The puer liquor is filtered through a Schleicher and Schulls hard filter, No. 602, 18-1/2 cm. diam.; 50 c.c. are taken, 4 drops of phenolphthalein solution (10 gr. dissolved in 300 c.c. alcohol) added, and titrated with N/5 acid or alkali as the case may be. If the acidity be determined by adding excess of alkali and titrating back, a larger amount of acid is found than by direct titration. In the above-mentioned case, an indirect titration showed an acidity equal to 18 c.c. N/10 acid per 100 c.c. of liquor. On adding the alkali to a fresh filtered puer liquor, a flocculent precipitate is produced, which appears to be due to the decomposition of proteid compounds of weak organic acids,[28] the alkali uniting with them and setting the proteids free. As these compounds are undoubtedly decomposed by the lime in the skins, it seems probable that the acidity available for neutralizing lime may be greater than that shown by direct titration. [28] Probably syntonin, or bodies of a similar nature (see Allen. Comm. Org. Anal. iv. p. 4). The supposition is supported by the fact that solutions of puer have a slight laevorotatory action on polarized light. See also note on Van Liers paper, Coll. 376, 1909, p. 823. A method which was suggested by me for the estimation of the acidity of tan liquors may be applied also to the examination of the bate liquors, namely, the method by which the electric potential between a hydrogen electrode dipping into the bate liquor, and a standard calomel electrode, is used to determine the neutral point (for details see Chapter III.), using the potentiometer devised by Dr. H. J. S. Sand.[29] [29] See paper entitled, “The Employment of the Electrometric Method for the Estimation of the Acidity of Tan Liquors,” by H. J. S. Sand, Ph.D. D.Sc., and D. J. Law, B.Sc. F.I.C., J.S.C.I. 1911, p. 3; also Part II., by Wood, Sand and Law, J.S.C.I. 1911, p. 872. Puer liquors, titrated in this manner with N/10 soda or HCl until a potential of 0·69 volts is reached (at which point phenolphthalein turns from colourless to red), gave the following results per 100 c.c. filtered liquor.