THE fact that a second edition of Dr. L. F. Hewitt's monograph on “Oxidation-Reduction Potentials“has been found necessary indicates the interest taken by biologists in the subject (“Oxidation-Reduction Potentials in Bacteriology and Biochemistry”: published by the London County Council. Pp. 81. 2s.). Since the first edition was noticed in our columns (NATURE, 128, 73; 1931) the subject has advanced steadily, and the author has taken the opportunity to revise and extend the text and bring the bibliography up to date. The study of electrode potentials is proving of assistance in the practical question of the rationalisation of culture media and cultural conditions, and is throwing light upon the biological behaviour of organisms, such as the maintenance or loss of virulence, questions of great importance in medical practice.