[Western 22] • Pickering

Excerpt from Pickering Sprays

Bedford and Pickering (3, 4) claimed that the fungicidal action of Bordeaux depends upon the re-formation of copper sulphate by the action of carbon dioxid from the air. They believed that the excess of lime present in Bordeaux is carbonated before the copper, and held that since the fungicidal action depends upon the carbonation of the copper and the re-formation of copper sulphate, the excess lime of Bordeaux delays the action. These investigator's stated that basic sulphates of copper are produced by the action of lime on cep per sulphate, and that the basic sulphates vary in composition according to the proportions of lime and copper used. Most of the basic sulphates are complex, and contain, in addition to the ele ments essential in a basic sulphate of copper, calcium sulphate or calcium oxid, sometimes both. For present, purposes the calcium sulphate in these compounds need not be considered. The basic sulphates of copper, being practically insoluble in water, can, of themselves have little Or no fungicidal action, but when exposed to the carbon dioxid of the air they are gradually decomposed to form copper carbonate and Copper sulphate The copper carbonate, being insoluble, is incapable of energetic action. According to Bedford and Pickering, the substances which are formed in the so called Pickering sprays by the action of lime on copper sulphate, omitting the calcium sulphate present.

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