  Dalton's atomism had given a line on just one property of atoms, their relative weight. But it is quite clear that they needed far richer properties to play there presumed role in chemistry. It was to be developments in chemistry, and later physics, that were to give further clues about what properties to ascribe to atoms. (We have seen how chemists came to ascribe the property of valency to them.) There was no viable atomistic theory of chemistry in the nineteenth century that was such that chemical properties could be deduced from it. The phenomenon of isomerism is often regarded as a success for atomism. (See Bird, (1998, p. 152) for a recent example.) There are reasons to doubt this. The fact that there are chemical substances with the same proportional weights of the elements but with widely different chemical properties was a chemical discovery. It could not be predicted by any atomic theory of the nineteenth-century because no theory contained within its premises a connection between the physical arrangement of atoms and chemical properties.Isomerism could be accommodated to atomism but could not, and did not, predict it.