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.

MY DEAR PROF. TAIT,—I gather from your letter of September 28 (NATURE, October 8, p. 546) that you admit the correctness of Van der Waals's deduction from the virial equation (1) when the ...