  Samarium sulfide (SmS) is a non-magnetic narrow-gap (0.06 eV) semiconductor
which undergoes a transition to a metallic intermediate valence state at 6.5
kbar. Europium sulfide (EuS) is a ferromagnetic semiconductor with a Curie
temperature of 16K and a gap of 1.6 eV. Here we present a study of the lattice
constant, magnetic susceptibility, and resistivity of the substitution series
$Sm_{1-x}Eu_xS$ for 0 < x < 1. We observe a smooth interpolation of magnetic
and transport behavior across the series, consistent with a virtual crystal
scenario and Vegard's law. Surprisingly, however, the lattice constant deviates
below Vegard's law in a manner that suggests parametric control of the Sm-Sm
distance by the Eu moment in the manner of a magnetic polaron.
