INCUBATION OF CULTURES. 25 
To make a stab culture take the other gelatin tube 
and proceed as before until you get to step 5. When 
you have passed the needle into the tube drive it steadily 
into the medium, taking care not to deviate from the 
axis of the tube. Finish the process as before. 
All this may seem involved. As a matter of fact it is 
very simple and need not take more than a minute to 
perform. But every step must be carried out, and the 
whole process must be learnt so thoroughly that it is 
performed automatically whenever a culture is made. | 
INCUBATION OF CULTURES. 
The limits between which bacteria can live are very 
wide; some grow best at one temperature, others at 
another, the limits for the great majority of organisms 
being about 16° C. and 40° C. In practice two tem- 
peratures are all that is used for ordinary work. The 
lower, or so-called ‘‘room temperature” is about 20° C. 
(68° F.) and is of most use for those bacteria which 
grow naturally outside the body, 7.¢., as saprophytes. 
The higher, or body temperature, is about 37° C. 
(98°6° F.), and is the best temperature for the majority 
of germs which live within the body, ¢.¢., the parasites. 
It is obvious that gelatin cannot be incubated at this 
high temperature as it melts at 25° C. or thereabouts; 
but all other media are available. 
The term ‘room temperature” must not mislead the 
practitioner, for the temperature of most rooms is 
rarely constant at or near 20° C. for periods sufficiently 
long to permit of its use for incubating bacteria. In 
the laboratory we use an incubator, the temperature of 
which is regulated by means of an automatic regulator, 
