LONDON PURPLE. 



141 



In looking back over an experience of five years we are 

 unable to see how any better method of destroying the young 

 larvae could have been devised from the knowledge of in- 

 secticides then possessed. 



Spraying with London Purple. 

 London purple was used both in-doors and m the field in 

 the season of 1891 but the results of the experiments with it 

 were not as satisfactory as those reached with Paris green. 

 While it was quite as efiective as Paris green in destroying 

 larvae, it injured the foliage more. London purple is more 

 finely powdered and lighter than Paris green and requires 

 less stirring. It also has the advantage of cheapness. The 

 great objections to its use are its partial solubility in water, 

 which causes burning of the leaves, and the unevenness of 

 the effects produced by it. While one sample may give fairly 

 satisfactory results, the next may not only burn the foliage 

 badly but even the young fruit on fruit trees. According to 

 Professor Bailey the arsenic in London purple is in the form 

 of arsenite of calcium, which in some samples is about seventy- 

 two per cent, of the whole compound. Over fifty per cent, of 

 this arsenite or nearly forty per cent, of the London purple is 

 quickly soluble in water.* The analysis of leaves injured by 

 Paris green shows no arsenic in the texture of the leaf ; but 

 when leaves injured by London purple are analyzed, arsenic 

 is found in their texture. This shows the penetrative powers 

 of the latter poison. The London purple was used in the 

 field to some extent, but the conclusion arrived at was that 

 while, as it is ordinarily used, one might be justified in ex- 

 perimenting with it upon his own premises, its use in public 

 work on the premises of others was unwarrantable. 



Spraying ivitJi Arsenate of Lead, 

 Although nearly all poisons known to us which can be 

 used as insecticides have been experimented with during the 

 past five years in the hope that something would be found 

 which would prove fatal to the gypsy moth, only one which 

 is more effective than Paris green has been discovered. This 



* See "Experiences in Spraying Plants" by Prof. L. H. Bailey, Report Agricultural 

 Experiment Station, Ithaca, N. Y., Bulletin 18, 1890, page 98. 



