Physics, the act by which a body is rendered rare; that is, brought to occupy more room, or appear under a larger bulk, without accession of any new matter. This is very frequently the effect of heat, as has long been universally known. In many cases, however, philosophers have attributed it to the action of a repulsive principle. But, from the numerous discoveries concerning the nature and properties of the electric fluid and fire, there is the greatest reason to believe that this repulsive principle is no other than elementary fire.