LIGHT AND ELECTRICITY 43 
lowest temperature to which it must be exposed for ten miiuites under 
constant conditions to ensure the sterihty of the culture. The deter- 
mination of the thermal death-point is influenced by many factors beside 
the kind of organism under observation and the temperature. Older 
cidtures are usually less resistant than younger cultures of the same 
kind. The reaction of the medium (acids particularly decrease thermal 
resistance), the presence of extraneous substances as mucin and other 
non-conductors of heat, all play a part. Certain modifications in the 
characteristics of bacteria are observed when they are exposed for 
several hours at the maximum temperature of growth or a degree or two 
above this point. For example, anthrax bacilli, which habitually 
form spores, lose this property when they are exposed to 44° C. for 
several hours. 
Dry Heat, Moist Heat.— Dry heat is less effective in killing bacteria 
than moist heat. This is shown by the high temperature to which 
glassware and other apparatus must be exposed in order to kill spores, 
a temperature of 160° C. for one and a half hours being required 
to ensure sterility. Moist heat, whiqh is best obtained by dry steam 
under pressure, will kill even the most resistant spores in fifteen 
minutes at 15-pounds pressure provided sufficient time is allowed for the 
material to reach the temperature and pressure before the time is 
counted. 
H. HEAT PRODUCTION. 
The energy liberated by bacteria during the decomposition of 
organic substances by bacterial growth is partly utilized by them 
for their anabolic requirements. A larger part, however, is dissipated 
as heat. The heat generated in actively growing cultures of bacteria 
can be detected with sensitive thermometers, provided losses due 
to radiation and evaporation are guarded against. The heat pro- 
duction is not great as a rule, although in certain fermentations it 
may rise as high as 12 to 15° above the uninoculated controls. The 
decomposition of protein and protein derivatives (putrefaction) 
usually gives rise to less heat than the decomposition of carbohydrates 
(fermentation) under the same conditions. 
I. LIGHT AND ELECTRICITY.^ 
The vast majority of plants possess a photodynamic pigment, 
chlorophyll. This pigment can synthesize inorganic substances, as 
CO2 and water, together with nitrates, into complex organic compounds 
through the energy of the sun's rays acting upon it. Plants possessed 
of this pigment, therefore, are the synthetic agents of Nature. Usually 
this pigment is green; it may, however, be brown or red, the latter 
pigment being characteristic of certain alga?. A group of the higher 
' Miiller: Ergeb. d. Physiol., 1904, 4, 138, for discussion. 
