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REFLECTIONS ON DISEASE TREATMENT 179 
(2) The copper sulphate solution must not be placed 
in vessels of tin, iron, or zinc, as it corrodes them and 
loses its strength. 
(3) Concentrated solution of copper sulphate and lime 
should never be mixed. 
(4) The lime and water should be added to the copper 
sulphate solution and not vice versa. 
(5) Hot water should not be used in the preparation. 
(6) The mixture can only be used when freshly 
prepared, as it loses its fungicidal powers after standing 
12 hours. 
(7) Bordeaux Mixture must be strained before trans- 
ferring to the spraying machine, and constantly shaken 
or stirred during use. | 
The action of the lime on the copper sulphate is to 
produce compounds called basic copper sulphates, which 
possess valuable fungicida] powers. If insufficient lime 
is used to combine with all the copper sulphate (which 
happens when the lime is not up to standard), a certain 
amount of the latter will remain free in the solution and 
will scorch the foliage when the solution is applied. It 
is important, therefore, to make quite sure that enough 
lime has been added and that no free copper sulphate 
remains, as shown when the yellow prussiate of 
potash test is applied. A 10 per cent solution of 
yellow prussiate of potash, which may be secured from 
any chemist, is used. If a drop of this solution be 
allowed to fall on to the surface of the mixture after 
thorough stirring the drop will turn a reddish-brown 
colour should any unchanged copper sulphate be present. 
If this happens more lime must be stirred in until the 
brown colour fails to appear when a drop of the test 
solution is added. 
As a rule, the normal mixture is used upon plants 
with strong foliage grown in the open, but plants under 
glass are generally more sensitive, and a modification of 
the mixture is used. 
For very tender plants 3 lb. of copper sulphate, 6 Ib. 
