88 BACTERIOLOGICAL METHODS 



rinsed in sterilized distilled water, using a fresh supply of dis- 

 tilled water for each rinsing. 



Draw the liquid to be plated into the pipette, place thumb 

 over the upper end of pipette, let liquid run out until one of the 

 O.I cc. marks is reached, then bring the lower end of the pipette 

 close to the surface of the Hquid in the diluting tube and let 

 just O.I cc. run out, the finer degrees of accuracy are attained 

 by using or not using the meniscus of the liquid projecting from 

 the lower end of the pipette after the last drop has fallen. This 

 correcting droplet is secured by touching the lower end of the 

 pipette lightly against the inside of the diluting tube at a point 

 near the surface of the liquid, without, however, actually touch- 

 ing the liquid. A similar adjustment may be made when taking 

 up the I cc. amount to be diluted, only in this case the pipette is 

 of course to be touched against the side of the tube or vessel 

 containing the liquid of which the dilution is to be made. 



Thorough mixing of the contents of the diluting tube is at- 

 tained by vigorous shaking. Place the thumb over the open- 

 ing of the tube, interposing a piece of sterilized rubber sheeting 

 such as is used by dentists. Some workers mix the contents of 

 the tube by rapidly rotating between the two hands and by tap- 

 ping against the palm of one hand. 



6. Incubation.- — The regulation incubators are to be used. 

 It is highly important that there should be ample ventilation, a 

 matter to which amateurs and even experienced bacteriologists 

 as a rule give little or no attention. All modern incubators are 

 supplied with ventilating openings at the top which should be 

 kept open most of the time. The air in the incubating chamber 

 should be practically saturated with moisture, which may be 

 accomplished by placing a fiat dish containing water in the lower 

 chamber. 



Two standard incubating temperatures are employed, namely, 

 20° C. and 37° C, the first corresponding to the ordinary room 

 temperature and the second to the body (human) temperature 



