176 SCIENCE AND FRUIT GROWING 



exposure to the air : but the air, or, rather, the carbonic acid in 

 it, helps to liberate soluble copper from all these compounds, 

 and not from D only, for it converts the basic sulphate into basic 

 carbonate, the other product of the change being ordinary copper 

 sulphate. The chemical equations representing such actions 

 are 



A. ioCuO,2-5S0 3 + 375CO a - 375(CuO) 2 CO 2 + 2-5CuSO 4 . 

 C. ioCuO,SO 3 + 4-5CO 2 = 4-5(CuO) 2 CO 2 + CuSO 4 . 



The proportion of copper sulphate thus liberated is different 

 in the different cases : the above equations represent the same 

 amount of copper as being present in the two compounds taken, 

 namely, loCuO, but the sulphate obtained from A is 2-| times 

 greater than that from B (2-5CuSO 4 , against CuS0 4 ). It is 

 evident, therefore, that the amount of copper contained in even 

 very similar substances, is by no means necessarily a measure 

 of their efficiency, and that growers who gauge the value of a 

 copper fungicide simply by the percentage of copper in it, may be 

 very much misled. 



The copper liberated in the case of the compound A will be 

 25 per cent, of that contained in it ; in the case of C, only 10 per 

 cent. : with ordinary Bordeaux mixture, although it contains 

 copper and sulphur in the same proportion as C (10 : i), the 

 proportion of copper liberated will be still less, for it contains 

 also excess of lime, and this will destroy some of the copper 

 sulphate formed, re-making some of the compound C, which, in 

 its turn, will be decomposed, yielding, again, only 10 per cent, of 

 the copper in it. Direct experiment verified these conclusions : by 

 the action of carbonic acid, 25 per cent, of the copper in A 

 became soluble in about two hours, and 10 per cent, of that 

 in C, when the main action was complete ; whereas with ordinary 

 Bordeaux mixture no soluble copper made its appearance till 

 after several hours, and the amount eventually passing into 

 solution was only 3 to I per cent, of the total present (XI, 31). 

 The results, however, are very much complicated by the presence 

 of by-products calcium sulphate and carbonate and after the 

 main action is complete, subsidiary actions occur, resulting, first 

 in a gradual increase, and then a decrease in the soluble copper 

 present (XI, 28, 62 ; XIV, App. 279 ; Proc. Roy. Soc., A. 94, 317) ; 

 moreover, if air, instead of undiluted carbonic acid, is used, the 

 actual amounts of copper liberated are much reduced, but the 

 general results are sufficient to indicate that, judging by the 

 amount of copper becoming soluble, the basic sulphate A should 



