364 BORDEAUX MIXTURE 



cereals, this explanation would seem to be sufficient to account for the 

 facts. It receives confirmation from the discovery, recently made, 

 that spraying with 15 or 20 per cent solutions of sodium nitrate, 

 ammonium sulphate, or potassium chloride salts which cannot be 

 suspected of having any chemical toxic effects is also effective in 

 destroying charlock. 1 (See also iron sulphate.) 



Copper hydroxide, Cu(OH) 2 , is also largely employed as a fungicide 

 for application to vines, potatoes and fruit trees. It is usually em- 

 ployed in the foi-m of ' ; Bordeaux mixture," which is made by adding 

 lime (best in the form of "milk of lime") to copper sulphate solution. 

 Various strengths of solution have been recommended, the proportion 

 of copper sulphate varying from 12 Ib. to 30 Ib. per 100 gallons of 

 water, that of quick-lime from 8 Ib. to 20 Ib. The lime should in all 

 cases be first slaked, made into a smooth cream with water, and then 

 poured into the copper sulphate solution with constant stirring. For 

 many purposes it is highly important that the lime should be in excess 

 and the mixture therefore free from any dissolved copper compounds, 

 since these act corrosively upon the foliage. This can readily be de- 

 tected by inserting a piece of bright iron or steel, when no deposit of 

 metallic copper should form ; or another easily applied test is to breathe 

 upon the surface of a portion of the mixture, when a film of calcium 

 carbonate should be formed, showing the presence of free calcium 

 hydrate. The mixture should be kept constantly stirred and be ap- 

 plied by the sprayer as soon after its preparation as possible. 



Bordeaux mixture is largely used in the preventative treatment of 

 vines and potatoes and is highly effective. 



According to P ckering 2 Bordeaux mixture contains its copper, not 

 as hydroxide but as basic copper sulphates. He finds that when lime 

 water is added to a solution of copper sulphate, the precipitate, at first, 

 consists of a basic copper sulphate, S0 3 .4CuO ; as more lime water is 

 added, the composition of the precipitate approaches that indicated by 

 SO 3 .5CuO. With still larger quantities of lime, S0 3 .10CuO is formed, 

 together with some co-precipitated calcium sulphate. 



The action of Bordeaux mixture as a fungicide, depends, according 

 to Pickering, upon the gradual action of "atmospheric carbon dioxide 

 upon the residue left on the foliage, whereby small quantities of copper 

 sulphate are formed : 



SO 3 . 4CuO + SCO, = 3CuC0 3 + CuSO 4 

 S0 3 . 5CuO + 4CCX - 4CuCO ;j + CuSO 4 

 S0 3 .10CuO + 900, = 9CuCO 3 + CuSO 4 



CaS0 4 + CuSO 4 . 



From these equations it is evident that the most effective mixture 

 would be one in which the precipitate approximates most nearly to 

 4CuO.S0 3 , since in this case the effective fungicide copper sulphate 

 formed by the gradual action of the atmospheric carbon dioxide, 

 would correspond theoretically to one-third of the total copper present, 



1 Heinrich, Jahresbericht iiber Aricultur-Cheraie, 1901, 351. 

 * Jjur. Chem. Soc., 1907. Trans., 1988. 



