GENERAL CHARACTERS OF BACTERIA. 
of 140 if maintained for half an hour. At this temperature, 
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 con- 
siderable pressure. If the steam is 
allowed to collect in such a chamber 
at a pressure of 15 pounds, the tem- 
perature, then, will be about 240. 
This temperature, kept up for half or 
three-quarters of an hour, destroys 
even the most resisting spores. Lab- 
oratories usually have a small appa- 
ratus designed for this purpose, called 
an autoclav (Fig. 12), and this is used 
constantly for sterilizing liquids. 
Sterilization. This is a process 
closely related to the question of death 
temperatures. Sterilization is some- 
times, to be sure, brought about by 
adding poisonous chemicals to the FIG. 12. An autoclav used in 
material to be sterilized; but more ^qib ""der pressure 
commonly, and almost universally, 
when we are dealing with food products, sterilization is accomplished 
by heat. If a material to be sterilized contains only active organisms, 
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 
accomplish the purpose. Even boiling is not sufficient to destroy 
spores, so, to be sure of complete sterilization, a temperature above 
boiling is necessary. If the object is a solid that can bear heat 
it is simply heated at about 300 F. for an hour or so. If it is a 
