102 LABORATORY EXERCISES IN BACTERIOLOGY. 



which are somewhat hotter than the interior atmosphere; and a thermometer should 

 be included with the tubes, which may be watched through the glass cover lest the 

 heat rise too high. A thermostat set several degrees higher than the temperature 

 desired in the interior chamber should be placed in the water-bath of the inspissator 

 and connected with the flame so as to prevent the temperature from becoming ex- 

 cessive (Fig. 31). 



(c} When the serum is to be used as a liquid medium, the preparation is precisely 

 similar to the above, save that the last step of the process that of coagulation 

 is omitted. The tubes in this case also should be tested for growth in the incubator. 



(d) In recent years it has become customary to omit many of the precautionary 

 steps of the above mode of operation and accept as a satisfactory product a white, 

 opaque, thoroughly coagulated serum susceptible of subsequent sterilization by frac- 

 tional steaming. With such purpose in mind the collection of the serum is simplified 

 to permitting it to flow from the severed vessels of the sacrificed animal into the sterile 

 jar, from which the cover has been completely removed for the purpose, only taking 



FIG. 31. BLOOD-SERUM INSPISSATOR, KOCH PATTERN. 



the ordinary precautions of reasonable cleanliness. When the jar has been filled, 

 the cover is readjusted, and the jar set aside until coagulation has taken place. The 

 cover is then again removed and with a sterile glass rod or wire the clot is broken 

 from the sides of the jar. This done, the cover is reapplied and the jar conveyed 

 with as little agitation as possible to the laboratory and placed in the refrigerator 

 The temperature of the refrigerator should not be very low lest it interfere with the 

 further clotting of the blood and the efficient shrinkage of the clot, but should be 

 sufficient to prevent the development of the microorganisms which have probably 

 gotten into the blood in its collection. After one or two days in the refrigerator the 

 serum will be found well separated from the shrunken clot and may best be removed 

 by means of. sterilized pipettes. .It is placed in a tall sedimentation jar for a few hours, 

 and after the blood cells have settled to the bottom and have left the supernatant 

 serum clear, the latter is transferred by means of sterile pipettes to sterile culture 

 tubes. These are then placed into the dry-air oven, each tube in a slanting position, 

 and heated cautiously to 90 C., when the serum will solidify into a dense, opaque, 



