HARD BODIES, such bodies as are absolutely inflexible to any shock or collision whatever.
This is the common meaning of the term ; but Huygens, by hard bodies (corpora dura) meant what others call per fectly elastic bodies: for he thus expresses himself: " Qme cunque sit causa, corporibus duris, a nmtuo contactu resiliendi cum se invicem impinguntur : ponimus, cum corpora duo inter se :ego:Ilia, celeritate, ex adverso ac directe sibi mutuo occurrent, resilire utrumque eadem qua advenit cele ritate." Huyg. De Motu Coy. ex Percuss. Hypoth. 2. But this hypothesis is consistent only with perfect elasticity, and not with the common supposition of hardness or inflexi bility, which produces no resilition. The laws of motion flar hard bodies are the same as for soft bodies, and these two sorts of bodies might be comprised under the common name of unelastic.
Some who follow Leibnitz's doctrine, concerning the measuro of the moving force of bodies, deny the existence of hard or inflexible bodies. And it is so far true. that no
experience ever taught us that there are any such. The hardest bodies to appearance do not preserve their figures in collision, such bodies being only elastic, yielding to the shuck, and then restoring themselves.
M. Bernoulli goes so far as to say, that hardness, in the vulgar sense, is impossible, being contrary to the law of continuity. For supposing two such hard bodies of equal masses, and with equal velocities, to meet directly, they must either stop or return after the collision. The first supposition is commonly admitted ; but then it follows, that these bodies must instantaneously pass from motion to rest, without going through successive diminutions of their veloci ties till they stop : but this is thought to be contrary to the fundamental laws of nature. Bence this author rejects per fectly solid and inflexible atoms, which others think a conse quence of the impenetrability of matter.