52 THE WHITE-FLY OF GREENHOUSES 



taining squares of 1-4 aud 1-16 inches. Let the 1-4 inch 

 squares represent square feet. Draw on this sheet a cross 

 section of your house, on the scale of 1-4 inch to the foot. 

 Count the enclosed squares, reduce them to feet, and multiply 

 by the length of the house. In this manner the exact cubic 

 contents of any house may be easily found no matter how 

 irregular its dimensions. 



HOW TO MAKE THE GAS 



The amount of cyanide used in these experiments was at 

 the rate of 1 ounce of cyanide of potassium to 400 cubic feet 

 of greenhouse space. 



Use an earthen pot ; do not use metal ; this vessel should be 

 large enough so that the liquid does not run over on to the 

 greenhouse floor. In our experiments an earthenware gallon 

 jar was used, and this was not any too large. 



Pour the water into the vessel, add the acid ; then weigh 

 the cyanide, put it in a paper bag. Close the ventilators, and 

 firmly fasten every door of the house except the one used by 

 the operator. This caution should not be neglected, as the 

 writers have repeatedly witnessed where a person unawares 

 approached and entered the greenhouse when filled with 

 tobacco fumes. Were this to hapi)en when the house is filled 

 with the hydrocyanic acid gas treatment, the result would be 

 serious, if not fatal. When all is ready, approach the pot 

 with the bag containing the cyanide ; fill the lungs with air, 

 hold your breath, drop in the cyanide, bag aud all, and leave 

 the house at once, lock the last door and notice the time. 

 After the number of minutes required to kill the adult white- 

 flies are passed open the doors and give the gas plenty of time 

 to escape before you enter. 



Hydrocyanic acid gas is very poisonous, and the house 

 should not be entered during treatment. 



These experiments related to tomatoes and cucunihei^s under 

 glass. We do not know what effect the gas might have o?i other 

 plants and advise 2^^eliini7iary trials before it is used. 



Greenhouses used for growing vegetables can often be given 

 a stronger treatment at times when a crop is matured and the 

 house is empty. At such times the gas may be generated in a 

 much larger quantity and left over night. It will thus be 

 pretty sure to kill all insects present. 



