METHODS OF EXAMINING MILK 265 



" The dilution bottles should have glass stoppers or 

 some other type of closing that makes shaking possible. 

 Cotton plugs are unsatisfactory because the dilution 

 water will soak into the cotton. 



" Straight-sided pipettes graduated to deliver be- 

 tween two marks are best, but pipettes marked to deliver 

 may be used if care is taken that the points are not broken, 

 and the tube is completely emptied. 



" In making dilutions the original sample and each 

 dilution bottle shall be shaken 25 times, each shake being 

 an up-and-down motion with an excursion of about one 

 foot. After the final dilution fill a pipette to the mark 

 and allow the contents to run into an empty Petri dish, 

 the end of the pipette touching the bottom of the dish 

 as the liquid runs out. If the pipette is one that delivers 

 1 c.c., be sure that the last drop is carried into the Petri 

 dish. (Pipettes should be placed immediately in water 

 after using to make subsequent cleaning easier.) " 



Mating. " The agar in the flasks (or test tubes) 

 shall be melted in boiling water or steam and after 

 melting should be cooled to 45 C. before using. 



" Pour 10 c.c. of the melted agar in each inoculated 

 Petri dish, and by a gentle rotary motion thoroughly mix 

 the agar and the diluted milk. As nearly as possible the 

 same amount of agar should be poured into each Petri 

 dish so that the depth of the agar in all be uniform. If 

 desired, 10 c.c. may be measured out from the flask with 

 a sterile pipette. 



" After dilution of the milk not more than half an 

 hour should elapse before the agar is poured into the 

 Petri dishes. 



" After the agar has thoroughly hardened place the 

 Petri dishes in an incubating oven, inverted, in order to 



