QUANTITATIVE BACTERIAL ANALYSIS 257 



a. thermometer for recording the temperature of the water in the 

 water-jacketed bath; sterile 1 c.c. pipettes; sterile petri dishes; and 

 sterile dilution water in measured quantities. 



For milk work porous earthenware petri dish covers (Fig. 63) 

 are much superior to glass covers, since they absorb the excess 

 moisture from the agar and prevent " spreading." 



It is quite essential to the best results that the porous covers 

 should be wet as seldom as possible. In sterilizing them the process 

 should be prolonged beyond the time necessary to kill the organisms 

 an order that the covers may be thoroughly dry. 



Straight-sided 1 c.c. pipettes are more easily handled than. 



Fig. 63. 



Petri Dishes. 



those with bulbs; they may be made from ordinary glass tubing 

 about 3/16 of an inch in diameter and calibrated in the laboratory. 

 They should be made about to inches in length. 



Plating Technique, — The agar after melting should be kept 

 in the water-jacketed water bath between 40° C. and 45" C. for 

 at least fifteen minutes before using, to make sure that the agar 

 itself has reached the temperature of the surrounding water. If 

 used too warm the heat may destroy some of the bacteria or retard 

 their growth. Below this temperature the agar tends quickly to 

 solidify. 



For routine work in cities in order to bring down the actual 

 number of colonies in a plate around the standard of two hundred, 



