PERSISTENCE OB PUTREFACTIVE AND INFECTIVE ORGANISMS. 
179 
Comparing these results with those obtained with the turnip-infusion, it will be 
observed that cucumber and turnip exhibit about the same resistant power: three 
hours’ boiling, and less, failed to sterilize both of them ; four hours’ boiling, and more, 
rendered both of them permanently barren. 
The cucumber-infusions prepared on the 22nd and 28th of February were connected 
with the atmosphere through the cotton-wool plugs ; but no attempt had been made to 
remove its floating matter from the air above the infusions. On the 22nd, however, 
four bulbs of the infusion were also prepared, charged with filtered air, left unplugged, 
and hermetically sealed. The same was done with four bulbs on the 28th of February. 
Each group was subjected to periods of boiling of 15, 30, 45, and 60 minutes respec- 
tively. All of them became turbid ; but it was interesting to notice the gradual and 
obvious fall of life from the 15-minute to the 60-minute period. • Could the Bacteria 
have been counted, and the result graphically represented, the ordinate corresponding 
to the abscissa 15 would have been found very considerably longer than that corre- 
sponding to the abscissa 60. 
The method of experiment here for the most part pursued was employed by Spallanzani 
and Needham. It was afterwards extensively applied by the late excellent Professor 
Wyman, of Harvard College, while in 1874 it was materially refined and improved upon 
by Dr. William Roberts of Manchester. The method is hampered by one grave doubt. 
The air, plus its floating matter, is imprisoned in the sealed bulbs, so that the heat applied 
has not only to destroy the germs clasped by the infusion, but also those diffused through 
the supernatant atmosphere. Now it is not certain whether an amount of heat which 
would be absolutely destructive to a germ embraced by a hot liquid may not be wholly 
ineffectual when acting on a germ floating in vapour or air. Throughout Spallanzani’s 
and Needham’s experiments, throughout those of Wyman and Roberts, and throughout 
my own, as reported in this section and the last, this possibility of error runs. Such 
experiments, in short, do not enable us to state with certainty the temperature at which 
an infusion is sterilized, because the germs which most pertinaciously oppose sterilization 
may not belong to the infusion at all, but to the adjacent air. 
The most astonishing cases of resistance to sterilization observed by 
Wyman were associated with this particular mode of experiment. The 
possible action of the uncleansed air, moreover, was in his case augmented 
by the fact that he employed quantities of liquid, very small in comparison 
with the size of his flasks. In some of his earlier experiments the volume 
of air was more than thirty times that of the infusion. These relative 
volumes are represented in the annexed figure (fig. 7), copied from 
Wyman’s Memoir of 1862*. 
* Silliman’s American Journal, vol. xxiv. p. 80. 
