In a revision of the proof a serious omission was made in the inadvertent dropping of "and 1 ml. of 0.1% CuSO4 5H2O solution" after "Aliquots of 2.0 ml are mixed with 6 ml of clear 12.5 per cent Na2C03 solution" on p. 405. Addition of copper is essential in enhancing the sensitivity of the Folin reagent, as already noted by others.