  In the early decades of the life of Dalton's atomic chemistry various attempts were made to solve the problem of the under-determination of atomic weights and formulae. We have already mentioned the appeal to the equal numbers hypothesis and vapour densities. The fact that chemists of the time did not have the resources to make this solution work has been explored in detail by Brooke (1981) and Fisher (1982). A second method was to employ an empirical rule, proposed by Dulong and Petit, according to which the product of the specific heats and the atomic weights of solids is a constant. The problem with this at the time was, firstly, that some atomic weights needed to be known independently to establish the truth of the rule, and, secondly, there were known counter-instances. A third method for determining atomic weights employed Mitscherlich’s proposal (Rocke, 1984, 154–6) that substances with similar formulae should have similar crystal structure. This method had limited application and, again, there were counter-examples.