174 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 16 
The smaller particles in a dispersion of a slightly soluble material 
upon standing will grow smaller while the larger particles will grow 
larger. If the material is a solid there will be difficulties in keeping 
it in suspension, if an oil, the emulsion will break. 
The particles in a suspension may carry a positive, a negative, or 
no electric charge. The character of the charge frequently controls the 
adherence of the particles to the sprayed plant. 
Surface tension determines whether the arsenical spray will form a 
thin film over the leaf surface or roll off in the form of drops. It also 
largely determines the effectiveness of the contact spray and the cattle 
dip. 
The phenomenon of adsorption plays an important role in limiting 
the value of soil insecticides, nicotine dusts, and certain fumigants. 
There appear to be several types of adsorption and it is only by a 
thorough knowledge of this phenomenon that its effect may be overcome. 
Two compounds of the same composition may be optically active or 
inactive. It has been shown that, of the optically active compounds, 
the laevo rotatory are more toxic to both animals and plants than the 
dextro rotatory. To what extent has this physical property been 
considered in the use of insecticides. ? 
Chemical Reactions of Insecticides 
In order that the cost of application be reduced entomologists fre¬ 
quently recommend the combination of two or more insecticides and 
fungicides. Some of these mixutres are made without regard to the 
incompatibility of the materials or the chemical reactions which may 
reduce their efficacy. Such an incompatible mixture as lead arsenate, 
lime sulphur, nicotine sulphate, and soap has been recommended. In 
other cases lime is added to lead arsenate ia-such large quantities as to 
almost entirely destroy its insecticide value. Lime sulphur is used 
with lead arsenate without regard to the many and complicated chemical 
reactions which occur. In these days when dry mixtures of lead 
arsenate, lime, sulphur, and calcium caseinate are coming to the front, 
the question of the reactions between these ingredients should be carefully 
considered. When mixed and stored for a short time in a tight container 
one series of chemical reactions occur, while if stored in the open an 
entirely different series of reactions take place. In neither case would 
the final product be similar to the freshly prepared material. 
It is not generally recognized that lead arsenate and calcium arsenate 
are fundamentaly different and that the conditions favorable for the 
