MATERIALS 45 



slightest description, and often not noticeable. Mix- 

 tures containing as much as 12 and 18 per cent, have 

 been found to be harmless to apples, while dormant ; 

 but it is probable that the repeated use of liquids 

 of this strength would ultimately do harm ; such, at 

 any rate, has been found to be the case in America, 

 though there, still stronger mixtures, often made with 

 crude petroleum, have been used. 



For use when the leaves are on the trees, emulsions 

 containing \\ per cent, of paraffin are the strongest 

 which should be used, though an increase to 2 per cent, 

 causes only an amount of damage to the foliage which 

 is generally too slight to be noticeable. While the 

 trees are in blossom, paraffin insecticides should not 

 be used at all. 



For incorporating paraffin with water, special pumps 

 are in use in America, but a much more satisfactory 

 method is to add some substance which will emulsify 

 the oil. In emulsification the paraffin is broken up 

 into microscopic globules, and there must be present 

 some substance which has the power of preventing 

 these from reuniting: the mixture then remains per- 

 manently milky. Soap, and also solids in an ex- 

 tremely fine state of division, have this power, and 

 several emulsions made with them have already been 

 mentioned. For use in winter, paraffin emulsions are 

 not of much service, unless mixed with caustic soda, 

 for without the soda they will not destroy moss and 

 lichen, and, therefore, those emulsions are preferable 

 which are not broken up (de-emulsified) when soda 

 is added to them. This is so with the emulsions 

 made with basic iron sulphate and basic copper 



