Effect of Structure on the Change in Colour of Vat Dyes on Soaping *

After a general survey of the problem of the changes in colour which accompany the soaping of vat dyes, it is shown that, at least for a limited range of dyes, the 1,4‐dibenzamidoanthraquinones, such changes must be due to changes of a molecular character. A working hypothesis has been evolved according to which the different forms are to he regarded as representing a new type of isomerism, termed contact isomerism , and can be visualised as partly existing in solid form, and in isolated cases even as crystalline modifications, in conformity with the earlier hypothesis of different polar forms. The marked effects of certain substituents on the colour, hitherto inexplicable, can be explained by a predominating effect on the resulting isomeric composition, acting alongside the normal effect of the substituents. Various anomalies that can occur during development can be readily explained on this basis.