74 Preparation of Substances 



sandths of one per cent, — while the amount 

 of soluble copper that a leaf can stand without 

 burning is much larger and is in the neigh- 

 borhood of 0.04 per cent. 



The ordinary Bordeaux mixture — con- 

 taining four times as much lime as is needed 

 for producing the insoluble copper compounds 

 — after being spread out on the plant does 

 not begin the liberation of soluble copper until 

 the carbon dioxid of the atmosphere has 

 acted on the excess of lime present. This 

 process requires several days. On the other 

 hand the Woburn Bordeaux having no excess 

 of lime is acted upon by the carbon dioxid 

 at once and soluble copper is available in a 

 short time. 



The increased vigor of plants, particularly 

 potatoes, which is noticed when they have 

 been sprayd with Bordeaux mixture, is due, 

 to the best of our knowledge, to the preven- 

 tion of minor insect ravages rather than a 

 stimulating action of the very dilute copper 

 solution on the chlorophyl. It has been 

 shown by Pickering that potato leaves im- 

 merst in dilute copper sulfate solution give 

 off iron and take on copper and from this it 

 was argued that the dilute copper solution 

 might have an accelerating effect upon the 

 chlorophyl action. Recent work has shown, 

 however, that the simpler explanation of in- 



