110 VEGETABLE FORCING 
which should be 98 to 99 per cent pure. Ordinary com- 
mercial sulphuric acid will be needed to produce the gas, 
and a glass or porcelain measure or dipper should be pro- 
vided to handle the sulphuric acid. Metal dippers are 
quickly destroyed by this acid. A suitable basket will be 
required to carry the packages of cyanide through the 
greenhouses. 
Numerous experiments have been made to determine 
the amount of cyanide of potassium which will prove 
effective in destroying the white fly and yet cause no 
injury to the plants. So many factors are involved that 
no general rule can be given. Much depends on the con- 
dition of the houses. Poorly constructed and old houses 
with many small openings between the panes of glass 
and wooden parts will require more cyanide than new, 
tightly built ranges. Young, tender plants are much 
more easily injured than older plants with tougher 
tissues. If it is necessary to fumigate on a windy night 
much of the gas will escape. Again, if the plants are 
wet or if the humidity of the house is very high the plants 
will be susceptible to injury. 
The earlier writers on this subject advocated one ounce 
of cyanide of potassitim to 5,000 cubic feet of space. 
Later, when fumigators became more skillful, one ounce 
to 8,000 cubic feet was often employed. More recently 
some of the most extensive and successful growers find 
that when proper conditions exist the plants are not in- 
jured if an ounce of cyanide is used to 1,000 cubic feet. 
This amount, however, is probably the maximum quan- 
tity which should be used under the most favorable con- 
ditions. Some of the most cautious growers prefer to 
make lighter treatments of one ounce to 2,000 to 2,500 
cubic feet, and to fumigate more frequently. Ordinarily, 
the treatment should be repeated at intervals of ten days 
until no insects can be found. As a preventive measure 
some greenhouse men fumigate at intervals of two wecks, 
