"In the eighteenth century... the word Caloric was introduced to signify heat as a measurable quantity. ...but the form of the word accommodated itself to the tendency of the chemists... to seek for new 'imponderable substances,' so that the word caloric came to connote... heat as an indestructible imponderable fluid, insinuating itself into the pores of bodies, dilating and dissolving them, and ultimately vaporising them, combining with bodies in definite quantities, and so becoming latent, and reappearing when, these bodies alter their condition."