26 

 Copper-Salt Fungicides. 



Bordeaux Mixture. 



This substance derives its name from the city of Bordeaux, in 

 France, as it was in the vineyard district surrounding this place that it 

 was first found useful. Therefore the name gives no" indication as to 

 what Ingredients are present. 



The source of the copper in this fungicide is copper sulphate, or 

 what is commonly known as "bluestone. " Now copper sulphate is an 

 "acid salt," i.e., it is a salt which shows an acid reaction and will turn 

 blue litmus paper red. This property of blue stone is due to the fact 

 that it is a compound formed by the combination of a weak alkali 

 (Cu(OH) 4 ) with a very strong acid (H 5 SO^), the outcome being that of 

 a salt in which the acid property predominates. Being acid, therefore, 

 copper sulphate cannot be applied to foliage, because it exerts a burning 

 or destroying influence. Like white arsenic, it must be changed to some 

 other form, a form which will not be injurious. Bluestone can be applied 

 t) foliage without doing harm, but in such a dilute form that it is not 

 very effective. 



The material used to overcome the acid property just mentioned is 

 slaked lime or milk of lime. A good sample of lime is secured and slaked 

 with a minimum quantity of water, thus changing it into the hydrate, as : 



CaO + H 2 = Ca(OH) 2 . 



Lime. Water. Slaked lime. 



This slaked lime is then added to the bluestone, which has been 

 dissolved in water, whereupon the following theoretical reaction takes 

 place : 



CuS0 4 + Ca(OH) 2 = Cu(OH), + Ca(SO) 4 . 



Copper sulphate. Copper hydrate. Gypsum. 



Providing enough lime has been used to act on all the copper, the 

 latter will now all be present as copper hydrate, a precipitate or sediment 

 of a beautiful sky-blue color, and which is practically insoluble in water. 

 In this form, as a solid in suspension in water, it is sprayed on to the 

 foliage. 



Being in a solid form, copper hydrate, or Bordeaux, as it is called, 

 is inert and will not act on the fungus. It must be changed to some 

 form on the leaf which will be soluble in water. This change is brought 

 abo it by carbon dioxide of the air and by that contained in dew and rain, 

 or even by that which comes from the leaf itself. 



How tc make the Bordeaux Mixture. The first thing to do in the 

 manufacture of the Bordeaux mixture is to decide on some recommended 

 formula. The formula which has long been advocated in Ontario is 

 known as the 4-4-40 formula. It is as follows : 



CuSO^ (crystallized copper sulphate) 4 pounds. 



CaO (quick lime) 4- 



Water 40 gallons. 



