IN reply to Prof. Tait's criticism (NATURE, December 31, 1891, p. 199) of my paper (December 17, p. 152), I wish to say that I certainly do not consider Van der Waals's b as an absolute constant.

Real Gases Real gases. Compressibility Factor. Van der Waals Equation. Condensation of gases. Principle of Corresponding States. Math Interlude Partial Differentiation. Thermodynamics First Law.