BUREAU OF ENTOMOLOGY AND PLANT QUARANTINE 107 
sulfur residues were conducted to check on the use of this element 
in the control of the citrus thrips. At the same laboratory residues 
of tartar emetic, used against the citrus thrips, were investigated, 
and a method for determining the antimony content was developed. 
Because of the importance of copper fungicides, methods for de- 
termining copper spray residues were studied. 
Experiments concerning removal of residues from cryolite sprays, 
conducted at Yakima, Wash., showed that dual washers were re- 
quired to remove the rather heavy residues of fluorine. 
New methods of removing residues preliminary to chemical de- 
termination were developed, and the work thereby expedited. Also, 
many of the residue studies were so arranged that the expected 
natural variation could be estimated and data collected that will 
permit of more critical judgment when the mean results are com- 
pared with one another. 
Work was done on the removal of nicotine spray residue from 
apples, in cooperation with the Bureau of Plant Industry at Yakima, 
Wash. Nicotine residues of 0.01 grain per pound resulting from 
the application of six coA r er sprays of nicotine bentonite were entirely 
removed in a dual-process machine using sodium silicate at 110° F. 
in one compartment and hydrochloric acid at 100° in another. 
Each of these solutions used alone is effective in reducing the nico- 
tine residue to 0.001 to 0.003 grain per pound. 
DEVELOPMENT OF INORGANIC INSECTICIDES 
A phase-rule study of the system sodium hydoxide-arsenious acid- 
water was conducted to obtain information concerning sodium ar- 
senite. which is used extensively in controlling the Mormon cricket. 
The data include solubility figures for all possible combinations of 
the three components, and indicate all the definite compounds that 
exist at the chosen temperature. A new compound was discovered. 
White arsenic, despite its high arsenic content, is not sufficiently 
poisonous to the Mormon cricket to warrant its use. It was surmised 
that its low toxicity might be due to a low solubility in water; a 
study was therefore made of the solubility in distilled water, in 
water adjusted to definite hydrogen-ion concentrations, and in solu- 
tions of colloidal materials such as glue and gum arabic. The solu- 
bility was found to be markedly affected by the conditions mentioned, 
and this knowledge may perhaps lead to improvement in the use of 
the compound. All three of the well-known forms were included in 
the solubility experiments, and it was found that the amorphous 
variety was more rapidly soluble than the others. Its use as an 
insecticide may be indicated. 
Cryolite has proved to be toxic to the European corn borer, and 
its commercial use might be feasible except for the fact that at times 
it severely injures the leaves of the corn plant. Because this injur}' 
is thought to be due to the solubility of the material, a study was 
made of ways of reducing the solubility by admixture with substances 
that contain common ions that will buffer the mixture to an optimum 
hydrogen-ion concentration, or that will precipitate the fluorine that 
dissolves. Eighteen samples of cryolite, comprising natural and both 
domestic and imported synthetic material, were examined. This in- 
vestigation is being continued. 
