INSECTICIDES, FUNGICIDES, ETC. 195 



action, and produced an effect only by becoming converted 

 into electropositive copper (XI, 132). 



The extent to which a fungicide is capable of wetting the leaf 

 or the fungus, must be an important factor in determining its 

 efficiency. Soap, as is well known, possesses great wetting 

 powers, but it cannot be used with copper compounds, as it 

 forms with them an insoluble copper soap. Saponin (or quillaia, 

 which contains saponin) possesses still greater wetting powers, 

 and so its effect on the scorching action of copper compounds 

 on leaves was examined. Leaves dipped into a solution of 

 saponin were found to retain two to four times as much liquid 

 adhering to them as when they were dipped into water : possibly, 

 however, this gives an exaggerated estimate of the wetting 

 powers of saponin, for some of the liquid is spread over the 

 leaf in the form of a film, stretching from point to point, without 

 really wetting intermediate portions : a rose leaf wetted, appar- 

 ently, by saponin, when put in a stream of water, comes out 

 practically dry, showing that no actual wetting of the surface 

 has taken place. The addition of saponin to copper sulphate 

 was found to have no appreciable effect on the scorching action 

 of the latter; the results of several series of experiments with 

 different sorts of leaves showed an increase of only a few units 

 per cent. (XI, 163). With the basic sulphates of copper, however, 

 the presence of saponin did produce some increase in the action, 

 though not uniformly so in every series; but this increase 

 was probably not due to any increase in the wetting of the leaf, 

 but to the fact that the saponin, when present in quantities 

 exceeding a certain proportion, dissolves the basic sulphate 

 entirely. Such a solution is, of course, open to all the objections 

 applying to the use of a mere solution as a fungicide (p. 175), 

 and thus the addition of saponin to copper fungicides cannot be 

 recommended. It might, of course, be of value in the case of 

 some insecticides. 



