Insecticides and Related Substances. 295 



substances as the free element, but as a constituent of "white 

 arsenic," technically called "arsenious oxide" or "arsenious 

 acid." Soluble compounds of arsenic were at first tested as in- 

 secticides, but they were found to cause serious injury to foliage. 

 Later experiments have demonstrated that arsenical compounds 

 insoluble in water produced the desired effect, probably by virtue 

 of the solvent action of the juices of the digestive tract of the 

 insect. The resulting effort to furnish the arsenic of insecticides 

 in insoluble form has been stimulated also by the passage of state 

 laws restricting the amount of arsenic permissible in soluble form. 

 Paris green has been a leading insecticide in America for fifty 

 years. It was first used, apparently, in an attempt to control 

 the Colorado beetle or "potato bug" which had made its ap- 

 pearance in the western United States. This stomachic poison 

 Contains arsenious acid, acetic acid and copper in a definite chem- 

 ical structure known as "Schweinfurt's green," and technically 

 known as "copper aceto-arsenite. " It is prepared by adding a 

 hot solution of arsenious oxide to a hot solution of copper acetate. 

 Paris green separates from the mixture and settles out as a rather 

 fine powder of a clear, green color. The pure compound is prac- 

 tically insoluble in water, but readily soluble in ammonium hyd- 

 roxide, or ammonia water, and has the following composition: 



Per cent 



Copper oxide 31 . 29 



Arsenious acid 58.65 



Acetic acid 10.0(> 



Scorching of foliage by applications of Paris green suspended 

 in water was frequently observed during its early use. Gillettte 

 showed, in 1890, that the use of lime water or Bordeaux mixture 

 with Paris green prevented this injury. A year later, Kilgore 

 found that the scorching effects were due to soluble forms of 

 arsenic and concluded that the preventive substances acted by 

 virtue of their lime, which fixed the soluble arsenic in insoluble 

 compounds. Experiments at the New York Experiment Station 

 with Paris green and sodium arsenite applied to potatoes led to 



