18 BULLETIN 1465, U. S. DEPARTMENT OF AGRICULTURE 
The temperature to which the cans were brought and the time of 
exposure were no more than enough to cause sterility. Peripheral areas 
were sterile, but the center sometimes was not. Usually an exposure 
of a can' to 65 pounds of steam for eight hours resulted in sterility. 
In a few cans sterilized at 60 pounds Pyronema developed. Bacteria 
were sometimes found. Fungi which developed usually grew luxu- 
riantly and could be detected when the cans were opened. When a 
can was once found to be contaminated the contents were discarded, 
as no success was attained in resterilizing contaminated soil. 
SOUECES OF CONTAMINATION 
The sources of contamination of the cans were many and varied . 
In addition to the risk of nonsterility due to faulty sterilization, 
there was the chance of introducing foreign organisms with the 
water, also with the seed piece. An imperfect joint or seal would 
let in bacteria or spores under the stimulus of wind, rain, or chang- 
ing temperature. It was only by the greatest care and drastic 
application of the gasoline torch that contamination was prevented. 
The elbows and small pipe of the vent cap (fig. 2) were filled with 
asbestos wool to exclude organisms. These were heated in place 
with the gasoline torch, to kill any possible entering contamination 
as well as to sterilize the thread joint of the top of the pipe. 
The top of the lamp chimney, plugged with cotton, was covered 
with a heavy paper cap, such as appears in Plate 5, to ward off rain, 
dust, hail, or curious visitors. 
The joint between the cover and the can offered considerable 
opportunity for contamination. The top rim of the can closed with 
a lap joint, which left an awkward opening under the cover. This 
was closed with iron cement and solder. The circumference of the 
cover was larger than the circumference of the rim of the can, so 
that in drawing a tape or strip of cloth around the joint the lower 
half was not tightly stretched and might be loose and unattached. 
Torn pieces of canvas well attached with waterproof glue and drawn 
almost to the point of breaking made excellent joints. Adhesive 
tape could not withstand the drawing pressure that canvas stood. 
In addition, the tape seemed to weaken on exposure to the sun, and 
the rivet heads in the rim would eventually burst through. By put- 
ting on three rounds of tape, gluing and stretching each one on, and 
tying the whole with a strong cord, a very satisfactory joint was made, 
and through this no contamination could ordinarily pass. 
In spite of all precautions, however, the loss of cans from contami- 
nation was considerable. The origin of the trouble could not al- 
ways be ascertained. Constant application of the torch and the var- 
nish brush were imperative, together with rigid care in handling at 
all stages of operation. 
LOSS OF SEED PIECES 
Different varieties of potatoes were used for seed purposes. The 
Early Ohio was found to be the most satisfactory. If the eyes could 
be germinated before paring away for sterility, this imparted a greater 
vigor to the part remaining. The eye of the potato contains folds 
and crevices impossible to clean by artificial means. They all had 
to be cut away. The last cut across the eye came close to the 
