the active esters submitted to optical examination included halogen, nitro-and nitro-halogen derivatives of benzoic acid. The results derived from the study of nine separate series of compounds clearIy indicated that, with the single exception of the ortho-isomeride of the iodobenzoic esters," the ortho-substituent had the greatest * We propose to study the effect of tcmperature on the rotation of the three isomeric iodobenzoic esters, View Article Online ISOMERISM TO OPTICAL ACTIVITY. PART

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... . If, according to Winther (Zoc. cit.), change of rotation in a compound is conditioned by change in internal pressure, it seems probable that a temperature widely removed from the crystallising point, and therefore from the region in which molecular aggregates may be formed, will afford a more correct basis for comparison. It is for this reason that provisionally we are disposed to regard the values at looo as the more satisfactory for the purpose of comparison until more light has been thrown on this aspect of the * The rule laid down by Frankland and Wharton (Trans., 1836, 69, 1309, 1583), in which the para-isomeride is supposed to exert the greatest effect, is not only contradicted by some of their own and others observations, but seems quite incapable of proof where solvents are used, owing t o the indeterminate influence of the solvent.