IXSECTICIDES, PREVENTIVES, AND MACHINERY. 451 



seeds. Where used in bins or closed receptacles, to kill insects 

 infesting their contents, one drachm should be counted for each 

 cubic foot of space, and a drachm may be roughly measured as a 

 teaspoonful. It may be placed in an open vessel on top of the 

 grain or other material, as the vapor, being heavier than the air, 

 will descend to the bottom. It is also useful in many cases 

 against plant-lice, and the range of the material has been already 

 given in the various cases for which it is recommended. It is 

 exceedingly inflammable and explosive, and in no building where 

 the material is used should a light be allowed or a fire of any 

 kind. Lighted pipes or cigars must be avoided, and even a 

 spark may cause an explosion. With ordinary precautions, how- 

 ever, the material is a useful one, since entire buildings can be 

 treated at once. 



Hydrocyanic acid gas is another vapor exceedingly destructive 

 to all hfe, but much more fatal to animals than to plants. It may 

 be used for fumigating nursery stock or for destroying scale and 

 other insects on trees. The vapor is produced as follows : in a 

 glazed earthenware vessel place in the proportion of three fluid 

 ounces of water and one ounce of ordinary sulphuric acid ; then 

 add one ounce of fused potassium cyanide of ninety-nine per cent, 

 strength. This will generate a very light and extremely poison- 

 ous gas which is fatal to animal life of all kinds. The quantity 

 above given suffices for one hundred and fifty cubic feet, and for 

 a larger or smaller space a proportionate amount should be used. 

 Trees to be treated with this gas must be covered by a tight tent 

 of some kind, and for this purpose an eight-ounce duck, soaked 

 with boiled linseed oil, may be used. Where shrubs or small 

 trees are to be fumigated, a covered frame may be used and 

 placed over the plants by hand ; but where large trees require 

 attention, it will be necessary to have machinery mounted on 

 wagons. In the chapter on insecticide machinery, one of these 

 tents will be found figured. This hydrocyanic acid gas seems to 

 be less fatal to plants and more effective against insects at night 

 and in cool weather, and treatment at such periods is recom- 

 mended ; or the tent may be painted black. It should never be 

 used in hot weather during the middle of the day. This is 

 another case where an insecticide seems to be more effective on 

 the Pacific coast than in the East ; but it has not been sufficiently 



