126 Control of Parasites 
neutralize the acid of the copper salt, which would injure 
the foliage, the reaction leaving a copper hydrate, which, dis- 
solved in the carbonic acid of the air, is the active principle 
in the fungicide. This solution should be used the day it 
is prepared, and before using should be strained to avoid 
trouble in the spraying nozzle. Since the Bordeaux mixture 
is a disinfectant so useful that every gardener or owner of 
trees should have it on hand and use it freely, it will save 
time to make a stock solution of blue stone in the propor- 
tion of one pound to two gallons of water, when two and a 
half gallons of this solution will be required to make a barrel 
of mixture. The lime may also be slacked beforehand, in 
the proportion of two pounds to one gallon, providing it is 
kept under water until used. In this condition it is not 
easily measured, although “half and half” will make about 
the proper mixture; but it is well to apply a simple test to 
show when sufficient lime has been added to the blue stone. 
For this test an ounce of yellow prussiate of potash, 
obtained from the drug store, is dissolved in half a pint of 
water; if a few drops of this, added to the Bordeaux mix- 
ture, produces a brown color, it shows that not enough lime 
has been added. A simpler test is to hold a bright knife 
blade in the solution for a minute or more: if it comes out 
copper colored, more lime is to be added. A small excess 
of lime does no harm. Keep the solutions covered to pre- 
vent evaporation. When the two solutions are mixed 
they should be used within twenty-four hours; separate, they 
may be kept through the season. 
There are now prepared limes in the market, powdered 
or partly slacked, which, if fresh, are more effective and 
more easily handled. 
Since the mixture is not really a solution but an emulsion, 
which consists of minute insoluble particles of the sub- 
