  The first theory aims to explain the properties of gases by assuming that they consist of a very large number of molecules in rapid motion. (The term ‘kinetic’ is meant to underline the vital importance of motion here, and to distinguish the approach from older static molecular gas models.) During the 1860s probability considerations were imported into this theory. The aim then became to characterize the properties of gases, in particular in thermal equilibrium, in terms of probabilities of various molecular states. This is what the Ehrenfests call “kineto-statistics of the molecule”. Here, molecular states, in particular their velocities, are regarded as stochastic variables, and probabilities are attached to such molecular states of motion. These probabilities themselves are conceived of as mechanical properties of the state of the total gas system. Either they represent the relative number of molecules with a particular state, or the relative time during which a molecule has that state.