292 INSECTICIDES AND FUNGICIDES 



Dilute ammonia dissolves more copper carbonate than strong 

 ammonia, so that it is better to use dilute ammonia in 

 dissolving the copper carbonate, rather than to use strong 

 ammonia for solution and to dilute it afterward. Excess 

 of ammonia harms vegetation, hence care must be used 

 in dissolving the copper carbonate. The soluble copper is 

 the active fungicide and in this form is not so dangerous to 

 foliage as is copper sulphate which is sometimes used in 

 dilute solution. 



When diluted considerably, basic copper carbonate is 

 apt to precipitate out and in such cases an agitator can be 

 used in the spray, although if the spray is used immediately 

 after dilution, it can be brought on the plants before pre- 

 cipitation begins. 



(b) Bordeaux Mixture is usually made by mixing 

 equal parts of copper sulphate and burnt lime, the former 

 previously dissolved in water, and the latter slaked 

 and mixed with an equal quantity of water. The pro- 

 portions vary with the different uses of the fungicide. 

 Formerly the reaction was thought to be a simple one by 

 which copper hydroxide and calcium sulphate were formed, 

 but this is impossible. The mixture would gradually turn 

 black if copper hydroxide were formed. Copper hydroxide, 

 Cu(OH) 2 , blue, changes to Cu(OH) 2 .(CuO) 2 , and finally 

 CuO, black. The color is, however, very permanent and 

 there is some evidence to show that the compound formed 

 is a double basic sulphate of copper and calcium to which 

 has been given the formula, (CuO)i .SO 3 . (CaO) 4 .S0 3 . Graphi- 

 cally it can be written as follows: 



O O— Ca^ "Ca" Ca^ ^Ca— O O 



s s 



/ \ / s 



O O— Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu— O 



000000000 



Burnt lime which has been air-slaked and hydrated lime 

 which contains considerable carbonate are not recommended 



