PICKERING SPEAYS. 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 
arid 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. 
