  While most atomists were able to rid themselves of scruples about accepting forces as ontologically primitive, the issue of the empirical foundation for the various unobservable forces hypothesised remained. The best arguments that could be mounted were hypothetical-deductive. Forces postulated at the atomic level were credited with some empirical support if they could serve to explain observable phenomena. The form of such arguments, as well as their inconclusiveness, can be illustrated by Newton's demonstration in the Principia (Bk. 2, Prop. 23) that a gas consisting of a static array of atoms repelling each other with a force inversely proportional to their separation would obey Boyle's law. The fact that some of these theories did indeed reproduce the experimentally established facts was certainly a point in their favour, but hardly served to establish them. Whewell brought the point home by identifying competing theories of capillarity, due to Poisson and Laplace, that were equally able to reproduce the phenomena but which were based on incompatible atomic force laws, as Gardner (1979, 20) has pointed out.