This paper summarizes ideas about the nature of gases proposed during the period following the discovery of Boyle's law. Particular stress is laid on the hypotheses of the Bernoullis, and later, on the equally speculative work of Herapath. Reasons for the success of Herapath's theory, and the neglect of Daniel Bernoulli's are discussed, but it has not been thought necessary to take the story beyond the initial acceptance of Herapath's theory by J. P. Joule, because the paper is concerned only (...) with the antecedents of the modern kinetic theory. (shrink)