April, ’23] 
MOORE: NEED OF CHEMISTRY BY ENTOMOLOGISTS 
173 
recommends mixtures of insecticides and fungicides which are not only 
incompatible but even useless and in many cases harmful to the plant. 
One could easily offer an excellent argument, showing the need of more 
chemical training in the course of study of the young entomologist, 
by presenting a series of quotations from the writings of current 
entomologists. Such a paper might not meet with hearty approval. I 
shall therefore try to outline some of the problems and difficulties en¬ 
countered in the use of insecticides and permit you to judge whether 
the economic entomologist has sufficient chemical knowledge to solve 
them. 
Physical Variations in Insecticides 
Solutions of electrolytes and non electrolytes, colloidal solutions, 
emulsions, and suspensions are all represented among the common 
liquid insecticides. The effectiveness of the spray often depends upon 
its physical rather than its chemical nature. Emulsions are often more 
effective than solutions, and even the character of the emulsion has an 
influence. Emulsions with very small particles will behave differently 
from one with large particles. Recently an oil company has prepared 
an oil which mixed with water forms a system with particles so small 
that it may be classed as a colloidal solution or emulsoid. The water 
can be evaporated from this colloidal solution without destroying the 
original properties of the oil. Lime may even be added to the solution 
and the oil will not collect in a film on the surface until several hours 
have elapsed. 
An emulsion of water in oil can easily be made and might be a far 
more effective insecticide than the ordinary emulsion in which the oil 
is the disperse phase. No one to my knowledge has ever tested such 
an emulsion to determine its effect on insects or plants. 
Recently it has been shown that by the use of a protective colloid, 
in the manufacture of lead arsenate, a more finely divided material could 
be prepared. The reduction of the size of the particles should increase 
their solubility and possibly their toxicity but may not the presence of 
the protective colloid more than neutralize this advantage? 
If a soluble salt is present in a suspension of a slightly soluble arsenic¬ 
al, having an ion in common with it, the solubility of the arsenical will 
be reduced. Such a salt may be present at the time the arsenical is 
tested for water soluble arsenic with the result that little or no soluble 
arsenic will be found. After the spray is applied to the foliage the 
soluble salt may be washed away by rains or dews or may be changed 
to a less soluble form with the result that the plant is severely burned. 
