18 THE GENERAL CHARACTERS OF MICROORGANISMS 
temperature is above 100°, most bacteria grow less rapidly than 
when it is a little lower, while at a slightly higher temperature they 
cease growing. A few species, however, grow best at unexpectedly 
high temperatures, some having been found flourishing at 140° 
or even higher. These peculiar bacteria are called thermophiles. 
How they can find conditions in nature warm enough for their 
growth is a question. 
The death temperature is a factor of great importance, since it is 
so closely associated with the matter of sterilization by heat. 
Most bacteria, when in an active condition, are killed by a tem- 
perature of 140° if maintained for half an hour. At this tem- 
perature, however, they die slowly; a temperature of 150° destroys 
them more rapidly still, while a temperature from 170° to 180° 
is proportionately more effective. A total destruction of bacteria, 
including their spores, can be brought about only by a temperature 
above that of boiling water, and this is usually accomplished, in 
the case of liquids, in a closed chamber where the steam can be 
generated under considerable pressure. If the steam is allowed 
to collect in such a chamber at a pressure of 15 pounds, the 
temperature, then, will be about 240°. This temperature, kept 
up for half or three-quarters of an hour, destroys even the most 
resisting spores. Laboratories usually have a small apparatus 
designed for this purpose, called an autoclave (Fig. 12), and this is 
used constantly for sterilizing liquids. 
Stertlization.—This is a process closely related to the question 
of death temperatures. Sterilization is sometimes, to be sure, 
brought about by adding poisonous chemicals to the material 
to be sterilized; but more commonly, and almost universally, when 
we are dealing with food products, sterilization is accomplished 
by heat. Ifa material to be sterilized contains only active organ- 
isms, it might be accomplished by subjecting it to a moderately 
low heat, 140° to 150°F. But it is almost always a fact that 
anything which we wish to sterilize is likely to contain spores, and, 
since these withstand a higher temperature, no moderate heat will 
