PERSISTENCE OE PUTREFACTIVE AND INFECTIVE ORGANISMS. 
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tubes ; but the spirit-lamp enables us to humour the infusions by occasionally with- 
drawing the flame and moderating the ebullition. The lamp, of course, may be employed 
alone without the preliminary immersion of the tubes in hot water. Usually the process of 
heating is repeated at intervals of twelve hours, but in the case of very nutritive infusions 
in a very warm room the interval ought to be shorter. Practice must inform the 
experimenter on this point. The reheating must always occur before the infusions show 
the slightest tendency to change. 
In the early days of February a closed chamber of six tubes was treated in the manner 
here described. Three of the tubes were charged with strong turnip and the three 
others with strong artichoke-infusion. After two days discontinuous heating night and 
morning, they were allowed to remain undisturbed in the warm room. The six tubes 
remain perfectly brilliant to the present hour. 
On the 12th of February a closed chamber of three tubes was charged with cucumber- 
infusion. Heated discontinuously in the manner described, and abandoned afterwards 
to a warm temperature, the three tubes remained perfectly sterile. 
Any process competent to sterilize very old hay can sterilize with greater ease any 
other infusion. The fact, therefore, that only a few days ago three closed chambers 
charged with our most refractory hay-infusions were sterilized by discontinuous heating 
proves the power of the method over infusions of all kinds. 
By this method very instructive comparative experiments might be made, and the 
resistant power of different germs might be expressed in terms of the heatings necessary 
for their sterilization. I possess, for example, two test-tubes, containing the same infusion 
and associated with the same closed chamber, one of which has been heated five times 
and the other six. The former is quite turbid, while the latter is perfectly clear. In 
this case five heatings had left some of the more resistant germs still unkilled, which 
were destroyed by the sixth heating. Of two other tubes charged with a different 
infusion, one has been heated seven times and is now full of life ; the other has been 
heated eight times and is perfectly barren. 
With due care the method of sterilization here described is infallible, however highly 
infective the surrounding atmosphere may be. But here, as elsewhere in these difficult 
inquiries, the sagacity which comes in great part from nature, the skill which comes 
from training, and the care which ought to root itself in his moral constitution are all 
necessary to save the experimenter from error and to lead him to the truth. 
§23. Mortality of Germs through defect of Oxygen produced by Exhaustion 
with the Sprengel Pump . 
An equally striking mode of sterilization is now to be described. The crowding 
together of the organisms so as to form in a multitude of cases a heavy, corrugated, 
fatty scum upon the surface of the infusions obviously indicated that air was a necessity 
of their life. In some cases the oxygen dissolved in the infusions sufficed to enable 
the Bacteria to cloud them from top to bottom ; but in many cases they gathered 
