112 VEGETABLE FORCING 
lieved that the tobacco fumes serve to disturb and dis- 
lodge the flies, and that the hydrocyanic gas is then more 
effective in killing them. 
There should be sufficient acid to react with all of the 
cyanide crystals,’ and sufficient water to dissolve the 
potassium acid sulphate which results from the reaction. 
A common formula is one ounce of cyanide of potassium, 
two ounces of sulphuric acid and four ounces of water. 
A more recent formula, which is wholly satisfactory and 
less expensive, is 1-1-3, which provides sufficient acid for 
chemical reaction. Sodium cyanide, apparently, is just 
as effective as potassium cyanide. When this is used the 
formula should be 3-4-6. Sodium cyanide will produce 
more gas, and only three-fourths as much is required per 
1,000 cubic feet as when potassium cyanide is used. 
The jars are usually placed 20 to 30 feet apart in the 
central walk of the greenhouse. Their distance apart 
depends on the width of the houses. A successful grower 
of cucumbers uses 12 jars in a 25 by 300 foot house. He 
places six ounces of water, six ounces of sulphuric acid 
and 114 ounces of cyanide of potassium in each jar. A 
good plan is to have three jars for a house 20 by 100 feet 
in size, four jars for a house 25 by 100 feet and five jars 
for a house 30 by 100 feet in size. In very wide houses 
or in ridge and furrow ranges there may be a number of 
rows of crocks or vessels. 
Preparations for the use of hydrocyanic gas are made 
before dark. The ventilators and all openings of the 
house are closed as tightly as possible. The valves regu- 
lating the heating pipes are adjusted so that they will 
require no further attention until morning. Care is 
exercised to keep all workmen, visitors, children and 
animals out of the houses. The jars are then properly 
distributed along the walks, and water is placed in them. 
The sulphuric acid is added about half an hour before the 
cyanide is to be used. Violent heat is caused by the 
