32 BULLETIN 130, U. S. DEPARTMENT OF AGRICULTURE. 



inches in depth, contained in a glass cylinder, and allowed to diffuse 

 upward through the carbon disulphid, thereby taking up the carbon" 

 disulphid in solution. This solution was immediately collected and 

 placed in sealed bottles until used at the rate of three-fourths gallon 

 per square foot for the control of the root form of the woolly apple 

 aphis. This extremely dilute solution, although less than 1 to 1,000, 

 gave perfect control. 



In consideration of the foregoing data, a dosage at the rate of one- 

 half ounce of carbon disulphid to 4 gallons of water was rated as 

 the most efficient, and this was verified subsequently in practice. 

 One-half ounce of carbon disulphid is more than can be dissolved in 

 4 gallons of water at ordinary temperatures. When the liquid is 

 agitated, the portion of the material not entering into solution forms 

 a mechanical mixture with the water. A margin of safety is thereby 

 secured which insures the success of the treatment. Under no 

 circumstances is it necessary to employ a stronger dosage than the fore- 

 going. In fact, when larger doses are used the excess carbon disul- 

 phid drops to the bottom of the container, despite agitation, thereby 

 resulting in an uneven strength and consequent danger of injury 

 to the roots. 



THE DIFFUSION OF THE LIQUID AND CONTAINED GAS IN THE SOIL. 



During the course of the work the question arose as to whether 

 the carbon-disulphid gas given off from the water diffused laterally 

 beyond the point in the soil attained by the water itself, and a series 

 of experiments was carried out to determine this point. Figure 1 

 shows the method employed. The soil treated with the liquid was 

 confined to the circular area 4 feet in diameter within the galvanized- 

 iron strip D. The galvanized-iron strip may therefore be taken to 

 represent the line of demarcation between the soil within the strip, 

 directly subjected to the action of the liquid, and the soil outside 

 the strip which remains in its natural state of dryness, except for 

 any possible lateral diffusion at this immediate line of demarcation 

 between the two. At points A, B, and 0, respectively, 2, 4, and 6 

 inches from the strip, screen-wire cylinders containing apple roots 

 infested with the woolly apple aphis were placed in the ground pre- 

 vious to the application of the liquid. When examined 5 days later, 

 the aphids at A, B, and C were alive and breeding. This experi- 

 ment was carried out in (1) light and (2) heavy soils, both in dry and 

 in moist condition, with the same results. On several occasions the 

 examination of treated trees showed that aphids on the roots located 

 just outside the treated area had escaped, thus corroborating the 

 results of the specific experiments previously outlined. It is apparent, 

 therefore, that the carbon-disulphid gas in the solution does not 

 diffuse laterally through the soil beyond the point reached by the 

 liquid. 



