To subordinate the principle to all reversible processes, Helmholtz introduced into it the conception of the "kinetic potential." In this form the principle has universal validity.
An offshoot of the mechanical theory of heat is the modern kinetic theory of gases, developed mathematically by Clausius,
Maxwell, Ludwig Boltzmann of Munich, and others. The first suggestions of a kinetic theory of matter go back as far as the time of the Greeks. The earliest work to be mentioned here is that of Daniel Bernoulli, 1738. He attributed to gas-molecules
great velocity, explained the pressure of a gas by molecular bombardment, and deduced Boyle's law as a consequence of
his assumptions. Over a century later his ideas were taken up by Joule (in 1846), A. K. Krönig (in 1856), and Clausius
(in 1857). Joule dropped his speculations on this subject when he began his experimental work on heat. Krönig
explained by the kinetic theory the fact determined experimentally by Joule that the internal energy of a gas is not altered by expansion when no external work is done. Clausius took an important step in supposing that molecules may have rotary motion, and that atoms in a molecule may move relatively to each other. He assumed that the force acting
between molecules is a function of their distances, that temperature depends solely upon the kinetic energy of molecular motions, and that the number of molecules which at any moment are so near to each other that they perceptibly influence each other is comparatively so small that it may be neglected. He calculated the average velocities