"Denoting by v and v^\prime the temperatures of two equal molecules m and n...p their extremely small distance [apart], and... dt, the infinitely small... instant, the quantity of heat which m receives from n during this instant will be... (v^\prime - v) \theta (p) \cdot dt. We denote by \theta (p) a certain function of the distance p which, in solid bodies and in liquids, becomes [zero] nothing when p has a sensible magnitude. The function is the same for every point of the same given substance... [but] varies with the nature of the substance."