394 



The reaction between copper sulphate and lime is shown to 

 yield four basic sulphates: 



1) 4 Cu O, SO 3 (or 10 Cu O, 2.5 SO 3 ). 



2) 5 Cu O, SO 3 (or 10 Cu O, 2 SO 3 )> 



3) 10 Cu O, SO 3 . 



4) 10 Cu O, SO 3 . 3 Ca O, and two other compounds. 



5) Cu O, 2 Ca O (or 10 Cu O, 20 Ca O), (existence doubtful). 



6) Cu O, 3 Ca O. 



The conditions of the formation of each of these basic sulphates 

 were investigated, and a series of the corresponding Bordeaux mix- 

 tures prepared. 



The properties of the sulphates were studied and in particular 

 their decomposition under the influence of water and carbonic acid 

 in presence of calcium sulphate or of organic matter, these being 

 the conditions on the leaf surface. 



The next problem was to ascertain the function of Bordeaux 

 mixture and so to settle which was the most useful of the possible 

 basic sulphates. These salts are insoluble and have to be converted 

 into soluble substance before they can exert a fungicidal action. 

 No evidence could be obtained that the plant leaf or the spore 

 excreted anything that could dissolve an insoluble substance, but 

 it was shown that the carbonic acid of the air decomposes these 

 basic sulphates, liberating the copper sulphate, which constitutes 

 the active part of the mixture. It acts in two ways. It directly 

 poisons the fungus cell developing from the spore. Some of the 

 copper sulphate gets into, the leaf, displacing a certain amont of 

 iron and entering into a remarkable combination not yet investi- 

 gated, which seems so long as it persists to afford the leaf immu- 

 nity against fungal attacks, etc. 



The object of the fungicide is to furnish a steady supply of 

 copper sulphate and therefore the reaction yielding the lowest basic 

 sulphate i), now known as the Woburn Bordeaux (though it has 

 been in use in Italy for many years) is to be considered the most 

 efficient of the series. In the reaction yielding the basic sulphate 

 4), owing to the excess of lime having to be disposed of first, there 

 will be a delay in the liberation of the copper sulphate. 



The fact that in Bordeaux mixture i) (Woburn Bordeaux) no 

 such delay exists any more than with 3), that it contains no gritty 

 particles of lime to clog the spraying nozzles, that it must be 2 l / 2 

 times more effective, or 2 l / 2 times more economical than the or- 



