HARDNESS, a quality of certain bodies, which consists in an intimate union and strong adhesion of their integrant parts, which cannot therefore be easily disjoined.—We cannot precisely determine what disposition of parts occasions the greatest hardness; but this probably depends on the perfection and extent of the contact of these parts: and this quality depends essentially upon their figure, which we do not know.—The hardest of bodies which we do know, are vitrifiable stones; and amongst these the hardest are also the most pure and homogeneous, that is, diamonds. Not any one of all known bodies is perfectly hard. This quality belongs undoubtedly to the primary or elementary particles of matter, which of all beings we know the least of.