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 a warm, 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 soils . 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 



