BUREAU OF ENTOMOLOGY AND PLANT QUARANTINE 89 
injected with zinc chloride, sodium arsenate, and mercuric chloride for the 
same purpose. These results were needed to show just how the various chemi- 
cals spread throughout the trees. 
CHEMICAL INVESTIGATIONS TO DEVELOP INORGANIC INSECTICIDES 
The investigation of calcium arsenate was actively continued, being directed 
largely to an attempt to discover the relationship between the chemical and 
physical characteristics of a given material and its toxicity to both plants and 
insects. The phase-rule study of the system CaO, As 2 5 , H2O was completed. 
This included an approximate evaluation of the concentration limits between 
which the compounds more basic than dicalcium arsenate can exist at 62° C, 
and X-ray studies of the samples in the basic region reveal the existence of 
both tricalcium arsenate and tetracalcium arsenate, with a continuous series 
of solid solutions between them. A complete analysis was made of the 22 
brands of calcium arsenate now on the American market, and the findings of a 
similar study made several years ago was confirmed, namely, that they all 
contain some basic arsenate, which in no case is more basic than the formula 
4CaO/ASi0 5 . The tests of solubility by a new method recently proposed by the 
New York Agricultural Experiment Station gave results ranging from 0.2 
to 11 percent, revealing the great differences that exist in spite of the apparent 
uniformity of the generally accepted superficial analysis. The claim that low 
solubility by the New York method indicates safety to foliage is rather offset 
by preliminary results, which indicate that three products representing the 
maximum, mean, and minimum solubilities are all showing considerable injury. 
The commercial products mentioned were also analyzed mechanically in a new 
sedimentation apparatus devised for the purpose. Great differences in size- 
frequency distribution of particle sizes were found, and the insufficiency of the 
so-called density test for estimating fineness was clearly demonstrated. Por- 
tions of a commercially prepared sample of basic calcium arsenate (approxi- 
mately 4CaO/As 2 OD), which itself is rather injurious to foliage, were subjected to 
various degrees of dry and wet heating for various periods. The insect and 
foliage tests so far made indicate that toxicity to both insects and foliage de- 
creases as the temperature is raised and the time of treatment extended, and 
that the action is more rapid in the presence of steam. The reason for this 
inactivation is being studied further. 
A survey of the paris greens on the market was also made, the samples ob- 
tained being analyzed both chemically and physically. Bibliographies concern- 
ing two other minor arsenical insecticides, namely, magnesium arsenate and 
manganese arsenate, were compiled, and an investigation of the copper and 
zinc salts of xanthic acid was begun. 
CHEMICAL INVESTIGATIONS ON FUMIGANTS FOR CONTROL OF INSECT PESTS 
The chemical investigations on fumigants were augmented by the reassign- 
ment, on September 1, of a chemist to work with the entomologists of the 
Bureau stationed at Whittier, Calif., for the study of the problem of the apparent 
development, by the California red scale on citrus, of a resistance to fumiga- 
tion with hydrocyanic acid. 
It appears from the fumigation tests conducted in the field that no simple 
relationship exists between scale mortality and concentration of hydrocyanic 
acid. Protective stupefaction seems to occur, and hence the rate of attainment 
of maximum concentration is very important. The relationship between maxi- 
mum concentration and such variables as temperature, humidity, wind velocity, 
tent porosity, surface volume ratio of tent. etc.. proved so complex that ii was 
decided to resort to laboratory experiments under controlled conditions, and 
two fumigatoriums, of 27 and 850 cubic feet capacity, respectively, have been 
constructed. 
The data on all the tent fumigations so far made have been studied criti- 
cally, and it has been found that in general the gas concentration decreases 
logarithmically with time. A consideration of 91 curves shows the rate of 
decrease to be greater at higher temperatures and with smaller tents, and 
to become smaller with increasing humidity. 
Considerable work was done on the subject of fumigation with nicotine. A 
technique was developed by which Quantities of nicotine from 0J to 0.5 milli- 
gram can be determined with an accuracy of about 1.5 percent and a suitable 
air-sampling device was constructed. Samples taken from various parts of a 
greenhouse in which the vapor from a boiling nicotine solution had been dis- 
persed showed a fairly even distribution, but at a much lower concentration 
