INSECTICIDES, FUNGICIDES, ETC. 175 



mixture, as well as that which forms part of the compound itself 

 (represented by 3CaO in the formula D), becomes converted into 

 carbonate (chalk) the residue will consist of ioCuO,SO 3 , which is 

 sufficiently soluble to enter into chemical reaction. The carbon- 

 ation of the lime necessary before any copper becomes soluble, 

 explains a fact which had been fully established by previous 

 observers, namely, that a considerable time elapses after spraying, 

 before the fungicidal action of Bordeaux mixture begins to occur : 

 thus, Millardet and Gayon 1 found that 18 days elapsed before Bor- 

 deaux mixture, which was being washed with water, yielded any 

 copper in the drip -water. The delay in actual spraying is, however, 

 probably considerably less than such a result (or the results 

 obtained at Woburn, to be mentioned on p. 19 1), would indicate ; 

 for, when the drops of Bordeaux mixture dry up on the leaves, 

 some of the particles of basic sulphate become separated from 

 those of the lime, and are decomposed by the carbonic acid, just 

 as if no excess of lime were present : by substituting a piece of 

 paper impregnated with potassium ferrocyanide, for a leaf, 

 red spots make their appearance here and there, just before the 

 mixture dries up. Thus there may be an almost immediate, 

 though limited, fungicidal action, even with ordinary Bordeaux 

 mixture. 



Soluble copper being necessary for fungicidal action, it might 

 appear at first sight that the ideal fungicide would be one in 

 which the copper was all initially in the soluble form, for its 

 action would be more prompt, and it would be a simple matter to 

 determine the strength suitable for use in any case. But general 

 experience has pronounced against such compounds, and it is 

 easy to see that, if they were used, any rain or dew would readily 

 wash them off, and necessitate further sprayings : added to which, 

 they would not really afford a coating of uniform strength over 

 the foliage, for when they dried up they would become concen- 

 trated into separate droplets of great strength, producing ex- 

 cessive action and scorching at a few points, and leaving the rest 

 of the leaves unaffected. What must be aimed at, therefore, in 

 a fungicide is to obtain a material which will yield a gradual 

 supply of soluble copper, sufficient to kill the fungus, or to prevent 

 its development, but insufficient (if possible) to injure the host 

 plant. 



Whichever of the basic sulphates of copper is used, this gradual 

 supply of soluble copper is obtained, owing to their being them- 

 selves slightly soluble, or, in the case of D, becoming so after 



1 Journ. d'Agric. prat., 1887, 701. 



