UNTIL four years ago, data for the proportions in which isomerides are simultaneously formed in aromatic hydrogen-substitutions contributed the only direct, quantitative experimental basis for the numerous theories of the process which had at various times been advanced. At the period mentioned the broad lines of the modern view had been laid down; and not only was this view no exception t o the preceding statement, but also many theoretical details which have subsequently been filled in have arisen from the determination of further d.ata of the same type. I n 1927, however, the view that continued development of the theory would require the consideration, not only of proportions of isomerides, but also of relative velocities of substitution, led to the publication of Part I (Ingold and Shaw, J., 1927, 2918), in which data of the latter kind were submitted for a number of benzene derivatives having single orienting substituents of widely differing structural types.Proportions of isomerides, which are also ratios of their rates of formation,? are regarded as giving only the ratios of magnitudes which individually relate t o the effect of the substituent on reactivity at the different nuclear positions. The speed of total substitution relatively to that of benzene, in short, the " reduced " velocity of substitution, yields the sum of such magnitudes. Only when both the ratios and the sum are known can the individual effects be defined, and the influence of the substituent on nuclear reactivity ROTHSTEIN, arid DENIS WARD. * The first of these studies, called Part I in the text, was published in another series under the specific title " A n Attempt further to define the Probable Mechanism of Aromatic Substitution." -f This is true provided that the simultaneous substitutions are reactions of the same order and are irreversible, which are the conditions contemplated ; it is known that normally these conditions are fulfilled for nuclear nitration and halogenation.

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“An Introduction to Probability Theory and its Applications, Vol. II,” by W. Feller. Wiley, xviii + 626. pp. £4 10s. 0d.

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