2 BULLETIN 866, U. S. DEPARTMENT OF AGRICULTURE. 



tested to a limited extent in England, where laboratory tests indicated 

 that they were superior to the Bordeaux sprays (3, 4). 1 Pickering 

 sprays, sometimes called Pickering limewater sprays, are prepared 

 by mixing saturated limewater with dilute solutions of copper sul- 

 phate, and contain their copper in the form of basic copper sulphates. 



RESULTS OF PREVIOUS INVESTIGATIONS. 



FUNGICIDAL ACTION. 



Bedford and Pickering (3, 4) claimed that the fungicidal action of 

 Bordeaux depends upon the re-formation of copper sulphate by the 

 action of carbon dioxid from the air. They believed that the excess 

 of lime present in Bordeaux is carbonated before the copper, and 

 held that since the fungicidal action depends upon the carbonation 

 of the copper and the re-formation of copper sulphate, the excess 

 lime of Bordeaux delays the action. These investigators stated that 

 basic sulphates of copper are produced by the action of lime on cop- 

 per sulphate, and that the basic sulphates vary in composition 

 according to the proportions of lime and copper used. Most of the 

 basic sulphates are complex, and contain, in addition to the ele- 

 ments essential in a basic sulphate of copper, calcium sulphate or 

 calcium oxid, sometimes both. For present purposes the calcium 

 sulphate in these compounds need not be considered. The basic 

 sulphates of copper, being practically insoluble in water, can of 

 themselves have little or no fungicidal action, but when exposed to 

 the carbon dioxid of the air they are gradually decomposed to form 

 copper carbonate and copper sulphate. The copper carbonate, 

 being insoluble, is incapable of energetic action. According to 

 Bedford and Pickering, the substances which are formed in the so- 

 called Pickering sprays by the action of lime on copper sulphate, 

 omitting the calcium sulphate present, are: 



Formula A— 4 CuO, S0 3 (or 10 CuO, 2.5 S0 3 ). 



Formula B.— 5 CuO, SO s (or 10 CuO, 2 S0 3 ). 



Formula C— 10 CuO, S0 3 . 



Formula D. — 10 CuO, S0 3 , 3 CaO (ordinary Bordeaux). 



Formula E— CuO, 2 CaO (or 10 CuO, 20 CaO) (existence 



doubtful). 

 Formula F — CuO, 3 CaO (or 10 CuO, 30 CaO). 



The following equations 2 express the changes which these sub- 

 stances undergo when acted upon by carbon dioxid in the laboratory, 

 the equations being so arranged as to represent the results when the 

 same initial weight of copper sulphate is taken in each case. The 



1 The figures in parenthesis throughout this bulletin refer to the bibliography on page 46. 



2 The formulas are expressed id terms of the English imperial gallon, which weighs 10 pounds, while the 

 U. S. gallon weighs 8.3389 pounds, and the English fluid ounce, which is equivalent to 1/20 pint, or 28 ce., 

 while the U. S. fluid ounce is equivalent to 1/16 pint, or 29.6 c-c. 



