42 FRUIT TREES AND THEIR ENEMIES 



potassium cyanide, and on that account is preferable to 

 it, though it is somewhat dearer. Weight for weight, 

 it hberates 30 per cent, more gas than the potassium 

 cyanide. 



The damage done to trees by the gas is usually 

 insignificant, and is generally confined to killing off 

 the ends of a few shoots ; but occasionally the 

 damage will be more serious, and trees are some- 

 times nearly killed, even with doses not strong 

 enough to destroy the eggs of the mussel scale. 

 This discrepancy in behaviour is doubtless connected 

 with differences in the water contents of the cells of 

 the trees, which would affect the results when dealing 

 with a gas soluble in water. 



14. Iron sulphate {Copperas : Green vitriol). — This 

 appears to have little or no value as a fungicide, and 

 cannot, therefore, be used as a substitute for copper 

 sulphate in this respect. Merely as a substance for 

 emulsifying paraffin oil, however, it is an efficient substi- 

 tute for the corresponding copper salt. (See pp. 34, 48). 



15. Paraffin {Petroleum : Kerosene). — Paraffin is a 

 generic name applied to certain substances pos- 

 sessing an analogous chemical constitution, but 

 differing considerably in physical properties, and 

 ranging from gases to solids. Those which are 

 of use in the treatment of trees are all liquids, 

 but these liquids vary largely as regards their 

 volatility. If they are too volatile, they will eva- 

 porate before having time to act on the pests 

 to be destroyed ; if they are too stable, they will 

 remain so long on the trees that the bark of these 

 may be damaged. For general use the best oil is 



