xxviii Reviedial Measitres and Insecticides. 



the action of sulphuric acid on potassium cyanide in the presence 

 of water. The required quantities 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 these are like powder, the re- 

 action is violent and immediate ; but, if in lumps, the reaction takes 

 place more slowly and continues for a minute or longer. The slow 

 reaction is desired, partly because less injury results to the foliage 

 immediately above the vessel. But the lumps must not be too 

 large, for then the reaction is liable to be imperfect owing to a 

 black coating (carbon ?) forming over the lumps and preventing 

 further decomposition by the acid. The water should not be 

 added too soon or part of the cyanide becomes dissolved and gives 

 a violent reaction. The residue which remains in the dishes is 

 buried ; and the dishes are washed in clean water before being 

 again used. 



' Time necessary for Treatment. — The cover is left over the tree 

 for thirty minutes in the case of small trees, and forty-five in the 

 case of those over twelve feet in height. At the expiration of this 

 period the generating vessel is removed, and the residue buried in 

 the soil. 



* A number of trees are fumigated together, the endeavour 

 being to treat as many at a time as can be covered and uncovered 

 duried the period of exposure. In this way the men are kept 

 continuously busy, the time for the removal of the first tent 

 arriving by the time that the last tree is covered. 



'■Absence of Sunlight necessary. — The originators of the fumiga- 

 tion process observed that the gas was most efficacious, and that 

 less injury resulted to the foliage when the operations were per- 

 formed at night than when they were carried on in sunlight. It is 

 said that chemical changes are produced in the gas by the action 

 of sunlight, and that the resulting gases are more injurious to the 

 plant life and less to animal than hydrocyanic acid gas. Whether 

 or not these theories are correct is of small practical importance, 



