52 BULLETIN 1147, U. S. DEPARTMENT OF AGRICULTURE. 
soluble arsenical is used in combination with a fungicide than when 
an insoluble arsenical is used. Accordingly it may be possible to 
explain chemically the increased activity or efficiency when sodium 
sulphid is used with arsenicals. 
Evidence seems to show that it is not always true that an insecti- 
cide containing a high percentage of arsenic is more toxic than one 
containing less arsenic, for the reason that toxicity depends not alone 
upon the amount of arsenic present, but also upon its form of combi- 
nation. The insecticidal role played by the base itself is small and 
sometimes nonexistent. 
When lime or Bordeaux mixture was combined with the arsenicals 
the toxicity of the arsenicals was reduced. The fact that the addi- 
tion of lime or Bordeaux mixture to the arsenicals reduced the 
toxicity of these insecticides to insects may be explained in two ways: 
(a) Leaves sprayed with the arsenicals combined with lime or 
Bordeaux usually contained less arsenic than those similarly sprayed 
with the arsenicals alone; (b) the toxicity was greater in the tests 
with honeybees fed honey containing the arsenicals alone than in 
tests in which bees ate honey containing the arsenicals with lime or 
Bordeaux mixture. These results support the theory that the cal- 
cium present prevents or counteracts the formation of soluble or 
more toxic arsenic compounds. 
Based on the reported results, it would appear that if all seven 
species of insects used had been tested under similar conditions, 
their susceptibility to an acid lead arsenate would probably be in 
the following order, beginning with the insect most susceptible: 
Honeybees, silkworms, grasshoppers, potato-beetle larvae, tent 
caterpillars, webworms (H. textor), and webworms (E. cunea). 
The arsenious oxid (" white arsenic") samples were crystalline; 
the other commercial arsenicals generally lacked crystal outline and 
were probably for the most part amorphous. The calcium arsenates 
used contained some small "octahedral" crystals, but were largely 
composed of apparently amorphous material. The arsenious oxid 
samples gave variable results in the toxicity studies and wide varia- 
tions were found in the results when calcium arsenates were used. 
On the other hand, the amorphous acid lead arsenates and the 
amorphous Paris green samples gave uniform toxicity data. The 
data show a relation between the uniformity of the products and 
uniformity of toxicity. Where the products were not uniform 
variations in toxicity were found. 
Commercial arsenicals used for spraying or dusting purposes are 
usually judged chemically on the basis of the total arsenious or arsenic 
oxid contents and on the percentages of the total amount of these 
oxids which go into solution under certain conditions. The per- 
centage of base present is also determined. Soluble arsenic oxids 
or arsenic rendered soluble after the application of arsenicals will 
burn foliage, the extent of the injury depending mainly on the 
amount of soluble oxid present or formed m the spray or solution 
applied. The results here reported indicate that it is the soluble 
arsenic or the arsenic rendered soluble by the insects that causes 
death. The rapidity with which arsenicals are made soluble in the 
bodies of insects seems to be the most important factor in connection 
with their toxicity. What happens to the soluble arsenic inside the 
insects is not known, except that part of it passes through the in- 
