Materials and Formulas. 127 



cal changes which take place have not been fully determined, 

 but Professor J. T. Willard has suggested the following: 1 



2 (Cu(OH) 2 , CuSO 4 ) + CO 2 = 



basic copper sulphate carbonic 



acid gas 



2 CuSO 4 -f Cu(OH) 2 + CuCO 3 + H 2 O. 



copper copper copper water, 



sulphate hydrate carbonate 



This theory appears to be very plausible and several facts 

 tend to support it. When dew or rain-water gathers upon a 

 leaf, the liquid always contains a certain amount of carbonic 

 acid gas in solution, obtaining it both from the air and from the 

 leaf itself. If the foliage has been sprayed with the Bordeaux 

 mixture, the carbonic acid comes in contact with the copper 

 sediment, and a certain amount of the copper is dissolved. 

 This much has been proved. That the dissolved copper may 

 be in the form of the sulphate is also very probable, since it is 

 well known that a solution of copper sulphate is injurious to 

 foliage, and well-prepared Bordeaux mixture has also caused a 

 similar injury. This alone is not very convincing, but when it is 

 considered that the injury following the use of the fungicide does 

 not take place immediately as a rule, but only after the sedi- 

 ment has been exposed to the air for some time, the position is 

 strengthened. If, however, the weather is of such a nature that 

 the foliage is constantly wet by light showers, not enough rain 

 falling to wash off the leaves but only to wet them, and if this 

 were continued for some time, much injury might be expected 

 to result from the use of even such a safe preparation as the 

 Bordeaux mixture. During the early part of the year 1894 

 such conditions did exist, and much complaint was heard 

 regarding the injury done to both apples and pears by the 

 use of this remedy. It seems very probable that the carbonic 

 acid should unite with some of the copper, and also with some 

 of the lime, although no mention is made of this in the above 

 reaction, and that pure copper sulphate should result. As the 

 amount of this compound gradually increases, the injury to 

 the foliage and the fruit naturally follows. 



i Cited by Fairchild, U. S. Dept. of Agric. J>iv. Veg. Path. Bull. 6, 14. 



