54 METHODS OF PREVENTION AND REMEDIES. ' 



Wardian cases, and may be treated separately. We 

 have in the ' Wardian case ' an airtight chamber 

 ready to hand, in which the plants can be fumigated 

 before their removal. After a large series of experi- 

 ments with various fumigating media, I find that 

 hydrocyanic acid gas remains by far the most efficient 

 insecticide, and the least injurious to the plants. But 

 with delicate succulent plants I find it has to be ap- 

 plied rather differently. A more concentrated dose of 

 the gas applied for a shorter period is most satisfactory 

 in its results. In a Wardian case containing about 

 sixteen cubic feet I find a dose of half an ounce cya- 

 nide, half an ounce acid, and one ounce water, with an 

 exposure of half an hour, will kill every individual of 

 a colony of Orthezia (the most resistent of all Coccids) 

 without in the least affecting the plants. ' The treat- 

 ment should be carried out only after sunset. Ac- 

 cording to Mr. Lounsbury's tables, these proportions 

 of chemicals should be sufficient for a space of 140 

 cubic feet with a longer exposure." 



Mr. Green further describes the treatment of orchard 

 trees on a large scale, quoting again from Mr. Louns- 

 bury's method. 



" Generation of the Gas. Hydrocyanic acid gas is 

 generated by the action of sulphuric acid on potassium 

 cyanide in the presence of water. The required quan- 

 tities of the cyanide and water are first placed in the 

 generating vessel, the cyanide being broken into small 

 pieces not above the size of lump sugar. The tree is 

 then covered with the tent or sheet, and the vessel 

 slipped under almost to the base of the tree. Reaching 

 in, the operator then adds the acid, pouring it slowly 

 into the vessel so as to avoid its splashing, and thus 

 burning his hand or the cloth. He immediately with- 

 draws, and the men shovel a little soil on the edges of 

 the cloth all around, to more thoroughly prevent the 

 escape of the gas. 



" The rapidity of the evolution of the gas depends 

 largely upon the size of the pieces of cyanide. If 



