294 BULLETIN 226. 



or by anything else that irritates the skin. To prevent injury from the 

 Bordeaux mixture plenty of lime must be used. Four pounds of blue 

 vitriol to fifty gallons of water is sufficient for the later sprayings. This 

 lessens the danger of russeting and seems to be effective in preventing 

 the scab fungus. Six pounds to fifty gallons of water are ordinarily used 

 for the first spraying. 



Damage to foliage from spraying. The foliage is sometimes burnt by^ 

 too strong a spray. This is particularly likely to occur in rainy weather. 

 However, most of the spots on the leaves that are attributed to the spray 

 are not due to the spray, but to fungi. (See page 340.) 



Professor G. W. Cavanaugh gives the following suggestions about 

 spraying in rainy weather: 



" In the preparation of Bordeaux mixture, which is made from a solu- 

 tion' of blue vitriol and lime, there is a definite chemical union brought 

 about between the copper of the vitriol and the lime. In order that this 

 union may occur, the lime must be in a water-slaked condition, chemically 

 known as calcium hydroxide. Should the lime be air-slaked, i. e. in the 

 form of carbonate of lime, this chemical union with the copper does not 

 take place. The action of the carbonic acid of the air on water-slaked 

 lime is to change the lime to the air-slaked form. 



:( The chemical union between the lime and the copper in the Bordeaux 

 mixture is not a very stable one. In fact, it is so weak that the carbonic 

 acid of the air can, under certain conditions, break up the union and 

 change even that lime which was combined with the copper into air- 

 slaked lime. This, of necessity, frees the copper and puts it in a condi- 

 tion similar to that where an insufficient amount of lime is used in the 

 orignal mixture. This action of the carbonic acid of the air is facilitated 

 if the mixture remains moist for a considerable time on the foliage. The 

 result of this is a burning of the leaves by the free copper sulfate. When 

 the mixture dries on the trees this action of the gas is so much retarded 

 that no injurious results have ever been noticed. It therefore seems a 

 wise precaution, during a wet season at least, to use more lime than the 

 formula ordinarily calls for." 



