In Lorentz Ether Theory, FitzGerald-Lorentz contraction affects all matter — containers, molecules, and the electromagnetic interactions between them. In a static gas, the anisotropic collision cross-sections of contracted molecules drive the density distribution toward the same anisotropy as in a solid, restoring the SR/LET equivalence. However, this equilibration proceeds at the diffusion timescale τ_diff ~ L²/D — hundreds to thousands of seconds for a laboratory gas cell. In a continuously rotating experiment with period T_rot ~ 5—60 s, the anisotropy cannot keep pace with the changing contraction direction: the gas density remains isotropic while the solid tracks the contraction instantaneously. This timescale separation creates a measurable difference: the directional density of a gas (molecules per unit length) is isotropic, while that of a solid is anisotropic. The resulting fringe shift on rotation is ΔN = k(L/2λ)(n−1)(v/c)², where k is the number of passes. The signal scales linearly with gas pressure — providing a built-in calibration. We predict null results for solid-dielectric interferometers regardless of refractive index.