Leucippus and his associate Democritus hold that the elements are the full and the void; they call them what is and what is not respectively. What is full is solid, what is not is void and rare. Since the void exists no less than the body, it follows that what is not exists no less than what is. The two together are the material causes of existing things. And just as those who make the underlying substance one generate other things by its modifications, and postulate rarefaction and condensation as the origin of those modifications in the same way that these men too say that the differences [in their elements] are the causes of other things. They hold that these differences are threeshape, arrangement and position; being, they say, differs only in "rhythm, touching and turning" of which rhythm is shape, touching is arrangement and turning is position; for A differs from N in shape, AN from NA in arrangement, and Z from N in position .
Aetius, I, 3, 18 (from Kirk et al., p. 421)
Democritus named two [properties of atoms], size and shape; but Epicurus added a third to these, namely weight . Democritus says the primary bodies (that is, the solid atoms) do not possess weight but move in the infinite as the result of striking one another .
Simplicius, de caelo 242, 21 (Kirk et al., 425)
As they [the atoms] move they collide and become entangled in such a way as to cling in close contact to one another, but not so as to form one substance of them in reality of any kind whatever; for it is very simple minded to suppose that two or more could ever become one. The reason he gives for atoms staying together for a while is the intertwining and mutual hold of the primary bodies; for some of them are angular, some hooked, some concave, some convex, and indeed with countless other differences; so he thinks they cling to each other and stay together until such time as stronger necessity comes from the surrounding and shakes and scatters them apart .