CULTURAL TECHNIQUE 3 



boiled in water or allowed to stand in a strong solution of 

 washing powder for twenty-four hours. The washing powders 

 which are so largely used in dairy work will be found prefer- 

 able to soap, rinsing from the glass much easier and produc- 

 ing no deposits of calcium salts with hard water. 



Pipettes which have been used for milk should be rinsed 

 at once, placed in a solution of washing powder for several 

 hours, removed, rinsed well, and drained. 



The destructive test-tube brush should be replaced by a 

 swab, made by slightly expanding the end of a piece of 

 glass tubing and fastening to this, with an ordinary rubber 

 band, a bit of soft sponge, which may be renewed as often as 

 necessary. 



Exercise. Clean all test tubes, Petri dishes, and flasks in your 



possession. 



Sterilization of glassware. Dry heat is not as effective a 

 germ destroyer as moist heat, but for certain purposes it is 

 preferable. It is especially adapted for sterilization of empty 

 culture dishes, pipettes, and other glassware. Organic matter 

 such as cotton must be carefully watched in sterilizing in 

 dry heat, as it chars when heated above 150 C. The slight 

 browning of the cotton plugs in test tubes is an indication that 

 the heating has been sufficient to destroy all bacteria present. 



All material that can be subjected to dry heat is sterilized 

 by heating to 150 C. for one and one-half hours in a hot-air 

 sterilizer. The latter consists of a double-walled box of sheet 

 metal so provided with burners as to heat rapidly and uni- 

 formly. The glassware must be thoroughly dry before being 

 placed in the sterilizer. In case the Petri dishes are placed 

 in solid piles, it should be remembered that it requires a con- 

 siderable period of time for the heat to penetrate into the 



