LABORATORY TECHNIQUE 41 



preparations for microscopic observation and note the 

 morphology of the plant. Many stains are in use for 

 demonstrating various characteristics. He will also 

 prepare what is known as a "hanging drop/' This 

 consists of a drop of fluid broth culture upon a 

 thin inverted glass (Fig. 13). He will discover from 

 this preparation under the microscope the presence of 

 motility and the manner of division of the bacteria. 

 From his tube cultures he chiefly finds out whether 

 the bacteria develop enzymes. To the solid media 

 (agar-agar) he may add various sugars to discover the 

 fermentative powers of the bacterium. The fermenta- 

 tive powers may also be observed when the germs grow 



FIG. 13 



Hollow slide with cover-glass. (Park.) 



upon bouillon containing the sugars. This broth is 

 placed in an apparatus called fermentation tubes, so 

 arranged that the percentage of sugar broken up by 

 the bacteria can be estimated. When he shall have 

 made all his observations he will sum up his results 

 and identify according to the classification of bac- 

 teriologists. 



Animal Inoculation. Another method of studying 

 bacteria is by injecting them into susceptible animals. 

 Thus can be discovered their power of producing 

 disease, its severity, called virulence, and the nature 

 of their action. Animal inoculation is used for keeping 

 alive certain viruses or increasing the virulence of 

 those whose strength has declined. When the presence 



