17 



computed from the time the water boils. The flasks of 

 bouillon should be boiled or steamed for 20 minutes on each 

 of the two succeeding days. When they have cooled, the 

 outside of the tubes should be carefully wiped with a moist 

 cloth and placed in the incubator until the next laboratory 

 day. Then carefully examine them and if any of the tubes 

 are contaminated, that is, if the liquid is clouded or has a 

 membrane on the surface, they must be rejected. Label all 

 others and place them in the locker. 



12. Labeling. Stick on each tube of media, about 3 cm. 

 from the top, an adhesive, white label about 2 cm. square. 

 On the upper lines should be written the name of the me- 

 dium, and the date of its preparation. Thus, Bouillon, 13- 

 VII-97. When the tube is used the name of the organism 

 or material with which it is inoculated, together with the 

 date of inoculation, should be written on the lower lines. 

 This applies to all media and cultures. 



EXERCISE IV. 



THE PREPARATION OF AGAR AND GELATIN. 



13. Explanatory note. Of the solid media employed 

 in cultivating bacteria, agar and gelatin are most commonly 

 used. They depend for their nutritive properties largely 

 upon the bouillon from which they are made, the agar and 

 gelatin forming simply the solidifying elements. The strik- 

 ing difference between the two is that the gelatin melts at 

 the body temperature whereas the agar is not liquefied below 

 the boiling point. For this reason gelatin cannot be used as 



just as good results as the customary 3 boilings. As the media is not 

 used for 2 or 3 days after its sterilization and during which time it is 

 kept in an incubator, the method is adopted, not that it saves time in 

 preparing the media, but it relieves the congestion in the sterilizer and 

 appreciably aids the student. 



