In recent years John Herapath has emerged as an important figure in the early history of the dynamical theory of gases. In a series of papers published in 1821 and 1822 he outlined an elaborate theory of the states of matter, specific and latent heats, vapours and gases, culminating in a theory of the gravitational ether. This work was based on the rejection of the caloric theory of heat and was founded instead on the idea that the particles of matter were in constant motion; the forces of caloric were replaced by the transfer of momentum. Herapath's is one of the earliest of dynamical theories of gases to be worked out in any mathematical detail.