MATERIALS 39 



to a certain extent mechanically, forming a coating 

 over the Q%^s>, through which the insects cannot break ; 

 but it also, no doubt, acts chemically, the comparative 

 weakness of its causticity being counterbalanced by 

 the large proportion in which it is used, and the long 

 time required for it to become entirely converted into 

 carbonate. 



In Canada this wash has been in use for many 

 years for cleansing trees from moss, lichen and rough 

 bark, and also for destroying scale ; and in Nova 

 Scotia it is being tried for reducing the attack of the 

 leaf-curling apple aphis ; but experiments in this 

 country show that for removing moss, etc., and scale, 

 it is inferior to the paraffin-soda washes. 



As already stated, a powerful spraying machine 

 fitted with a special nozzle is required for applying 

 this and other lime-washes. 



All these lime-washes appear to delay the expansion 

 of the blossoms for a few days, which, on the whole, is 

 an advantage, by increasing the probability of their 

 escaping damage from frost. But other washes also 

 (soda-paraffin washes) have a similar action, when 

 applied late in the winter. 



13. Hydrocyanic acid {Pnissic acid). — This is a gas, 

 and is used for destroying various insects in green- 

 houses (scale, mealy bug, thrips, weevils, etc.), and 

 those on young nursery stock (woolly aphis, especially) 

 before planting out. "When used with the precautions 

 mentioned below, and under favourable conditions, it 

 should destroy also the eggs of these insects ; but if not, 

 a second fumigation, in the case of greenhouse plants, 

 will be necessary, to kill the insects hatched from the 



