Composition-dependent long range forces from varying m_p/m_e

We relate possible cosmological variations in the mass ratio mu \equiv m_p/m_e and the fine structure constant alpha to long-range composition-dependent forces mediated by a scalar field. The differential acceleration eta in Eotvos-type experiments is bounded below by 10^{-14}, except in cases where one or more scalar couplings vanish. We consider what values for these couplings could arise from unified theories. By considering the contribution of the scalar field to the cosmological energy density we use bounds on eta to put upper bounds on the current rate of change of mu and alpha.