  We present systematic measurements of the Casimir force between a gold-coated
plate and a sphere coated with a Hydrogen Switchable Mirror (HSM). HSMs are
shiny metals that can become transparent by hydrogenation. In spite of such a
dramatic change of the optical properties of the sphere, we did not observe any
significant decrease of the Casimir force after filling the experimental
apparatus with hydrogen. This counterintuitive result can be explained by the
Lifshitz theory that describes the Casimir attraction between metallic and
dielectric materials.
