BLOOD SERUM 179 



to be had. It is important that the temperature be raised 

 slowly so that the blood gases escape gradually. Three to 

 five hours or longer should be allowed for the temperature 

 to reach the boiling-point. If the tubes are heated too 

 rapidly the serum is filled with bubbles and badly torn 

 since the gases are driven off suddenly. Loffler's serum is 

 made by adding one part of dextrose broth to three parts 

 of serum and then coagulating as above. The solidified 

 serum in either case is best sterilized discontinuously, 

 though with care the autoclave at 15 pounds' pressure may 

 be used for a single sterilization. This is very apt to cause a 

 greater darkening of the serum and frequently also a lacera- 

 tion of the solid mass by escaping gases. 



Blood serum tubes may be solidified and sterilized at the 

 same time if a good pressure sterilizer is available. Place 

 the tubes in the steriHzer in a sloping position, quickly raise 

 the pressure to 15 pounds and keep it constant {this is the 

 essential point, to allow no variation in the pressure) until 

 the tubes are sterile, usually forty-five minutes. Allow the 

 pressure sterilizer to cool gradually before opening. 



Blood serum is also used in the liquid state. For this 

 purpose it is best to collect it aseptically; or it may be 

 sterilized discontinuously at a temperature of 55° or 56° on 

 seven to ten consecutive days. Novy has suggested dialyzing 

 the serum to free it from salts and thus prevent its coagula- 

 tion when heated. Whether the removal of the various 

 "extractives" which diffuse out with the salts deprives the 

 serum of any of its advantageous properties remains to be 

 ascertained. 



From the discussion of the physiological activities of bac- 

 teria in Chapters IX-XII it is apparent that a very great 

 variety of culture media other than those described is neces- 

 sary for the study of special types of bacteria, but such 

 media are beyond the scope of the present work. 



The ideal culture media are without doubt the synthetic 

 media, that is media of definite known chemical composition, 

 so that the various changes due to the growth of bacteria 

 can be accurately determined and thus a means of sharply 

 differentiating closely related organisms be secured. Such 



