16 



the copper salts as fungicides. 

 By p. D. Chestee. 



In order to make an intelligent comparison between the several well 

 known fungicides containing copper, it is important to understand 

 what salts of copper occur in each and in what relative proportions. 

 This in turn involves some inquiry into the chemical reactions which 

 take place in their preparation and during their stay upon the vine. 



For much valuable assistance in the preparation of these notes I am 

 indebted to Prof. 0. L. Penny, the Chemist of this Station. 



THE BOEDEATJX MIXTTJEE. 



Formula. — Copper sulphate, 6 pounds; lime, 6 pounds; water, 22 

 gallons. In the addition of milk of lime to a solution of copper sul- 

 phate, the lime in solution precipitates the copper as cupric hydroxide, 

 forming at the same time a slightly soluuble sulphate of lime. These 

 two salts, together with an excess of lime, remain in suspension in the 

 Bordeaux mixture. 



The reaction is simple: 



CuSO^, SHsO + CaO.HsO = Cu (OH)j + CaSO< + .5H,0. 



From this formula a simple calculation shows that to precipitate the 

 6 pounds of copper sulphate, there would be required 1.34 pounds of 

 lime (CaO), which would in turn produce 2.34 pounds of cupric hy- 

 droxide. 



The weight of lime to be used should be considerably increased above 

 this amount, owing to its impure character as ordinarily purchased, 

 but it is likely that 3 or 4 pounds of commercial lime will suffice to 

 satisfy this reaction. 



The 22 gallons of water is capable of dissolving approximately .235 

 pounds of lime, an amount sufficient to precipitate practically 1 pound 

 of the copper sulphate. But since this quantity of lime is immediately 

 thrown down as a nearly insoluble sulphate, the water is free to dis- 

 solve another portion of lime, which in turn precipitates another portion 

 of the copper, until all of the copper is thrown down. It is found that 

 this complete precipitation of the copper takes place quickly, or by the 

 time the matter in suspension has fully settled, leaving a clear super- 

 natant liquid, which does not react for copper ; hence a long standing 

 of the Bordeaux mixture before use is hardly necessary. 



In drying upon the plant the cupric hydroxide in the Bordeaux mix- 

 ture undergoes no change, hence it is probably this salt of copper which 

 is the active principle. 



