CULTIVATION. 149 
water between its walls in order that the heat applied may be distribu- 
ted equally. It is provided with double doors, is covered with felt and 
has in the top a small aperture in which a thermometer is placed to 
record the temperature. To heat the chamber a Bunsen burner is most 
often used ; the supply of heat is controlled by one of the many thermo- 
regulators in use. 
In what way are the different varieties of bacteria separated one 
from the other? 
By making plate cultures or Petri-dish cultures as before described. 
What points are of value in distinguishing the different varieties ? 
In a properly prepared plate or dish culture the bacteria will be seen 
to develop as little spots or foci called colonies. Each colony does not 
come in contact with its neighbor. The points of difference to be 
observed that are apparent to the naked eye are as follows: Shape of 
colonies whether regular or irregular, color, and if on nutrient gelatin 
whether that is liquefied or not. On microscopical examination with.a 
low-power objective (one-third inch) note whether the colonies are 
smooth or finely or coarsely granular. Also note whether the members 
of a colony are arranged in radiating lines or concentrically, or in 
masses resembling cotton wool. These are some of the differences by 
which one variety of bacteria may be distinguished from another. 
What other points are to be considered in taking the life history of 
a given organism ? 
Its form must always be the same at the same stage of growth. 
Its behavior toward different culture media must be the same under the 
same conditions, and the reactions in different media must follow a fixed 
rule. It must be observed whether the presence of oxygen is needed 
or not, whether it posserses the power of motion or not, and whether 
or not it is pathogenic. 
What further steps are necessary to identify an organism ? 
The examining of cover-glass preparations, stained and unstained. 
The staining processes will be spoken of later. The unstained speci- 
men may be examined as a hanging drop or as an impression cover-glass 
preparation. 
How are the hanging-drop specimens prepared ? 
It is necessary to have a glass slide having a shallow concavity 
ground into one side of it. A drop containing the organisms to be 
studied is put upon the centre of a cover glass, with a sterilized plati- 
num wire. The cover glass is turned over so that the drop hangs free 
