INSECTICIDES, FUNGICIDES, ETC. 173 



namely, the compound present in it. This question was attacked 

 at Woburn, 1 and was found to be one of great complexity. 



The addition of lime, preferably in the form of clear lime- 

 water, to a solution of copper sulphate, forms a series of different 

 compounds (basic sulphates) according to the proportions taken. 

 The simplified chemical formulae of these, and the proportions 

 of sulphate (CuSO 4 ,5H 2 O) and lime required for their formation, 

 are as follows 



Proportions required. 

 Sulphate. Lime. 



A. ioCuO,2-5SO 3 (or 4CuO,SO 3 ) . i : 0-169 



B. ioCuO,2SO 3 . . . . i : 0-18 



C. ioCuO,SO 3 . . . . i : 0-203 



D. ioCuO,SO 3 ,3CaO . i : 0-270 

 F. ioCuO,aoCaO . . . . i : 0-674 



The compound A is formed when the lime is just sufficient to 

 precipitate the whole of the copper; C is the product when the 

 proportion of lime is increased up to the point when any further 

 addition would make the liquid alkaline, and F is the product 

 finally obtained with excess of lime. In ordinary Bordeaux 

 mixture, the proportion of lime taken is greater than that 

 required to form any of these compounds, but the whole of this 

 lime will not dissolve in the water taken, and the greater part of 

 it remains inoperative, so that it is mainly the compounds C 

 arid D which are formed, and these are only gradually, after 

 several days, transformed into F, as the excess of lime acts on 

 them. 



In instructions for making Bordeaux mixture, great stress is 

 alwa}^s laid on the necessity for insuring the complete precipita- 

 tion of the copper : doubtless this is quite right, for carelessness 

 in manufacture might result in there being enough copper left 

 in solution to do serious damage to the trees : but the insistence 

 on this point is probably responsible for a very general mis- 

 conception on the elementary principle which must govern 

 fungicidal action; for, such action, being fundamentally a 

 chemical one, is subject to the laws of chemistry, and one of 

 these laws is that, for any chemical reaction to be possible, one 

 or more of the reagents must be in the liquid (or dissolved) 

 condition. A reaction between an insoluble solid outside the 

 cells of a fungus spore, with the contents of those cells while 

 still enclosed within the cells, is a chemical impossibility. Either 



1 Pickering, Trans. Chem. Soc., 1917, Vol. IIT, p. 86. 



