the cell is lost is known as the _thermal death
point_. This limit is not only dependent upon the nature of the
organism, but varies with the time of exposure and the condition in
which the heat is applied. In a moist atmosphere the penetrating power
of heat is great; consequently cell-death occurs at a lower temperature
than in a dry atmosphere. An increase in time of exposure lowers the
temperature point at which death occurs.

For vegetating forms the thermal death point of most bacteria ranges
from 130 deg.-140 deg. F. where the exposure is made for ten minutes which
is the standard arbitrarily selected. In the spore stage resistance is
greatly increased, some forms being able to withstand steam at 210
deg.-212 deg. F. from one to three hours. If dry heat is employed, 260
deg.-300 deg. F. for an hour is necessary to kill spores. Where steam is
confined under pressure, a temperature of 230 deg.-240 deg. F. for 15-20
minutes suffices to kill all spores.

~Drying.~ Spore-bearing bacteria like anthrax withstand drying with
impunity; even tuberculous material, although not possessing spores
retains its infectious properties for many months. Most of the dairy
bacteria do not produce spores, and yet in a dry condition, they retain
their vitality unimpaired for considerable periods, if they are not
subjected to other detrimental influences.

~Light.~ Bright sunlight exerts on many species a powerful disinfecting
action, a few hours being sufficient to destroy all cells that are
reached by the sun's rays. Even diffused light has a similar effect,
although naturally less marked. The active rays in this disinfecting
action are those of the chemical or violet end of the spectrum, and not
the heat or red rays.

~Influence of chemical substances.~ A great many chemical substances exert
a more or less powerful toxic action of various kinds of life. Many of
these are of great service in destroying or holding bacterial growth in
check. Those that are toxic and result in the death of the cell are
known as _disinfectants_; those that merely inhibit, or retard growth
are known as _antiseptics_. All disinfectants must of necessity be
antiseptic in their action, but not all antiseptics are disinfectants
even when used in strong doses. Disinfectants have no place in dairy
work, except to destroy disease bacteria, or preserve milk for
analytical purposes. Corrosive sublimate or potassium bichromate are
most frequently used for these p