GROWING BACTERIOLOGICALLY STERILE POTATO PLANTS 
17 
melted agar, the tubes were rolled between the fingers, and the agar 
was poured into Petri dishes, any resulting growth indicating a con- 
taminated can. 
In the water test 2 cubic centimeters of water was added to a 
dextrose-broth tube with a small vial inverted in it to form a fermen- 
tation tube. Dextrose-agar plates to which a few cubic centimeters 
of water had been added were also poured. When no growth appeared 
it indicated sterility. 
THE EXPERIMENT IN OPERATION 
REGARDING SOIL STERILIZATION 
In the operation of the kettle the success of the work depended in 
part on the rapidity with which the steam could be admitted and in 
part on the initial temperature of the cans. As the work of sterili- 
zation was frequently done in the early spring, the cans were some- 
times frozen, or at least thoroughly chilled. Cold cans condensed a 
great deal of steam, making boiler management difficult, tending to 
fill the soil in the cans with excess water, and in a few cases to pud- 
dle them. The cans needed to be warmed as much as possible. So 
the kettle was usually started in the afternoon and after being 
unloaded was filled again, the cans warming considerably over night. 
On starting in the morning the difficulty resulting from condensation 
was thus partially avoided. During the day the sun warmed the 
cans which were to be used next. Usually two lots were run in one 
day, the time commonly required for a lot of six being about nine 
hours. Occasionally the kettle was operated continuously. The tem- 
perature in the interior of a can was always recorded in each lot. 
Whether the can was in the lower or upper tier in the kettle seemed 
to make no difference in the amount of heat received, as indicated 
by many readings. Figure 6 shows the curves of temperature at the 
center of a can and the steam pressure in the kettle for a representa- 
tive sterilization. 
Table 1 gives the temperature of live steam under pressure, in the 
range of pressures under which the kettle operated. 
Table 1. 
■Temperature in degrees centigrade and Fahrenheit of live steam under 
-pressure of SO to 70 pounds 
Pressure 
to the 
square 
inch 
Temperature 
Pressure 
to the 
square 
inch 
Temperature 
Pounds 
50 
55 
60 
°C 
139. 6 
143 
146.5 
°F 
283.2 
289.3 
295.6 
Pounds 
65 
70 
149.4 
152.5 
°F 
301.3 
306.4 
The kettle was usually operated at a pressure of 60 pounds but 
frequently ran to 65 and 70 pounds. 
The air in the soil was slow to leave. Had a vent been provided 
at the bottom of the can, this would have assisted in the operation. 
However, as such a vent or cock would offer opportunity for contam- 
ination and make handling the cans difficult, it was omitted. Instead, 
an iron rod was pushed down to the bottom of the can, leaving a 
small round passage which assisted in the escape of the air. 
