ABSTRACT
Hardness is measured according to what will scratch the surface being investigated; this provides a relative measurement. But atoms were supposed to be absolutely hard, so as never to wear away or break in pieces as Newton put it. Such particles could not bounce, because that would involve parts coming together and separating; so elasticity and hardness were seen as opposite qualities, and it was not until well into the nineteenth century that a coherent kinetic theory of gases appeared.
