18 GENERAL BACTERIOLOGY 



EXERCISE 9. PREPARATION OF POTATOES (Bolton). 



General Directions. 



a. Select a number of rather large test-tubes (150x20 mm.), 

 place a small wad of absorbent cotton^ in the bottom 

 of each (Fig. 7 a), plug and sterilize as usual. 



h. Wash a large potato, then with a cork borer 

 slightly smaller than the test-tubes punch out cylinders 

 about 5-6 em. long. 



c. Divide these diagonally and trim to shape indi- 

 cated in Fig. 7 h. 



d. Add a few drops of distilled water to each test- 

 tube and place pieces of potato in position. 



e. Sterilize on three consecutive days for 30 to 45 



minutes. Pia.7. Bolton's 



Unless the tubes are to be used immediately, they po'^'*" *"•>«■ 

 should be sealed. (11.) The dark color can be prevented by im- 

 mersing the pieces between c and d in running water for 12-18 hoiirs. 



References. A. 107 ; H. 49 ; M. & . 47 ; McF. 190 ; P. 216 ; P. 

 B. C. 28 ; S. 47. 



Special Directions. Prepare 15 test-tubes of potato, sterilize 

 and seal with paraffin. (11-2.) 



EXERCISE 10. PREPARATION OF WATER-BLANKS. 



General Directions. Water-blanks are prepared by placing 

 exactly 10 cc. of a physiological salt solution (6 gms. per 1,000 cc. 

 water) in test-tubes and sterilizing in autoclave 15 minutes at 120° 

 C., or in steamer 15 minutes on three successive days. 



Special Directions. Prepare and sterilize 10 water-blanks. 



EXERCISE 11. CARE OF CULTURE MEDIA. 



When sterile culture media (or test-tube cultures) are to be 

 kept for some time they must be protected from evaporation and 

 stored in a dark, cool place. Evaporation may be checked to a con- 

 siderable extent, (1) by storing them in tin cans, e. g. quinine cans. 

 Care must be taken, however, that these do not become too damp in 

 which case the mould fungi frequently grow through the cotton 

 plugs; (2) flasks and test-tubes may be sealed by removing the 



' Gage recommends glass beads. A smaller cylinder of potato may also be 

 used instead of the cotton; in this case the tubes would be sterilized empty. 



