In 1857 Rudolf Clausius published The Nature of the Motion which We Call Heat. In it he derived the relation p = n m v 2 ¯ ¯ / 3 for the pressure, p , in a gas with n particles per unit volume (number density), mass m , and mean square speed v 2 ¯ ¯ . He then noted that using the classical laws of Boyle and Charles one could write m v 2 ¯ ¯ / 3 = k T with k a constant of proportionality. Hence temperature was proportional to the average kinetic energy of the particles. This article inspired further work based on the twin ideas that substances are composed of indivisible particles, and that heat is a consequence of the particle motion as governed by Newton's laws. The work, known as the kinetic theory of gases, was done principally by Clausius, James Clerk Maxwell, and Ludwig Boltzmann. At about the same time J. Willard Gibbs also contributed, and advanced it by converting it into statistical mechanics.