80 
Into the soil at some depth below the surface the liquid evap- 
orates as it does in the open air, only much more slowly. The 
vapor tends to diffuse through all the air spaces of the soil. 
It thus produces an atmosphere which is fatal to all insects 
living within its reach. The rapidity of evaporation, the extent 
of diffusion, and the persistence of the vapor in the soil vary 
widely in soils of varying characters and conditions, so no 
one rule of application can be employed in all cases, and it 
thus becomes necessary to understand the influence of various 
factors that proper allowance may be made for them and the 
destruction of the insects attained without injuring the plants. 
Moisture. — Carbon lrisulphid evaporates more rapidly in a 
warm, dry, sandy soil, and the persistence of the vapor is also 
shortest in such soil. In fact it diffuses so rapidly that most 
insects will survive an ordinary dose: and if the dose is 
increased so as -to kill the insects, it is likely to kill the vines 
as well. The treatment cannot be successfully applied on such 
a soil in its dry condition. On the other hand, diffusion is 
slowest in heavy, wet, clay soil; and, when such soil is 
saturated with water, it is almost entirely prevented. Moisture 
lowers the temperature and decreases the permeability of the 
soil; it also prevents the evaporation of the liquid, and thus 
retards diffusion. Between these two extremes there is a 
medium condition of moisture which is most favorable for 
treatment. 
Character of soil. — 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 considerable 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. 
Depth of soil. — 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 produce a death atmosphere than will be needed 
in a soil of much greater depth. In soils of the same character 
and condition the amount needed will be proportional to the 
permeable depth of the soil. In heavy, compact soils increase 
the number of injections and diminish the dose; in light, deep, 
permeable soils decrease the number of holes and increase the 
dose. 
Amount to use. — In field experiments with the grape, using 
plain carbon bisulphid in “quite fresh’’ soil, vines were found 
to withstand 105 c. v. of carbon bisulphid (4.4 ounces, nearly), 
divided equally among 3 holes placed about 16 inches from the 
base of the vine and at a depth of about 20 inches; but 180 
c. c. (7Mt ounces) proved fatal to the vines. In a warmer, drier, 
more shallow soil a dose of 90 c. c. per vine, similarly placed, 
proved fatal. After considerable rain, when the ground was 
quite wet, a vine withstood 2C0 c. c. of carbon bisulphid, and 
some vines are said to have withstood 400 c. c. 
