INSECTICIDES, FUNGICIDES, ETC. 153 



be confined to the results so far as they bear on the question of 

 the action of paraffin itself as an insecticide. 



Undiluted paraffin is considered in this country, and very rightly 

 so, to be too dangerous a substance to apply to fruit trees, even in 

 the dormant season, and even the most carefully selected oil cannot 

 be used, as a rule, on apple trees in leaf. As regards insecti- 

 cidal value, measured by the destruction of the eggs of mussel 

 scale, it was found that the more volatile oils naphtha, petrol, as 

 well as wood spirits and methylated spirits were practically 

 useless, and that for a petroleum oil to be effective, at least 40 per 

 cent, of it must have a boiling point of over 250 C. This con- 

 dition is fulfilled by most of the oils used for lighting purposes, 

 and to a more satisfactory extent by a heavier oil, known as 

 solar distillate, which at the time of these experiments was much 

 cheaper than the lighting oils x (VI, 75). 



The dependence of the results on the boiling point of the oil 

 used is illustrated by the mortality caused in the eggs by using 

 oils of different degrees of volatility : thus 



j ,. n - 7c Medium Oils. Heavier Oils. 



Ben~ine Petrol etc Ordinary Crude Petroleum, 

 ne, Petrol, etc. Lighting Oils ^ Solar Distillaie . 



Mortality . n 88 97 



Also by applying the different fractions of a particular oil 

 obtained during the distillation of sample of it 



Fraction boiling at 



i65-22o 22o-285 above 285 



Mortality . . . .40 50 99 per cent. 



When this same oil was used in the diluted form of an emulsion, 

 the results were, naturally, less satisfactory, and showed no 

 probability of becoming more so until the proportion of oil 

 was raised to about 25 per cent.; thus with 



Emulsions with i per cent. Soap and Mortality. 



1 per cent, of oil o per cent. 



2 O 



4 o 



6 15 



8 13 



10 40 



J 5 43 



1 Its initial boiling point was 240 C., 94 per cent, of it boiling above 

 250 C. 



