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, S0 3 (or 10 CuO, 2 S0 3 ). 
Formula <?.— 10 CuO, S0 3 . 
Formula ZX— 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 in 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 cc, 
while the U. S. fluid ounce is equivalent to 1/16 pint, or 29.6 cc. 
