EXPERIMENT STATION BULLETINS 



239 



37^° C. the mercury would expand and decrease the size of the hole and 

 thus decrease the supply of gas to the flame. In this way it is possible 



to hold a good incubator within a slight variation. 



Safety In connection m ith an incubator is supposed to 



burner. be a safety burner. The ordinary gas burner 



could be used to obtain the heating results, but a 

 small draught would blow it out and allow the gas to escape 

 into the room. To prevent this escape of gas if blown out, 

 a spring is attached near the flame; when it is hot it expands 

 and holds up a lever which acts as a stop-cock for the gas; 

 when it cools the spring contracts and the lever falls, shut- 

 ting off the gas. In lighting the burner, the lever is held up 

 with a catch, until the spring expands and holds it; as soon safet'y 'burner. 

 as the flame goes out the lever falls, when the spring cools and retracts. 



CULTIVATION OF BACTERIA. 



To isolate and cultivate the various species of bacteria is the object at 

 which we have been aiming from the very beginning of our discussion of 

 laboratory methods. This is necessary for an investigation of their mor- 

 phological and chemical natures. We are now ready for a systematic lab- 

 oratory treatment. 



The isolation of bacteria is the first step in their study. To do this, the 

 material which is to be examined must always be guarded from outsid9 

 contamination lest the results be vitiated. Assuming that it is ready, a 

 bit of it is transferred to a liquefied gelatin tube and thoroughly stirred. 

 We will call this tube No. 1. i^rom No. 1, three loopfuls are taken and 

 mixed with a liquefied gelatin tube No. 2; from No. 2, three loopfuls are 

 taken and mixed with another liquefied gelatin tube. No. 8. The object 

 of using three tubes is to dilute the bacterial solutions so that there will 

 be but few bacteria in No. 3 tube. The reason for this will appear as we 



go on. The tops of these tubes are sterilized in the flame and 

 Petri dishes, the tubes put aside to cool. Three sterilized Petri dishes are 



gotten ready. These dishes are round, about five-eighths of an 

 inch high, and four inches in diameter. The bottom fits 

 directly into the cover as the bottom of a telescope fits 

 into its cove-r, and thus precludes any contamination 



from outside. The contents of the cooled 

 Plating. tubes are poured into these Petri dishes and Fiersn. Petri dish. 



spread over the surface of the bottom by the lip of the tube. 

 The Petri dishes are marked Nos. 1, 2, and 8 to correspond with the tubes. 

 This operation is called plating and its purpose is to separate the bacteria 



and scatter them over a large surface. Each micro-organism 

 Colony. upon growing will give rise to a mass of its own kind, called a 



colony, which will appear upon the gelatin as minute whitish 



