PRODUCTS AND DEVELOPMENT OF BACTERIA. 185 
may be seen as bubbles: n the substance in which they are located. 
Exactly what the nature of these gases is has not been definitely 
determined. 
In connection with this production of gas what other feature is 
noticeable ? 
The odors, some of which are intensely penetrating and offensive. 
They may also be produced without any apparent gas formation. This 
disagreeable feature of putrefactive change, then, is due to the activity 
of the bacteria acting as scavengers in their work of tearing apart 
dead organic materials. 
In what way does temperature influence the growth of bacteria ? 
Most forms, especially of the parasitic varieties, grow best at the 
temperature of the human body. A great many of the saprophytic 
forms do not require such a high temperature. If the temperature that 
is the most favorable to the growth and development of a given bac- 
terium be gradually raised, at first the organism becomes more and 
more active until finally a point is reached when its activity ceases and 
it dies. With a gradual reduction of temperature these organisms 
become legs and less active and either lie dormant until the tempera- 
ture is raised again or they die. 
At what extremes of temperature have bacteria been found to 
grow ? 
They have been found in sea water and in earth growing at O° C. 
the freezing point of water. On the other hand there are forms which 
require a temperature of 60° C. in order that they may thrive. 
At what temperature do the majority of these organisms develop 
the best ? 
The purely saprophytic varieties thrive the best at about 27° C. 
or ordinary summer heat. The parasitic forms are most favorably 
located for active growth at points ranging from 35° to 40° C. 
How are bacteria affected by light ? 
Ordinary daylight seems to have no particular effect upon them, but 
if they are exposed to strong sunlight they are liable to lose vitality 
and finally to be destroyed. Experiments with anthrax spores show 
that when exposed to strong sunlight for a few hours they lose their 
power of development. 
How are bacteria affected by dryness ? 
They may die or they may remain for long periods inert. As soon 
