I cannot assume that the students of philosophy who read this magazine are thoroughly versed in modern molecular physics, and therefore it is proper to mention that the governing principle in this branch of science is Clausius’s law of the virial. I will first state the law, and then explain the peculiar terms of the statement. This statement is that the total kinetic energy of the particles of a system in stationary motion is equal to the total virial. By a _system_ is here meant a number of particles acting upon one another.[5] Stationary motion is a quasi-orbital motion among a system of particles so that none of them are removed to indefinitely great distances nor acquire indefinitely great velocities. The kinetic energy of a particle is the work which would be required to bring it to rest, independently of any forces which may be acting upon it. The virial of a pair of particles is half the work which the force which actually operates between them would do if, being independent of the distance, it were to bring them together. The equation of the virial is (Transcriber’s Note: Italics have been removed from the formulæ for readability.) ½Σmv² = ½ΣΣRr. Here _m_ is the mass of a particle, _v_ its velocity, _R_ is the attraction between two particles, and _r_ is the distance between them. The sign Σ on the left hand side signifies that the values of _mv_² are to be summed for all the particles, and ΣΣ on the right hand side signifies that the values of _Rr_ are to be summed for all the pairs of particles. If there is an external pressure _P_ (as from the atmosphere) upon the system, and the volume of vacant space within the boundary of that pressure is _V_, then the virial must be understood as including ³⁄₂_PV_, so that the equation is ½Σmv² = ³⁄₂PV + ½ΣΣRr. There is strong (if not demonstrative) reason for thinking that the temperature of any body above the absolute zero (-273° C.), is proportional to the average kinetic energy of its molecules, or say _aθ_, where _a_ is a constant and _θ_ is the absolute temperature. Hence, we may write the equation aθ = ½m̅v̅²̅ = ³⁄₂PV̅ + ½ΣR̅r̅ where the heavy lines above the different expressions signify that the average values for single molecules are to be taken. In 1872, a student in the University of Leyden, Van der Waals, propounded in his thesis for the doctorate a specialisation of the equation of the virial which has since attracted great attention. Namely, he writes it aθ = (P + (c⁄V²))(V-b).