PICKERING SPRAYS. 5 



whereby the copper of Bordeaux prevented fungous infection of 

 plants. Gimingham (10) and Barker and Gimingham (1, 2) do not 

 accept the theory of Pickering that the carbon dioxid of the air 

 renders the copper soluble in Bordeaux, but believe that the per- 

 meable cell walls of the spores absorb copper from insoluble copper 

 compounds in the spray. 



In 1902 Clark (8) stated that the process of rendering soluble por- 

 tions of the copper hydrate (Cu(OH) 2 ) of Bordeaux mixture, which 

 under orchard conditions is of fungicidal value, is accomplished 

 chiefly by the solvent action of the fungus spores, which have the 

 power to dissolve enough copper to kill themselves. The host plant 

 has a certain power of dissolving copper hydrate deposited on its 

 leaves. 



PHYSICAL PROPERTIES. 



A spray must so distribute the copper compound it contains as to 

 completely cover the trees or plants in a thoroughly uniform manner 

 and must possess the proper adhesive properties. If either of these 

 physical properties is lacking, the spray fails to accomplish its pur- 

 pose. 



Various settling tests with Bordeaux prepared in different ways 

 have been made, and numerous adhesives have been tried. Haw- 

 kins (12), who gives a detailed description of such tests, states that 

 Pickering sprays remain in suspension better than ordinary Bordeaux. 

 Lutman (13), in his data, which include descriptions of the precipita- 

 tion membranes formed in freshly prepared Bordeaux, states that 

 the slow settling properties and the presence of the precipitation 

 membranes in freshly prepared Bordeaux are in a great measure 

 responsible for its superiority as a protective agent against fungous 

 diseases. This investigator studied the areas covered by 1 cubic 

 centimeter of the various sprays tested on glass slides, as a result of 

 which he concludes that "very dilute solutions such as. Pickerings 

 possess a greater covering power for the amount of materials used." 



The physical properties of the Pickering, or limewater Bordeaux, 

 sprays are not described by Pickering, but have been studied by 

 Lutman (14) and by Butler (6). Butler, who has investigated the 

 formation of sphere crystals in various copper sprays, claims that the 

 rate at which they form depends on the concentration and tempera- 

 ture of the mixture. Both of these investigators state that the film 

 membranes of the Pickering sprays do not deteriorate as do those in a 

 regular Bordeaux spray, and that the sphere crystals are not formed 

 in Pickering sprays, even after long standing. This is an important 

 point, as the formation of such crystals signifies a breaking down of 

 the precipitation membranes and a deterioration of the spray. 



