CIRCULAR 3 6 4, U. S. DEPARTMENT OF AGRICULTURE 



directed upward, the concentration of gas would be high at the top 

 of the house and a good deal lower along the floor, requiring from 15 

 to 25 minutes for uniform distribution. With the fans placed at a 

 height of from 5 to 8 feet from the floor and the air current directed 

 straight down, however, the gas rising from the floor is forced down- 

 ward again, circulating through the lower beds and rising in the side 



aisles. In this case, 

 4 ' 1 1 '" also, uniform distri- 



bution of gas requires 

 from 15 to 25 min- 

 utes, but in the mean- 

 time the higher con- 

 centration remains at 

 the floor level, where 

 it does the most good. 

 In the case of sodium 

 cyanide and acid, a 

 fan should be placed 

 directly above each 

 generator at a height 

 of about 8 feet. Fig- 

 ure 1 shows a com- 

 parison of typical 

 concentrations se- 

 cured at the top, bot- 

 tom, and middle of 

 mushroom houses, (A) with fans directed upward, and (B) with 

 fans directed downward. It will be observed that the concentration 

 becomes uniform in about 25 minutes in both cases, by which time it 

 has dropped considerably. It will be seen further that the concen- 

 trations are reversed by the fans, the downwardly directed fans 

 keeping the highest concentrations of gas at the floor. 



Figure 1. — Typical curves shoeing the concentration of 

 hydrocyanic acid gas during fumigation of mushroom 

 houses : A, Fans directed upward ; B, fans directed down- 

 ward ; t, m, and 6 indicate concentrations at top, middle, 

 and bottom of the house, respectively. 



or 



















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10 



20 



40 50 



MINUTES 



60 



70 



80 



Figure 2. — Average concentrations of fumigant obtained in mushroom houses from the 

 use of different sources of hydrocyanic acid gas : a, Gas generated with sodium cyanide 

 and acid ; 6, gas from liberation of liquid hydrocyanic acid ; c, gas generated from cal- 

 cium cyanide in powder form ; d, gas generated from granular calcium cyanide. 



In comparing the concentrations of gas obtained from the various 

 types of cyanide, the concentrations about half way between floor and 

 ceiling of the house, along the center walk about 12 feet from the door, 

 were used. At this point the concentration usually represents a fair 

 average between that at the top and bottom of the house, regardless 

 of the material used and the position and direction of the fans (fig. 2) . 



