Studies on Fungicides — III 27 



either to completely prevent the germination of conidia of Peronospora 

 in this water, or at least so that it cannot take place in a normal manner." 

 Again, to quote these investigators (1887b: 700, translation): "We 

 have proved that the copper hydroxide contained in a drop of the mixture 

 is soluble in rain and dew only when the lime which in the actual mixture 

 is found in enormous excess is completely neutralized, especially by the 

 carbon dioxide of the atmosphere." Millardet and Gay on based this 

 conclusion on an experiment in which they prepared five types of bordeaux 

 mixture, ranging from copper sulfate very slightly in excess, to that con- 

 taining lime eight times in excess, of neutralization. These mixtures 

 were exposed to the rains. The time of the first appearance of copper 

 in the solution varied from five to eighteen days. Clark (1902:43), 

 Crandall (1909:225), Bedford and Pickering (1910:72), Barker and 

 ( rimingham (1911:94), and Lutman (1916:17) all criticize this conclusion, 

 and point out that there is no delay in the fungicidal action of the copper 

 but that it is effective from the time of application. Clark (1902:43), in 

 fact, thinks this should be " sufficient to have set aside the hypothesis 

 which accounted for the solution of the copper of the bordeaux mixture 

 by atmospheric conditions." 



Sostegni (1890), experimenting with bordeaux mixture, passed carbonic- 

 acid gas through the copper hydrate and basic sulfates and succeeded 

 in bringing copper into solution. He therefore assumes that the CO2 of 

 the atmosphere has a solvent action on the precipitated copper compound. 

 These conclusions are accepted by Lodeman (1892). 



Fairchild (1894) is inclined to the view that the copper compounds are 

 slowly dissolved by the rains and dews. He also was able to obtain soluble 

 copper on the addition of C0 2 . 



Though performing no experiments himself, Swingle (1896) suggests 

 various possibilities. He mentions the possible role of atmospheric 

 agencies, but says (pages 19-20 of reference cited) : "It seems scarcely 

 possible that the minute quantities of C0 2 contained in rain water or 

 dew alone could dissolve enough copper to poison any but the most delicate 

 plants. Probably, as claimed by Gayon, the copper hydroxide present 

 is slightly soluble in pure water." 



Bain (1902) seems to have shown conclusively that the injurious action 

 of soluble copper on peach leaves, resulting from spraying with bordeaux 

 mixture, copper hydroxide, and dicupric hydroxide (Cii 3 2 (OH)2), is 

 most pronounced when the plants have been exposed to the rains and 

 dews. This injury was found to be further accentuated in the presence 

 of sunlight. 



Ruhland (1903), studying the effect of plant excretions on the solution 

 of basic copper, found that rain water containing CO2 and other sub- 

 stances from the air could dissolve; very small amounts of copper. How- 



