14 



§ 6. General Directions. Plug all of the tubes and flasks 

 with absorbent cotton and sterilize them, together with the Pe- 

 tri dishes, in the hot air sterilizer. After they are sterilized 

 store them in the locker until they are needed. The Petri 

 dishes must not be opened until they are used. 



§ 7. Plugging the tubes and flasks. The rolls of ab- 

 sorbent cotton are cut in short segments of from 5 to 7 cm. 

 in length. A piece of sufficient length of this narrow strip 

 to give cotton enough for the plug is torn off. The quantity 

 varies, of course, with the size of the mouth of the tube, or 

 flask, but a little experience will enable one to estimate the 

 quantity accurately. The torn edges of the cotton are 

 turned in and it is rolled up to form a firm plug which should 

 snugly fit the neck of the tube or flask. It should be in- 

 serted into the tube for about 2 cm. and the end should be 

 nearly flat and smooth. The projecting part should be 

 about the same length and be of equal firmness. 



§ 8. Sterilizing glass ware. Place all of the tubes, 

 flasks, and Petri dishes in the hot air sterilizer, close the 

 door tightly and light the gas. Heat the air in the sterilizer 

 to a temperature of from 135 to 150" C. and keep it there 

 for one hour, ( the temperature must not be allowed to go 

 above 150° C. ) Then turn the gas off and when the tem- 

 perature of the air in the sterilizer goes down to or below 

 45 ° C. the door can be opened and the apparatus removed. 



EXERCISE III. 

 THE PREPARATION OF BOUILLON. 



§ 9. Explanatory note. Bouillon is the liquid medium 

 most commonly employed in cultivating bacteria. It is prac- 

 tically a beef tea containing peptone. There are several 

 methods recommended for making it. (i) it may be made 

 directly from simple meat infusion or, (2) it may be made 

 from meat extract. The meat infusion is prepared either by 



