521 



copper. Careful experiment was made as to the effect of carbonic 

 acid in solution in the water, and it was found that very appreciable 

 quantities of copper could be washed out of the precipitate in solution, 

 but that the solvent action of the water containing CO 2 gradually 

 diminished owing to the conversion of the whole of the copper into 

 carbonate. 



Alkaline Bordeaux Mixtures. It was found that, using the same 

 quantities of the same lime, it was possible to obtain two mixtures in 

 which the constitution of the precipitate differed, (a) Blue Mixture : 

 Concentrated milk of lime prepared from good, freshly slaked lime 

 is poured rapidly into the solution of sulphate of copper or vice versa ; 

 the reaction in either case takes place in a strongly alkaline medium ; 

 the quantity of lime used should be such that there is an excess of 

 O'l per cent, when the reaction is complete. Technical chemical proof 

 is given that the precipitate formed under these conditions consists 

 of a mixture of copper hydrate associated with calcium sulphate and 

 traces of basic sulphates ; the supernatant liquid, though strongly 

 alkaline, contains some copper in solution (23 parts in 10,000) ; the 

 precipitate, when treated with water containing CO 2 in solution, yields 

 an appreciable proportion of dissolved copper. (6) Green Mixture : 

 Dilute or concentrated milk of lime, but, if concentrated, prepared 

 from a lime which is partially carbonated, is poured in small quantities 

 at a time into the sulphate of copper solution, which is violently 

 shaken up after each addition ; the quantity of lime used is the same 

 as before, but the reaction now takes place in an acid medium ; the 

 precipitate formed is no longer blue, but has a definite greenish colour 

 and consists almost exclusively of basic sulphates of copper, but 

 probably of a higher basicity than those formed in the acid mixtures, 

 as they are not so soluble and the reaction between them and the 

 lime is much slower. Chemical proof is given that the constitution 

 of the precipitate in the green mixture is unstable and that it has a 

 natural tendency to revert more or less to the composition of the 

 blue type, depending on the amount of lime in excess. It is not certain 

 whether these changes have time to take place on the leaves before 

 the lime is completely coiwertcd into carbonate. 



It is thus obvious that green and blue alkaline Bordeaux mixture 

 are two entirely different products, and it is not difficult to understand 

 why it is almost impossible to prepare the blue mixture with the 

 ground lime of commerce, each grain of which is encased in an 

 envelope of carbonate which greatly delays solution. The composition 

 of the supernatant liquid (blue or green mixture) is practically the 

 same in both cases. The general results of these experiments show 

 (1) that the acid mixtures contain basic sulphate of copper and not 

 hydrates as generally supposed ; (2) that of the two alkaline mixtures, 

 the blue contains hydrates and the green basic sulphates ; (3) that 

 alkaline mixtures, contrary to the generally accepted opinion, all 

 contain at the moment of use sufficient copper in solution to prevent 

 the germination of the vine mildew ; (4) that alkaline mixtures, though 

 not yielding much copper in solution at first to the action of CO^, 

 gradually yield a considerable amount, whilst the acid mixtures yield 

 a great deal at first and but little afterwards, as the whole of the 

 copper is rapidly converted into carbonate which is not readily soluble. 

 After thirty years of use, the vineyard owner is still in doubt as to 



