"The equation of Clausius to which I must now call your attention is of the following form:pV=\frac{2}{3}T-\frac{2}{3}\sum\sum(\frac{1}{2}Rr).Here p denotes the pressure of a fluid, and V the volume of the vessel which contains it. The product pV, in the case of gases at constant temperature, remains, as Boyle's Law tells us, nearly constant for different volumes and pressures. ...The other member of the equation consists of two terms, the first depending on the motion of the particles, and the second on the forces with which they act on each other. The quantity T is the kinetic energy of the system... that part of the energy which is due to the motion of the parts of the system. ...In the second term, r is the distance between any two particles, and R is the attraction between them. ...The quantity ½Rr or half the product of the attraction into the distance across which the attraction is exerted is defined by Clausius as the virial of the attraction. ∑∑(½Rr)... indicates that the value of ½Rr is to be found for every pair of particles and the results added together. Clausius has established this equation by a very simple mathematical process... it indicates two causes which may affect the pressure of the fluid on the vessel which contains it... We may therefore attribute the pressure of a fluid either to the motion of its particles or to a repulsion between them."