126 The Spraying of Plants. 



hydrate, water, and the sulphate of lime, in accordance with 

 the following reaction : 



CuS0 4 , 5H 2 + CaOH 2 = Cu(OH) 2 + CaSO 4 -f <5 H 2 O. 



copper sulphate crystals +water-slaked lime=copper hydrate+lime sulphate-fwater. 



It was soon found, however, that these changes would not 

 account for the various phenomena noticed during its manufac- 

 ture. Careful observation has shown that when lime is added 

 to a certain point, the mixture assumes a greenish tinge, due 

 to the formation of precipitates, probably basic sulphates of 

 copper, in which this color predominates. It is a popular idea 

 that the addition of lime is necessary in order to neutralize any 

 free sulphuric acid which may be present. If such were the case, 

 the precipitate would be largely a sulphate of lime, with no hy- 

 droxide of copper. When lime is added to a solution of copper 

 sulphate, the latter compound is entirely broken up and new ones 

 formed. The presence of the basic sulphate of copper is thus 

 explained. When sufficient lime is added so that the copper 

 sulphate is entirely neutralized, most of the copper is probably 

 precipitated in the form of a hydrate. But at least one other 

 compound is sometimes formed in the Bordeaux mixture. This 

 appears most commonly when an excess of lime has been added 

 to a concentrated form of the mixture. It ma} 7 be a double 

 basic sulphate of copper and lime, but so little work has been 

 done regarding its formation and action that no definite state- 

 ments can be made. 



Whatever may be the composition of the Bordeaux mixture, 

 it is certain that all but a trace of the copper is in the form of 

 a precipitate or sediment which is practically insoluble in water. 

 This settles to the bottom, leaving a clear solution above. This 

 solution is of no value as a fungicide, for the sediment contains 

 all the compounds useful for this purpose, and therefore the 

 mixture should be kept thoroughly stirred that the sediment 

 may be uniformly applied. 



But the reactions which take place in the Bordeaux mixture 

 do not cease when the material is applied to the plant. It is 

 well known that carbonic acid will cause considerable quanti- 

 ties of copper to enter into solution again if the acid comes in 

 contact with the copper sediment of the mixture. The chemi- 



