SUMMARY1The initial product of the interaction ol cupric sulphate and calcium hydroxide solutions, at ordinary temperatures, is the 4CuO. S03basic sulphate.2With amounts of calcium hydroxide in excess of 0–75 equivalent, the 4CuO. S03salt is slowly decomposed with elimination of the sulphate radicle and the formation of a blue hydrated cupric oxide or cupric hydroxide, a product called, for convenience, the hydroxide.3The cupric hydroxide formed retains, by adsorption, part of the sulphate radicle. Direct evidence by analysis of the precipitate, which might be considered to indicate the formation of a basic copper‐calcium complex, is therefore unreliable.4In the analogous interaction of calcium hydroxide with cupric chloride, the formation of cupric hydroxide in the presence of amounts of calcium hydroxide in excess of 0–75 equivalent, is indicated by the dehydration of the precipitate to form nigger brown hydrated cupric oxide, and by the definite point of inflection on the hydrogen‐ion concentration curve which corresponds to equimolecular amounts of calcium hydroxide and cupric chloride.5The hydrogen‐ion concentration curve for the cupric sulphate‐calcium hydroxide reaction shows that permanent alkalinity is not‐reached until equimolecular amounts of calcium hydroxide have been added.6The cupric hydroxide, formed from the 4CuO.SO3basic sulphate by the action of calcium hydroxide, is stabilised by adsorbed sulphate ions. If the adsorbed sulphate and excess calcium hydroxide be removed by continued washing with carbon dioxide‐free water, the Bordeaux mixture precipitate undergoes dehydration to form the nigger brown hydrated cupric oxide.7Dehydration of the Bordeaux mixture precipitate when sprayed upon foliage, is therefore prevented by some chemical or physical change of the precipitate which occurs on drying.8Dehydration is not prevented by physical changes which occur on drying.9Carbon dioxide is a factor in the process whereby dehydration is prevented.10It is suggested that the formationin situof the protective copper‐containing deposit by the action, primarily, of carbon dioxide, accounts for the excellent adhesive properties of the Bordeaux mixture precipitate; that inertness towards the chemical changes which occur after spraying may be the cause of failure of some Bordeaux substitutes; that these changes may profoundly affect the value of sedimentation tests as criteria for the fungicidal efficiency of various