58 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 
microérganism 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 
Fic. 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 
of the most important pathogenic bacteria produce toxic substances 
during their growth which may cause the death of susceptible ani- 
mals independently of the living bacteria ; and this demonstration 
