It is obvious that the amount of the optical activity of a given compound containing an asymmetric atom of carbon depends upon the amount of difference in character among the four radicals united to the asymmetric carbon atom, so that if two of them are very nearly equal we come very near to a compound of a symmetric carbon atom, in which the optical activity is zero. The question suggests itself, How are we to measure this difference of character ? We shall assume that there is a function, capable of numerical representation, derivable from the composition and constitution of the radical and the temperature of the substance, and that it is the difference between the values of this function in the case of two radicals which gives us the difference of character referred to. For the sake of brevity, we may call this function the K of the radical. The object of this paper is to show that, if there be such a function, there are methods by means of which we may hope to ascertain its value in each case.