In earlier papers (this Journal, vol. IV. pp. 69 and 76) we have detailed the experimental evidence which led us to conclude (1) that the view as to the fungicidal action of Bordeaux mixtures favoured by Pickering, viz. the liberation of copper sulphate by atmospheric carbon dioxide, is untenable; and (2) that contact between the fungus and the copper compound present in the mixture will account largely for its efficiency owing to a solvent action on the part of the organism under certain conditions. Pickering (this Journal, V. p. 273) has criticised our general conclusions and the deductions which we have drawn from certain of our experiments; and therefore before describing our further work on the subject, a brief reference to some of the points which he has raised is desirable.