The theory of transport processes in gases—such as diffusion, heat conduction, and viscosity—is developed on the basis of the assumption that the molecules behave like point-centres of force. The method of investigation consists in calculating mean values of various functions of the velocity of all the molecules of a given kind within an element of volume, and the variations of these mean values due, first, to the encounters of the molecules with others of the same or a different kind; second, to the action of external forces such as gravity; and third, to the passage of molecules through the boundary of the element of volume.
The encounters are analysed of molecules repelling each other with forces inversely as the nth power of the distance. In general the variation of mean values of functions of the velocity due to encounters depends on the relative velocity of the two colliding molecules, and unless the gas is in thermal equilibrium the velocity distribution is unknown so that these variations cannot be calculated directly. However, in the case of inverse fifth-power forces the relative velocity drops out, and the calculations can be carried out. It is found that in this special case the viscosity coefficient is proportional to the absolute temperature, in agreement with experimental results of the author. An expression for the diffusion coefficient is also derived, and compared with experimental results published by Graham.
A new derivation is given of the velocity-distribution law for a gas in thermal equilibrium. The theory is also applied to give an explanation of the Law of Equivalent Volumes, the conduction of heat through gases, the hydrodynamic equations of motion corrected for viscosity (Navier-Stokes equation), the relaxation of inequalities of pressure, and the final equilibrium of temperature in a column of gas under the influence of gravity.