Methods of Determination of Pathogenic Jlicrobes. 133 



the gas taken from tlie pipe passes first into a regu- 

 lator which corrects the variations of pressure at the 

 gasometer at clifterent periods of the day, and then 

 arrives at the apparatus by passing through a tem- 

 perature regulator. This last is of various forms; it 

 is usually a kind of mercurial thermometer sur- 

 mounted by a chamber into which the gas enters; 

 when the temperature rises in the oven in which it 

 is placed, the mercury, in expanding, partially ob- 

 structs the entrance of the gas, diminishes the flow 

 and the combustion, and depresses the temperature. 

 This depression results in the retraction of the mer- 

 cury, a larger inrush of gas, and the temperature 

 rises again: thus the regulation takes place con- 

 stantly between temperatures so close to each other 

 that the oscillations are inappreciable. 



The regulation of the ovens may also be obtained 

 by the expansion of the water between the double 

 walls. The water, in heating, expands, rises in a 

 small tube, and presses against an elastic membrane 

 of caoutchouc or thin metal, which membrane dimin- 

 ishes the orifice of entrance of the gas. 



Aerobic cultures. — All that is needed in order to ob- 

 serve the development of aerobic germs is to place 

 the inoculated media in the oven, or, as we may now 

 call it, the incubator, at 37°; this incubator is so ar- 

 ranged that the air in its interior is continually re- 

 newed. 



All the media admit of cultures being carried on 

 at 37°. ISTutritive gelatin, however, being fluidified 

 above 25°, should not be carried above this tempera- 

 ture if it is desired to obtain the advantages arising 

 from the use of solid media. 



