The fungicidal properties of the insoluble copper compounds, present in Bordeaux mixtures, have long been known, and it is an important point to ascertain the precise manner in which the poisonous action is exerted. A variety of theories have been put forward, but the matter cannot be regarded as settled. It has been generally accepted that, in order to account for the fungicidal action, it is necessary to look for some means by which the insoluble copper in Bordeaux mixture is rendered soluble when the material is sprayed on to foliage. Atmospheric carbon dioxide has frequently been suggested as the most important agent by which this is effected, and recently this view has been urged by Mr S. U. Pickering, F.R.S., who has also investigated in detail, the chemical nature of the compounds found in Bordeaux mixtures.