  There is no doubt that Newton shared the assumption of the Ancient and mechanical atomists that there is just one kind of homogeneous matter of which all atoms are composed. This is clear from the way in which Newton explained differing densities of observable matter in terms of the amount of space intervening between the component atoms.  Newton argued, for instance that the ratio of space to volume occupied by atoms was seventeen times greater in water than in gold on the grounds that gold is seventeen times more dense. The fact that thin gold films transmit light convinced Newton that the atoms of gold already contains enough space to permit the transmission of light particles. The preponderance of space between the atoms of matter, however bulky or solid they might appear at the observational and experimental level, became a characteristic feature of Newtonian atomism, as Thackray (1968) has stressed.