THE Faraday lecture of the Chemical Society was delivered at the Royal Institution on March 29 by Prof. Peter Debye, of the University of Leipzig. He took as his subject “The Relations between Stereochemistry and Physics” and gave a brilliant exposition of certain methods used for the elucidation of molecular shape and dimensions, methods to which his own contributions, both in theory and in experimental technique, have been all-important. Until comparatively recently, the two factors which essentially determine the dielectric constant of a substance were not clearly distinguished from one another. Prof. Debye showed how the total polarisation in an electric field is compounded of the natural polarisation inherent in the molecules themselves and of an induced polarisation, due to their electrical deformability; how the relative contributions of these two effects can be estimated by investigating the influence of temperature on dielectric constant; and how the magnitude of the natural dipole moment thus obtained can give valuable information on the degree and the nature of symmetry of the molecule. Examples were quoted from the work of Errera, Smyth and Williams.