436 AA^ ECONOMIC EXTOMOLOGY. 



are some instances when the dry appHcation is to be preferred 

 to the spray, as in the cotton-field against the cotton-worm and 

 against potato-beetles. The essential points in powder applica- 

 tions, where the substance is to be used undiluted, is to have it 

 perfectly dry, and to put it on very thinly and evenly. Paris 

 green, to have the greatest effect, should be extremely fine ; the 

 finer the particles the more effective will be the application and 

 the greater the surface covered by an equal amount of poison. 

 Where used in water it remains in suspension better in propor- 

 tion as the material is in good mechanical shape ; the larger the 

 grains the more rapidly they sink to the bottom. In applying 

 Paris green it must always be remembered that there is no solu- 

 tion, but merely a mechanical mixture ; that the substance is 

 heavy and will sink to the bottom unless it be kept constantly 

 stirred. 



London purple is a waste product, and its composition is not 

 in all cases quite the same, but under ordinary circumstances 

 does not vary much from the analysis given. It should always 

 be mixed with lime, as already described, before being used. It 

 is effective against most insects at the rate of one pound in one 

 hundred and eighty gallons of water. Against the plum curculio 

 or the codling-moth, one pound in one hundred and fifty gallons 

 of water is more effective ; while against the potato-beetle, one 

 pound in one hundred gallons is generally satisfactory. London 

 purple can also be used dry, but should never be used pure in this 

 way, because even a small degree of moisture may have the efiect 

 of liberating some of the free arsenic and causing injury. It is 

 best combined with air-slaked or dry hydrate of lime, and is 

 then both effective and harmless. The material is very much 

 finer and lighter than Paris green, and remains in suspension in 

 water very much better ; therefore, less stirring is required ; but 

 all the other points that have been referred to under Paris green 

 apply here equally well. 



Arsenate of lead, or "gypsine," stands on a difierent footing, 

 yet acts in the same way. This must be made, as used, of arse- 

 nate of soda four ounces and acetate of lead eleven ounces, or 

 in that proportion for a larger or smaller quantity. These sub- 

 stances dissolve readily, and the quantities given can be mixed 

 in one gallon of water for dilution, or can be placed in one 



