ESSAYS ON BA.CTEEIOLOGY. 121 



insure against rapid fluctuations of temperature. It 

 is heated by a self -regulating gas burner, so that tlie 

 temperature is maintained for days or weeks at the de- 

 sired point. Many bacteria which grow best and most 

 rapidly in the incubator may also be successfully 

 though more slowly and somewhat imperfectly grown 

 without, any special incubating apparatus, providfed 

 the temperature be kept fairly near that, of the body. 



Inoculation of bacteria-containing material onto the 

 culture medium is usually made with a platinum or 

 other wire, sterilized before and immediately after 

 use by heating in a clean flame. In rapidity of 

 growth bacteria vary, like other seeds, and under sim- 

 ilar conditions. If the conditions are favorable, most 

 bacteria present visible cultures in from one to three 

 or four days. A. few of the known varieties require 

 from one to three weeks for their development. 



To illustrate the method of bacteriological investi- 

 gation in its simplest form, let us suppose that we wish 

 to discover the germ of diphtheria. Having prepared 

 the culture material and been assured, by protracted 

 observation, of its freedom from bacterial growth, we 

 take upon a sterilized wire from the throat a little of 

 the membrane presumed to contain the specific germ. 

 Removing for a moment the cotton plug from a tube, 

 this material is deposited in the previously warmed 

 and liquefied agar-glycerin. The contents of this 

 tube, or of another into which a portion has been 

 transferred from the first for the purpose of dilution, 



