REPORT OF THE SOCIETY 43 



As you all know, calcium arsenate can only be used on foliage which is 

 fairh' resistant to arsenic injury. Calcium arsenate, though very insoluble in 

 water, is unfortunately reacted upon by carbon dioxide. Consequently, when 

 exposed to the air a certain amount of soluble arsenic is produced, the reaction 

 being of the following nature : 



CagCAsOJ 2 +CO2 +H2O = 2CaHAs04 +CaC03. 



T\vat is to say, the more basic insoluble calcium salts are converted into 

 the mo re soluble hydrogen salts. Naturally, in the presence of an excess of free 

 lime the above reaction cannot occur, so that calcium arsenate is satisfactory in 

 such mixtures which contain an excess of free lime. 



White arsenic 



White Arsenic AS2O3, is too soluble to permit its use on foliage. Mixed 

 with lime, it forms calcium arsenite, Ca(As02)2. and this also, though a relat- 

 ivelj- insoluble salt is so readily decomposed that it is highh' injurious to foliage. 

 When added to Bordeaux mixture, however, white arsenic loses, under certain 

 conditions, its injurious properties. The arsenic ceases to be soluble in water. 

 Whether this is due entirely to chemical union or to absorption by the copper 

 precipitate is not altogether known. 



The use which can be made of white arsenic in Bordeaux mixture is obvious- 

 H' of considerable economic importance, and instructions have been issued by 

 e Entomological Branch regarding its use. 



Bordeaux mixture 



Any mixture of copper sulphate and lime is termed Bordeaux mixture. 

 The composition of the copper compounds of Bordeaux mixture varies according 

 to the relative quantities of copper sulphate and lime present. The compound 

 present in the ordinary 4.4.40 Bordeaux mixture has the formula IOCUO.SO3. 

 3CaO together with free lime and free calcium sulphate. If more lime is pres- 

 ent, then there is more lime in the molecule ; if less lime is present, then there 

 is less lime in the molecule, and there exist lower basic sulphates of copper. 



The compound lOCuO.SO3.3CaO, which of course has also a certain amount 

 of water in its constitution, is typical of the compounds of Bordeaux mixture. 

 This salt is insoluble in water and there seems little doubt that its fungicidal 

 nature is dependent upon the solvent action of the fungus itself. 



Although this compound is insoluble in water, yet after it has been exposed 

 to atmospheric conditions for a considerable time, copper commences to make its 



