386 Diseases of Greenhouse Crops 



span house compute the number of square feet in 

 the rectangle and in the right angles, and multiply 

 the sum of the three by the length of the house. 

 To secure the cubical contents of a three-quarter 

 span house multiply the sums of the areas of the 

 rectangles, and the areas of the right angle triangles 

 by the length of the house. In estimating the cubi- 

 cal contents of a greenhouse it is not necessary to 

 make allowances for the space occupied by pots or 

 benches. Fumigation should never be attempted 

 during high winds. It should never be done during 

 the day and not earlier than one hour after sunset. It 

 is never wise to fumigate when the outside weather is 

 near the freezing point. Nor is it well to fumigate 

 during humid nights. The best time to fumigate is 

 when the temperature ranges from $S to 68 degrees 

 F. 



The chemicals required for fumigation are either 

 sodium cyanid (NaCN) or potassium cyanid 

 (KCN), sulphuric acid (H2SO4), and water (H2O). 

 Sodium cyanid is preferred. It should be free from 

 chlorin and contain not less than 51 per cent of 

 cyanogen. Cyanid is a violent poison. It should 

 be stored in airtight cans and carefully labelled 

 "Violent Poison." Commercial sulphuric acid of 

 66 degrees Baume or 1.84 specific gravity will answer 

 the purpose. Upon referring to Tables 20 and 21 

 we see at a glance the amount of cyanid per each 

 1,000 cubic feet necessary to kill the particular in- 

 sect and the amount each plant can stand. For ex- 

 ample, if one-half ounce of cyanid as indicated in 



