Chemical potential of water from measurements of optic axial angle of zeolites

Values of the uncorrected optic axial angle (2H_α) of a crystal of the calcium zeolite stellerite (CaAl₂Si₇O_l8 · 7H₂0) immersed in calcium chloride solutions of known activity of water (a_w) are directly proportional to log a_w. A general relationship between the chemical potential of water in the crystal and the optic axial angle is obeyed.