The Bethe-Weiss method in the statistical theory of Heisenberg ferromagnetism is generalized and applied to the case of subtitutional solid solutions of random distribution. Calculations are carried out for binary solutions containing magnetic atoms of spin 1/2 and nonmagnetic ones. The Curie temperature v.s. composition and the paramagnetic susceptibility v.s. temperature curves are obtained for simple and body-centered cubic lattices. The degree of magnetic short-range order at the Curie point becomes higher as the concentration of non-magnetic atoms increases. The results are qualitatively applicable to solutions of two different kinds of magnetic atoms. It is also concluded that the method gives an useful approximation only for higher concentrations of magnetic atoms.