer, that is bringing it to expand or occupy more room or space, without the accession of new matter: and it is opposed to condensation. The more accurate writers restrict the term Rarefaction to that kind of expansion which is effected by means of heat: and the expansion from other causes they term dilatation; if indeed there be other causes; for though some philosophers have attributed it to the action of a repulsive principle in the matter itself; yet from the many discoveries concerning the nature and properties of the electric fluid and fire, there is great reason to believe that this repulsive principle is no other than elementary fire.

The Cartesians deny any such thing as absolute Rarefaction: extension, according to them, constituting the essence of matter, they are obliged to hold all extension equally full. Hence they make Rarefaction to be no other than an accession of fresh, subtile, and insensible matter, which, entering the parts of bodies, sensibly distends them.

It is by Rarefaction that gunpowder has its effect; and to the same principle also we owe eolipiles, thermometers, &c. As to the air, the degree to which it is rarefiable exceeds all imagination, experience having shewn it to be far above 10,000 times more than the usual state of the atmosphere; and as it is found to be about 1000 times denser in gunpowder than the atmosphere, it follows that experience has found it differ by about 10 millions of times. Perhaps indeed its degree of expansion is absolutely beyond all limits.

Such immense Rarefaction, Newton observes, is inconceivable on any other principle than that of a repelling force inherent in the air, by which its particles mutually fly from one another. This repelling force, he observes, is much more considerable in air than in other bodies, as being generated from the most fixed bodies, and that with much difficulty, and scarce without fermentation; those particles being always found to fly from each other with the greatest force, which, when in contact, cohere the most firmly together. See Air.

Upon the Rarefaction of the air is founded the useful method of measuring altitudes by the barometer, in all the cases of which, the rarity of the air is found to be inversely as the force that compresses it, or inversely as the weight of all the air above it at any place.