QUANTITATIVE EXAMINATION OF MILK 



51 



4, Take the first tube from the beaker and well roll it in the 

 hand (inclining it occasionally) in order to obtain an even distri- 

 bution of the organisms. Raise the lid of the Petri dish marked to 

 correspond with it, remove the cotton - wool plug, pass the tube 

 orifice through the flame, and pour out the gelatine into the dish 

 with an even flow. Place aside on a level surface until solidifi- 

 cation has taken place. 



5. Proceed as rapidly as possible with the other tubes in a 

 similar fashion, and when solidification has taken place on the four 

 plates, place them at a suitable temperature for the development 

 of the colonies. We have ourselves found that the temperature 

 of an ordinary room, 15°- 18" C. gives the best result 



In practice it will be found that under ordinary circumstances 

 the plate containing -5 c.c. of the final 

 dilution will be superfluous. But, especially 

 in the examination of town milk, it should 

 on no account be omitted, for in the event 

 of the addition of antiseptics to the milk, 

 it will often be found that the colonies on 

 the plates containing a less quantity of the 

 dilution will be so few in number that error 

 will be likely to occur if they are relied 

 upon alone. 



A very convenient form of wire stand 

 or cage, fitted for Petri dish cultivations 

 when made is shown in Fig. 4. The 

 arrangement of the stages is such that 

 the lids are held close down and cannot 

 be removed without withdrawing the dish 

 itself from cage.^ 



Coutiting the colonies. — The colonies 

 should be counted upon the second, third, 



and fourth days, and an estimate made, not only of the total 

 number of organisms per c.c. of milk, but also of those producing 

 liquefaction of the gelatine. For the enumeration we recom- 

 mend the circular card form of counter such as that of Pakes, 

 illustrated on Plate 8. The plates should be carefully adjusted in 

 the centre of the black disc, the covers removed, and the organisms 

 counted with the aid of a magnifying glass. Unless this latter 

 precaution is taken it is almost impossible to distinguish between 



These and other apparatus of a like nature, referred to in this work, can 

 be obtained from E. Cogit & Cie., 49 Boulevard St Michel, Paris. 



Fig. 4. — Wire cage stand for 

 Petri dishes. 



