Formerly the present author worked out an elementary theory which could explain qualitatively the relation between the Young\u27s modulus and the degree of order regarding β-brass. By an analogous reasoning, we examined in this paper the relation between the Young\u27s modulus and the composition of binary solid solutions. Denoting the force constants in regard to the interaction-energy between A atoms, that between B atoms and that between atoms A and B, by ξAA, ξBB and ξAB respectively, we obtained the following results. (1) In case that ξAB is equal to the mean value of ξAA and ξBB, a linear relation between the Young\u27s modulus and the composition is obtained. (2) In case that ξAB is not equal to the mean value, a non-linear relation is obtained. (3) The theoretical curves obtained with appropriate values of ratios of the force constants agree well with the experimental ones. (4) Using the value of ξAA obtained from the measured values of the lattice constant and the compressibility, we are able to estimate the magnitude of the Young\u27s modulus of molybdenum in polycrystalline form which agrees within about ten per cent with the measured value
