96 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE, 1939 
The work with nicotine and tobacco took mostly an analytical turn. 
A new and delicate colorimetric method was devised which permits 
the determination of the nicotine on an individual sprayed apple, and 
is proving of great benefit in studying the various spray schedules 
that continue to -how the value of the tank-mix nicotine-bentonite in 
controlling the codling moth. A variation of the method was de- 
veloped which will permit at least a rough measurement of the i 
tine content of a leaf selected from a growing tobacco plant, and is 
i used by the Bureau of Plant Industry in the study of the 
nicotine-producing power of hybrids and crosses of 
plant. 
The fruit of the Amur cork tree (PheUodendron an 
previously been found to contain an insecticidal principle, a detailed 
investigation of it was begun. It- stability toward alkali shows that 
it is quite different from either the pyrethrins or ro i the 
absence of nitrogen proves that it is not an alkaloid. 
CHEMICAL INVESTIGATIONS TO DEVELOP SYNTHETIC ORGANIC ENSB4 TI( IDES 
The Division continued its search for synthetic i 2 - ides, 
preparing several hundred compounds and submitting them to regular 
testing services supported by several other divisions of the Bureau. 
Those showing any marked insecticidal efficacy were made the subject 
of patent application- to guarantee their free use to the public; two 
such patent- were secured. 
P lenothiazine received further laboratory attention. Search for 
suitable -ticker- to use with it was continued, and the fact that it de- 
composes more rapidly in mixture with some accessory materials was 
noted : it was definitely recognized that fine subdivision of the mat 
increases both it- adherence to foliage and its toxicity to insects, and 
a study of its volat ility was begun. A review of the literature concern- 
ing the rate of evaporation of slightly volatile substances Led to the 
belief thai phenothiazine may disappear by volatilization. The vapor 
pressure-versus-temperature curve of thi >und has been deter- 
mined, and actual evaporation studies are now under way. 
Renewed attention was given to phenazine, for which no simple 
method of preparation has been known. Two new methods of syn- 
thesis have been devised, and it- economical production by one or both 
of these is being further studied. 
New compounds are regularly tested now against larvae of the screw- 
won : fly, and during the year the toxicity of about 500 materials of 
jtructure was studied. Hopes of finding enough structural 
relationship between active compounds to permit formulation of a 
working theory of toxic groups were not realized. 
CHEMICAL INVESTIGATIONS OP SPRAT RESIDUES AND THEIB RSM0VA1 
The Yakima laboratory took part in it> usual fruit-washing program 
ooperation with the Bureau of Plant Industry. Ail lead residues 
reduced below the new tolerance of 0.025 grain of lead per pound, 
Loads as nigh a- <».r.*n grain per pound being reduced satisfactorily by 
even the single-pi achme. All fluorine residues except one were 
.leaned well belOW the new tolerance of 0.020 grain of fluorine per 
pound. At Yincenne- residues of arsenic, nicotine, and phenothia 
