INSECTS 203 



Thus, with anything above from 2 to 3 per cent, of soap the 

 mortality is about 70 per cent., and, when an allowance is made 

 for the probability that many of the apparently unaffected eggs 

 have been too much injured to hatch out, the real effectiveness 

 might be. placed even as high as 90 per cent. 



But the addition of so much soft soap to a solution of soda 

 presents mechanical difficulties, for a curd of hard soap separates, 

 occupying quite half of the liquid, and rendering its use as a 

 spray- fluid impracticable. This difficulty might be obviated by 

 keeping the mixture warm during use, or keeping the constituents 

 separate till they reached the trees, by using two jets : but both 

 these devices would complicate spraying operations considerably. 

 The substitution of the more expensive caustic potash for caustic 

 soda does not improve matters, for, though little actual separa- 

 tion of soap takes place in such a case, the mixture is too viscid 

 for spraying purposes, and eventually gelatinises. 



Observations made in connection with these soap-soda solu- 

 tions suggested that the liquid filtered from the curd gave 

 nearly as high an insecticidal value as the mixture itself. This 

 filtered liquid would contain none of the soap, and only such 

 constituents of the soap as were soluble, the chief of which 

 would be glycerin. This, however, would be present in the liquid 

 to the extent of only 0*12 per cent., at most. Some independent 

 observations seemed to confirm the view that glycerin, even in 

 such small quantities, considerably enhanced the action of the 

 alkalis, but that no proportionate further enhancement occurred 

 when the proportions were increased to i, and even 10 per 

 cent. The possibility of traces of certain substances modifying 

 ins'ecticidal action, though not, perhaps, fully established in this 

 case, is certainly one which should be borne in mind (VI, 65 ; 



VIII, 43). 



The results obtained with undiluted paraffin oils have already 

 been referred to (p. 153), and they showed that, provided the oil 

 contained a sufficient proportion of high-boiling paraffin, com- 

 plete destruction of the eggs of the mussel scale could be effected. 

 But the danger to the trees attending the use of such undiluted 

 oils, puts them out of court, and their use in the diluted con- 

 dition as an emulsion was not attended with success. Even 

 when the emulsion contained as much as 15 per cent, of the oil, 

 less than half the eggs were killed, and, judging by the results 

 obtained in America with ,San Jose scale, it would require the 

 presence of 25 per cent, of oil to produce satisfactory results 

 (VI, 83; VIII, 44). 



