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 potassium to 5,000 cubic feet of space. 

 Later, when fumigators became more skillful, one ounce 

 to 3,000 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 weeks, 



