BLOOD-SERUM. 



467 



thermo-regulator is employed to secure a constant temperature. 

 Heat is applied till a temperature of about 75° is reached and this is 

 then maintained for 1 hour. This is repeated on each of three or four 

 successive days.. The time necessary to secure sterilization can be 

 shortened by transferring- the tubes, for a few hours after each heat, 

 to an incubator at 37° as described in connection with the preceding 

 method. 



Sterilization of serum at 100°. — Serum sterilized at this 

 temperature is opaque white. This, howevei:, does not in- 

 terfere with its value as 

 a nutrient medium. The 

 loss of transparency is 

 counterbalanced by the 

 ease with which it can 

 be prepared in larg-e 

 quantities. If inclined 

 blood-serum is heated 

 rapidly to 100° it will 

 solidify, but the mass 

 becomes torn up by gas 

 bubbles due to the expul- 

 sion of carbonic acid. 



The formation of 

 these bubbles or spaces can be avoided if the serum is first 

 maintained at a temperature of about 80° for some time. 

 For this purpose, they can be inclined in an ordinary air- 

 bath (Fig. 25), or in the apparatus shown in Fig. 65. After 

 keeping the serum at 80° for some time, the temperature 

 should be gradually raised till that of 100' is reached. The 

 tubes are then steamed for 30 minutes on each of 3 suc- 

 cessive days. 



Fig. 65. Koch's serum sterilizer. 



Solidification of serum. — The sterile, fluid serum can be 

 used as such, but more often it is coagulated in an inclined 

 position and employed for streak cultures. The fluid serum 

 should be kept in sealed tubes to prevent evaporation ( see 



