CRYPTOGAMS 259 



Dissolve the agar in boiling water and add the first solution, 

 stirring constantly until they are thoroughly blended. 



Test with litmus and neutralize again if necessary with 

 sodium carbonate. Add water to make up to 500 cc. Boil 

 the solution and filter it hot through absorbent cotton. 

 Pour it into sterilized Petri dishes or test tubes as desired. 



Sterilizing. (a) Dishes, needles, tubes, etc., are sterilized 

 by being boiled for fifteen minutes in water in which wash- 

 ing soda has been dissolved, and then being thoroughly 

 rinsed in boiling or very hot water. They should be dried 

 in an oven or over a Bunsen burner or alcohol lamp 

 flame. 



(6) Needles, platinum wires, etc., may be sterilized by 

 being held in a flame until red-hot. 



(c) Absorbent cotton, used for the 'plugs in test-tube 

 cultures, is sterilized by being placed in an oven and heated 

 until it is on the verge of scorching. When the cotton 

 begins to assume a brownish hue, it should be removed 

 and placed in a sterilized fruit jar ready for use. 



221. BACTERIA 



Object. To prove the presence of bacteria in the air. 

 Apparatus. Petri dishes and a culture medium. 



Method. Having sterilized the Petri dishes, put a small 

 quantity of the culture medium in each. There should be 

 only enough to flow over the bottom of the Petri dish so as 

 to leave a thin, even layer when the agar sets. 



Cover the dishes to exclude dust and set them away until 

 the material has become firm. 



Examine the dishes and note the clear, transparent char- 

 acter of the culture medium. 



