162 



LABORATORY EXERCISES IN BACTERIOLOGY. 



robic varieties. Salomonsen's capillary tubes drawn full of the inoculated medium, 

 and the ends sealed, will often show marked growths of the anaerobes; and if in the 

 fermentation tube growth be limited to the closed arm of the apparatus, it may be 

 inferred that the organism present is capable of anaerobic development. 



5. Light. Direct sunlight is destructive to most organisms, even diffuse light 

 if moderately strong being restrictive to many. On the other hand, absolute and 

 constant darkness is not favorable for their best development, the most suitable con- 



FIG. 49. KIP GAS GENERATOR AND WASH-BOTTLE. 



dition being an extremely weak, diffused light alternating with periods of darkness. 

 Cultures in the incubator are thus properly placed ; and it should be a rule that cultures 

 growing at room temperature be placed in a rather dark nook for their best growth. 



THE NUMERICAL ESTIMATION OF BACTERIA. 



Sufficient reference has been made in connection with the above instructions as 

 to the mode of analysis of air, water, milk, and soil to have the student understand 

 that for this purpose diffusion inoculations of known small quantities of the original 

 infected material or of known dilutions of such matter in sterile diluents are first to 

 be made. As suggested, it is best to make a series of such dilutions, diffusing each 

 in the medium so as to have a series of cultures for comparison of results. After 

 diffusion in the medium (usually liquefied gelatine) in a culture tube, the mixed con- 

 tents are to be plated, either in plates, in Petri dishes, or in the interior of the tube 

 (Esmarch tube), the inoculated bacteria being thus disseminated over the surface 

 upon which the film of medium is distributed. It is to be presumed that upon placing 

 these plated cultures in proper temperature, atmosphere, and in a dark place, each 

 bacterium will grow into a colony more or less separated from its fellows. The time 

 permitted for growth must vary somewhat with circumstances, but forty-eight or 



