INSECTICIDES, FUNGICIDES, ETC. 193 



Moreover, the strengths of solutions effective in two cases 

 are of the same order of magnitude, and must, in many cases, 

 be identical. Thus, the scorching action, which in the strongest 

 solutions amounted to about 90 per cent., did not entirely dis- 

 appear with the weakest solutions, but at a certain strength 

 became constant, the residual effect being evidently due to the 

 natural dying of the leaf. On plotting out the results, it was 

 possible to determine the point at which this constancy was 

 reached, that is, the point at which the scorching action of the 

 copper became nil ; this varied, according to the nature of 

 the leaf investigated 



from -00016 to -0025 ; mean, -0007 per cent, copper. 



Clark's values for the smallest effective dose (Lee p. 186), on 

 different fungi varied- 

 from -00004 to -00024, mean, -00017 per cent, copper, 



or, taking the mean of his values for the smallest effective dose 

 and smallest lethal dose, the variation was 



from -00015 to -0026, mean, -0008 per cent, copper, 



values practically identical with those for the scorching action. 



Such close concordance is, of course, accidental, but its exist- 

 ence is sufficient to show how closely the scorching and fungicidal 

 action hang together. This does not mean, however, that it is 

 impossib'e to have fungicidal action without scorching, for both 

 fungi and leaves vary so much in their sensitiveness a variation 

 of ten- to twentyfold that a sensitive fungus on an insensitive 

 leaf might well be affected or killed, without the leaf being 

 perceptibly injured. Taking the particular fungi examined by 

 Clark, and the particular leaves examined at Woburn, the 

 minimum dose of copper which causes scorching would be four 

 times as great as the minimum producing a fungicidal effect. 



When the various series of results on scorching were examined 

 by plotting them out in a certain way, it was found that they all 

 lay on straight lines, the nature of the equation expressing these 

 lines l implying that a large increase in strength produces only 



1 The effect is represented by the equation log. a x -~ j- , p being the 



percentage of copper, and a and b, constants which would vary with the 

 particular leaves taken, the former being the percentage strength at which 

 the action ceases, and the latter the rate of increase of the action with the 

 strength. 

 O 



