METHODS OF PEEVENTION AND REMEDIES. 55 



these are like powder the reaction 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 pre- 

 venting 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 during 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 fumigation process observed that the gas was most 

 efficacious, and that less injury resulted to the foliage 

 when the operations were performed 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, for the 

 foliage of a tree will suffer serious injury if the tree is 

 left covered with an airtight oiled tent for half an hour 

 in sunlight without the gas being present. Having 



