FUMIGATION WITH CARBON BISULPHID 263 
produces an atmosphere fatal to all insects within its 
reach. ‘The rapidity of evaporation, extent of dif- 
fusion, and persistence of the vapor in the soil vary 
widely in different soils. It evaporates most rapidly 
in awarm, dry, sandy soil. The persistence of the 
vapor is also shortest in such a soil, and it diffuses so 
rapidly most insects will survive an ordinary dose. 
The treatment cannot be successfully applied on such 
a soil in its dry condition. On the other hand, diffu- 
sion is slowest in heavy, wet, clay soil; and when such 
soil is saturated with water it is almost entirely pre- 
vented. Moisture lowers the temperature and de- 
creases the permeability of the soil; it also prevents 
the evaporation of the liquid, and thus retards diffu- 
sion. Between these two extremes there is a medium 
condition of moisture which is most favorable for treat- 
ment. 
Action in different sotls,—Sandy soils permit an even 
but too rapid diffusion of the vapor. Rocky soils are 
not of even texture, and naturally the vapors follow 
the lines of least resistance. Heavy clay soils, when 
very dry, are usually much broken by cracks and 
fissures, which may run from the surface to a consider- 
able depth. Through such fissures the vapor escapes 
rapidly without permeating the soil to any extent, and 
its insecticidal value is therefore slight. But when 
such a soil is well moistened it is even in texture and 
very favorable to treatment. The depth of the soil is 
an important factor in determining how much carbon 
bisulphid must be used for a given area. If the soil is 
shallow and the subsoil very dense and impervious, it 
is evident that much less liquid will be required to 
