John Herapath (1790-1868), a self-taught scientist, was the first to present a comprehensive description of the structure of a gas, in which the molecules moved and collided with each other, instead of vibrating in a fixed lattice. His model made the wrong assumption that the momentum of the molecules, instead of their kinetic energy measured the temperature. He used his model to calculate for the first time the velocity of a molecule, the velocity of sound and to describe effusion phenomena through small pores, and adiabatic compression. His speculative approach put him at odds with the empirical approach of the British scientific establishment of his time.