DISEASES OF CITRUS TREES ] 97 



The gas employed is of course, hydrocyanic acid gas 

 which is evolved by sodium cyanide when acted upon 

 by diluted sulphuric acid The proportion of chemicals 

 for one good gassing should be 5 grammes of sodium 

 cyanide to 15 cubic c.m. of commercial sulphuric acid or 

 oil of vitriol diluted with at least an equal volume of 

 water, for every cubic metre (stere) of space enclosed by 

 the tent. The required quantity of sodium cyanide, 

 broken in small pieces or roughly powdered is placed 

 under the tree in a strong earthenware receptacle, and 

 the sulphuric acid properly diluted poured into the 

 receptacle. The fumes of this gas are highly poisonous, 

 and the operator should step out of the tent at once 

 after pouring the acid into the receptacle. There are 

 now appliances by which the acid and the cyanide may 

 be mixed together when the operator is outside the tent, 

 in order to prevent the possibility of fatal mishaps. It 

 is also possible to have the hydrocyanic acid gas prepared 

 and condensed in strong metallic cylinders from which 

 the flow of gas into the tent may be regulated by the 

 operator standing on the outside. The gas is allowed to 

 perform its work for about one hour, after which it 

 becomes partly absorbed, and the tent after having been 

 duly opened and ventilated may be removed and th 

 process repeated for other trees. 



