56 



STERILIZATION OF CULTURE MEDIA. 



heat as soon as the serum is solidified. Koch's apparatus for coagu- 

 lating blood serum is shown in Fig. 28. It is customary to place the 

 test tubes in an oblique position, so that a large surface may be ex- 

 posed upon which to cultivate the tubercle bacillus or whatever 

 microorganism may be under investigation. A form of apparatus 

 designed for both sterilizing and coagulating blood serum is shown 

 in Fig. 29. It is manufactured by Miincke in accordance with the 

 directions of Hueppe, and special precautions have been taken to se- 

 cure a uniform temperature in all parts of the air chamber. We 



FIG. 29. 



may remark that since it has been shown by Roux and Nocard that 

 the tubercle bacillus grows very well in agar-agar jelly to which 

 five per cent of glycerin has been added, blood serum is not so 

 largely used as a culture medium in bacteriological laboratories. 



Sterilization by Filtration. This method is especially useful 



for separating the soluble substances contained in a liquid culture of 



bacteria from the living cells. It has been demonstrated that several 



e most important pathogenic bacteria produce toxic substances 



during their growth which may cause the death of susceptible ani- 



mdependently of the living bacteria; and this demonstration 



