MATERIALS 41 



Trees growing in the open may be fumigated by 

 covering them with dome-shaped tents. 



The experience of both the writers of this volume 

 has led them to state at various times that fumigation 

 is not effectual in killing insect eggs, but further evi- 

 dence shows that hydrocyanic acid is really fatal to 

 eggs, provided it is applied in suificient quantity and 

 under sufficiently favourable circumstances, though 

 success does not seem to be a certainty. The fact that 

 it is soluble in water accounts for many of the dis- 

 crepancies noticed, and this would always render its 

 application to trees while in the ground very un- 

 certain in this country ; for the soil here is never dry 

 during the winter. A hard frost is the best type of 

 weather in which to treat such trees. For killing 

 insect eggs, the maximum quantity of cyanide men- 

 tioned above should be employed, and, if it is the 

 potassium cyanide which is used, this must be the 

 "lump" cyanide, which is practically pure, and not 

 "stick" cyanide, which contains only 40 per cent, of 

 true cyanide. The operator should also make certain 

 that the action has been complete, for occasionally the 

 lumps of cyanide become coated with a deposit of 

 acid sulphate, which protects them from further 

 action before more than half of the cyanide has been 

 attacked by the acid. To obviate this, the cyanide 

 should be broken up into pieces not larger than beans, 

 and it is best not to use paper for wrapping it up, as 

 is generally recommended, for some paper becomes 

 converted by the acid into parchment paper, and the 

 packet of cyanide reinains intact. 



Sodium cyanide dissolves more easily than the 



