128 The Spraying of Plants. 



The formation of the carbonate of lime may take place as 

 soon as, or even before, the appearance of the copper sulphate, 

 since the acid would probably act more energetically upon the 

 hydrate of lime than upon the copper compounds ; therefore 

 the appearance of the copper sulphate would depend to a 

 certain extent upon the absence of the hydrate of lime. This 

 well explains the tardiness with which the Bordeaux mixture 

 injures foliage. 1 



Several corollaries follow from the above, and these have 

 considerable practical bearing on the method of making the 

 mixture. 



If a larger amount of lime is used than is required to satisfy 

 the immediate chemical changes which take place, the more 

 slowly will the fungicidal action of the Bordeaux mixture ap- 

 pear. In wet seasons this is an advantage, since the mixture 

 will retain its efficiency longer, and less injury will be done. 

 The disadvantages of using much lime are very easily realized 

 by all who have applied the mixture. The machinery is apt 

 to be clogged, and the liquid becomes more difficult to handle 

 and to apply uniformly. The particles of lime probably also 

 offer more resisting surface to rain in heavy showers, and more 

 of the material will be washed from the trees. The use of as 

 small an amount of lime as possible would therefore appear to 

 be desirable, but such is the case only to a limited extent. 



The use of the ferrocyanide of potassium test, or Patrigeon's 

 method, has already been mentioned on page 46. This test 

 shows exactly how much lime is necessary to satisfy all imme- 

 diate chemical changes, and it serves, therefore, as an index of 

 the minimum amount required. The Bordeaux mixture so 

 prepared is of an intense blue, and, as more commonly made, 

 the amount of sediment is comparatively small. When applied 

 to plants it is easily handled, and is in this respect the most 

 satisfactory preparation. Its fungicidal action probably begins 

 as soon as the application is made, and the copper is more ener- 

 getic than when it is in the presence of considerable quantities of 



1 For more detailed accounts of the chemistry of Bordeaux mixture consult the 

 work of Chester and of Sostegni. An abstract of Sostegni's article is in Cornell 

 Agric. Exp. Sla. 1892, Bull. 48. Fairchild has written an exhaustive article on 

 this fungicide, which was published as Bull. 6 of the Division of Vegetable Pathol- 

 ogy, U. S. Department of Agriculture, and entitled " Bordeaux Mixture as a Fungi- 

 cide." 



