84 PRACTICAL BACTERIOLOGY. 



fully prepared. First, without regulating the supply 

 of gas to the flame, the incubator is heated nearly up to 

 the required temperature. The screw of the thermo- 

 regulator is then turned, until the flame begins to 

 become smaller. The temperature is then noted, and 

 every two or three hours observations are taken as to 

 whether a rise or fall in the temperature has taken 

 place. In the first case the thermo-regulator is closed 

 still furthei', in the latter it is somewhat opened, 

 until finally the required constant temperature, in 

 which a variation of not even ^° C. takes place in 

 twenty-four hours, is obtained. Then and then only 

 can the cultivations in question be placed in the 

 incubator. 



Nutrient gelatine may be used, if a lower tempera- 

 ture, such as from about 20°-24° C, is sufficient. The 

 medium in this case is prepared with. 10-15^ of 

 gelatine instead of the usual h°/, in order to make 

 it less easily liquefied. Above 25° C. no prepara- 

 tion of gelatine can be relied upon, as it becomes 

 semi-fluid, so that the colonies run into one another, 

 and lose their cbaracteristic appearance, hence 

 bouillon may as well, or even better, be used. In 

 many cases it is necessary to cultivate at the tem- 

 perature of the human body, 37° C. In these cases 

 agar-agar must be used instead of gelatine. 



It must however be observed, that agar-agar dries up 

 much more rapidly at this temperature than at lower 



