252 BACTERIOLOGY 



other apparatus which is heated day and night, within a 

 good hood. 



The Koch safety burner is shown in Fig. 39. It is a 

 small Bunse.n burner provided with two steel springs. 

 These are heated by the flame and expand. The motion, 

 thus induced, is communicated to a small arm which can be 

 set, so that, when the burner is lighted, it just supports the 

 weighted lever. The latter is connected with a valve. 

 When the light goes out the springs cool and draw the 

 small arm from under the weighted lever. This then falls 

 and shuts off the flow of gas. The tube of the Koch 

 burner is not infrequently wider than it should be, because 

 the minimum supply of gas from the thermo-regulator is too 

 small to keep the light burning above the burner. In that 

 case, the flame drops down into the tube. This defect can 

 be easily remedied by inserting a short piece ( ^ in. ) of brass 

 tubing just inside of the end of the burner. The flame is 

 thus made narrower and is, therefore, less liable to drop 

 down. 



Thermometer. — ^Any kind of a glass thermometer can be 

 used in connection with a thermostat, but it is preferable 

 to have one which would indicate fractions of a degree. 

 The thermometer usually employed is graduated from 

 to 60° and reads tenths of a degree. A Kappeler maximum 

 and minimum thermometer is, at times, very useful to ascer- 

 tain the limits of variation in the temperature. 



