AGf~ICULTURAL RESEARCH, PUSA.

BIOLOGY OF 1HICRO-OIWANIS1I1S 17
-Thus the thermal death-point of the 11llenlllococcWi is
apPl'oximate(',; 52° C.
Bacteria are more snsceptible to moist than dry
heat. Spores are much morc resistant to heat than
vegetative bacteria, but their degree of resistance
varies in diHerent species. l"urthel' data all this subject
are given in regard to sterilisation (p. (7).
Some species die if kept at 0° C., but others may
survive even lower temperatures.
MOISTU.RE.-Four-fifths by weight of the baderial
cell consist of water, and, as in the case of other
organisms, moisture is necessary for growth. Drying
is generally injurious; spores, however, can resist
this influence for long periods-e.g. spores of B.
antlimcis, for several years. Certain non-sporing
bacteria may also exhibit considerable resistance to
drying-c.g. the tubercle bacillus (q.11.).
LIGHT.-Thc optimum condition for growth and
viability is darkness. Ultra-violet rays are markedly
bactericidal-e.g. direct sunlight or radiation from an
arc or mercury-vapour lamp.
INFLUENCE OF CHEMICALS.-Various inorganic and
organic chemicals, acting as protoplasmic poisons, are
either inhibitory to growth or bactericidal, depending
on thc concentration brought into contact with the
particular organism.
Substances possessing markcd growth-inhihitory
or bactcricidal properties aTC llsually designated antiseptics,
and their practical applications are now well
lmmvn. A great variety of chemicals exhibit antiseptic
action and have been llsed for that purpose:
acids-e.g. nitric acid; alkalies-c.g. caustic soda;
metallic salts - c.g. perchloridc and biniodide of
mercury; organic mctallic compounds-c.g. mercurochrome,
silver salvarsan; halogens-e.g. chlorine (as
dcrived from bleaching powder, etc.), iodine; alcohols
-e.g. ethyl alcohol; aldehydcs-e.{!,. formalin;
eycNc hydJ'OwTbons-e.g. benzol; cyclic alcoholse.g.
phenol, cresol; volatile oils and thei!' p!'od'llctse.g.
thymol; anaesthctic sllbsfances--c.g. chloroform;
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