172 EXPERIMENTAL FARMS. 



Since the efficacy of copper sulphate as a fungicide has been well established — as 

 illustrated by its beneficial action on seed wheat infected by smut spores — and since it 

 is the basis or source of the copper in the spraying mixtures, and a material easily and 

 cheaply obtained, the question is often asked, " Why cannot a simple solution of blue- 

 stone in water be used for spraying ?" The answer may be very briefly stated. A solu- 

 tion of copper sulphate sufficiently strong to prevent the growth of fungous diseases 

 would, so far as our present experience shows, prove injurious to foliage. The corrosive 

 character of this chemical must, by precipitation or neutralization, be rendered inno- 

 cuous before bluestone can be of practical value for spraying when the leaves are out. 

 Fungicides, properly prepared, allow the application of an adequate amount of copper 

 to prove effective and at the same time are non-injurious to foliage. 



Bordeaux mixture is perhaps the best known and the most highly valued of all the 

 copper compound fungicides. The formula as now advocated is as follows : — 



Copper sulphate 4 pounds. 



Lime 4 pounds. 



Water 50 gallons. 



Briefly, the directions for manufacture are : The freshly burnt lime is slaked 

 by the addition of successive small quantities of water, and then well stirred" with 

 sufficient water to make a thin, creamy mixture. This is now strained through coarse 

 sacking into a barrel containing the dissolved copper sulphate, and then well stirred and 

 made up with water to 50 gallons. 



In order to produce a Bordeaux mixture that does not readily " settle out," the 

 milk of lime and the solution of bluestone should both be well diluted before mixing. 

 Concentrated solutions cause a heavy granular precipitate, which quickly separates and 

 falls to the bottom of the barrel. 



Iron or other metal vessels should not be used for dissolving the bluestone in, nor 

 as receptacles for the resulting mixture, since such cause decomposition, injuring 

 both vessel and fungicide. 



The reaction that takes place on mixing, is that the lime in solution precipitates the 

 copper from the solution of bluestone as an insoluble compound — cupric hydrate — the 

 sulphuric acid combining with the lime to form sulphate of lime, which on account of its 

 slight solubility remains to a very large extent in suspension. This reaction is repre- 

 sented by the following chemical equation : — 



CuS0 4 , 5(H 2 0) + Ca(OH), = Cu (OH), + CaS0 4 + 5H Q 

 Copper sulphate. Slaked lime. Cupric hydrate. Sulphate of lime. Water. 



By reason of the slight solubility of lime — 1 part in 750 parts of water — the fifty 

 gallons cannot hold in solution at once the amount of lime necessary to precipitate, or 

 throw out of solution, the four parts of bluestone. Since, however, the sulphate of lime 

 for the most part separates out as it is formed, the same water again takes up lime, 

 which causes a further precipitation of cupric hydrate. The reaction is continuous and 

 rapid until all the copper is precipitated. Finally, we have insoluble cupric hydrate, 

 lime (from the excess used) and sulphate of lime in a Liquid containing small quantities 

 of the two latter materials in solution. 



For the sake of simplicity the reaction has been represented as a simple one, re- 

 sulting in the formation of cupric hydrate and sulphate of lime. But although the 

 above equation depicts the chief and final reaction when the Bordeaux mixture 

 is properly made, several other compounds may be formed, according to the condition 

 of preparation. Before the reaction is complete, or if sufficient lime is not present, 

 basic sulphate of copper, [Cu (OH) 2 , CuS0 4 ] is most probably produced. As pointed 

 out by Professor Willard and others, the exposure of this compound to the atmosphere, 

 as upon the foliage, results in the re-formation of corrosive sulphate of copper. 



Chemistry therefore shows the necessity of an excess of lime, and further explains 

 why an insufficiently precipitated Bordeaux mixture, though it contains no copper in 

 solution, may prove injurious to foliage. The injury arising from the presence of the 

 basic sulphate of copper, as pointed out by Professor Fairchild, may not appear for 

 some time after the application, or until after the first shower. 



