  Alloys expand or contract as concentrations change, and the resulting
relationship between atomic volume and alloy content is an important property
of the solid. While a well-known approximation posits that the atomic volume
varies linearly with concentration (Zen`s law), the actual variation is more
complicated. Here we use an apparent size of the solute (solvent) atom and the
elasticity to derive explicit analytical expressions for the atomic volume of
binary solid alloys. Two approximations, continuum and terminal, are proposed.
Deviations from Zen`s law are studied for 22 binary alloy systems.
