108 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE, 194 
The particle size of 16 samples <>t' commercial Lead arsenate, deter- 
mined by sedimentation analysis, was found to be mostly below ■ 
diameter of 2 micron-, with from L0 to 60 percent of materia] below 
a diameter of l micron. The Loose bulking value (cubic inches per 
pound) of a sample of lead arsenate does not give an indication of 
the particle size In cooperation with the Division of Control Inves- 
tigations it was shown that paris green particles with an average 
diameter of l.l microns caused higher mortalities of the Mexican 
bean beetle and permitted less feeding when applied to bean foliage, 
either as a spray or as a dust, than did particles of 1*2 micron-, which 
were intermediate, and of 2*2 microns, which caused the Lowest mor- 
tality and permitted the greatest amount of feeding. 
Heating calcium arsenate sufficiently to make it relatively safe to 
hean foliage likewise made it nontoxic to silkworm larvae. In general, 
ignition decreased the percentage of soluble arsenic present. How- 
ever, there doc- not appear to be any appreciable correlation between 
the percentage of soluble arsenic as determined by the method <>f the 
Association of Official Agricultural Chemists and the toxicity t<> in- 
sects or plants. 
FUMIGANTS FOR CONTROL OF INSECT PESTS 
Chemical studies on the fumigation of stored grain and cereal 
products for control of the insects infesting them were resumed at the 
Manhattan, Kans., laboratory. Investigations were begun on th* 
composition of commercial fumigants offered for this purpose, of the 
adsorption of hydrocyanic acid by flour fumigated with it, of the 
effect of the moisture content of wheat on the results obtained with 
certain fumigants, and of the minimum lethal concentrations of var- 
ious fumigants. 
At the Whittier, Calif., laboratory attention continued to be de- 
voted to the control of the so-called resistant strain of the California 
red scale with hydrocyanic acid and other fumigants. The chemical 
work consisted in the establishment and maintenance of known concen- 
trations of the fumigant under conditions that would eliminate the 
influence of protective stupefaction brought on by accidental exposure 
to sublethal concentrations. Similar work was conducted with methyl 
bromide, to see if this recently developed and versatile fumigant 
might find another application here. 
Work on dispersing insecticides in the form of smokes (see the 
discussion of derris) was extended to ortho- and paradichlorobenzene, 
naphthalene, and tetrahydronaphthalene. A high concentration of 
these materials can be built up by dropping solution- of them in saf- 
role on a heated metallic surface. 
Studies on chlorine iras as a seed disinfectant, done in cooperation 
with the Bureau of Plant Industry, showed that, to obtain satisfac- 
tory killing of smut spores borne on the surface of the seed without 
causing pronounced seed injury, the gas concentration should n<»i be 
less than 3 or greater than 9 percent, the time of exposure From l to 
2 hours, and the volume of pure chlorine 4 gas not les< than 20 percent 
or more than W percent of the net volume of the seed being treated. 
Tn experiments with weevils it was found thai some active adult 
grain weevils survived after exposure to concentrations of 10, 20, and 
50 percent of chlorine gas for 60, 20, and 10 minute-., respectively. 
