730 Ji'nn/al of Ai^nc/il/z/rc. [lo Nov., 1910. 



decomposed, the sulphuric acid uniting with the lime and forming sulphate 

 of lime, while the copper formed a hydroxide. To put the action sup- 

 posed to take place in the form of a chemical equation : — 



CuSO, + (.HlUH), = CaSO^ - Cu(<)H), 



(Copper Sulphate) (CaUiuni Hydroxide; (Sulpliate of Lime) (C<ii>per Hydroxide) 



the lime taking the place of the copper and the copper that of the lime. 

 Quicklime is chemically named calcium oxide (CaO), and when it unites 

 with water, as in the process termed " slacking " of lime, it forms calcium 

 hydroxide (Ca(OH).-,\ which is the soft white powder known as '' slacked 

 lime. " ' 



Instead of the chemical action being so simple as the equation would 

 indicate, it is found to be very complex, and an in\estigation of the com- 

 pounds formed by the acticjn of lime on copper sulphate has shown that as 

 many as six different substances ma) be produced, according to the propor- 

 tions taken. The chemistry of Bordeaux mixture has only quite recently 

 been inxestigated by Spencer U. Pickering, FT<..S., and as he says, " It 

 is remarkable that the chemistry of a fungicide in such general use should 

 never have been elucidated." Such an investigation has show'n, among 

 other things, that the cost of the substance may be considerably reduced, 

 without impairing its efficiency. Onlv the essential points can be given 

 here, but those who are interested will find details in the Eighth Report of 

 the Wobitni Experimental Fruit Farm (1908). 



The normal copper sulphate may be represented as a compound of 

 copper oxide (C'uO) and sulphur trioxide (SOy ) or CuO. SO^. \Mien 

 lime is added to this in solution, it only takes away a portion of the 

 sulphur trioxide and leaves what is known as a basic copper sulphate, 

 that is to say, a copper sulphate containing relatively a larger amount of 

 copper oxide. \\'hen lime is added in just sufficient quantity to precipi- 

 tate all the copper from the liquid, the basic sulphate of copper which is 

 formed contains only one-quarter of the sulphur trioxide present in the 

 normal sulphate. But when lime is added in the proportions usually 

 taken for Bordeaux mixture, then only one-tenth of the sulphur trioxide 

 is left. 



When the spraying mixture is applied to the fruit trees, it is acted on 

 by the carbonic acid of the air, and since the ordinary Bordeaux contains 

 basic calcium sulphate, which the other does not. and since this is attacked 

 more easilv than the basic copper sulphate, it follows that until all the 

 former is converted into carbonate of lime, the latter is not acted on. 



The result is that the ordinary Bordeaux mixture takes some time before 

 ii; begins to act as a fungicide. That is the first advantage to be gained 

 by using only the proper proportions of lime, for then no basic calcium 

 sulphate is formed and no time is lost in the starting of the fungicidal 

 action. I^ut there is another advantage. It is found by experiment that 

 the amount of normal sulphate of copper liberated by the action of the 

 carbonic acid of the air is about twelve times as much, when the proper 

 proportion of lime is used, as in the other. 



It will now be evident why lime-water is employed instead of milk of 

 lime, for then we know exactly how much lime is in solution and can 

 adjust the quantity required in proportion to the amount of bluestone 

 used. 



The carbonic acid of the air in the presence of moisture has a double 

 action on the copper compounds formed. It unites with some of the 

 copper to form a carbonate of copper as well as liberating some of the 

 copper in the form of the sulphate. Thus although in Bordeaux mixture. 



