50 THE BACTERIOLOGICAL EXAMINATION OF MILK 



times as in the former case, withdraw the pipette, replace the 

 cotton-wool plug and shake for some minutes in order to obtain 

 an even distribution of the organisms. The flask now contains 50 

 c.c. of liquid containing ^ of the original milk, or a dilution of i 

 in 500. This is termed \h^ final dilution. 



It will easily be seen that by means of these test-tubes and 

 flasks of water almost any desired dilution can be made. Thus if a 

 dilution of y^^j^j- is desired, it is only necessary to transfer i c.c. 

 of the primary dilution to a second tube containing 9 c.c. of water, 

 and again i c.c. of this admixture to a third tube under the same 

 conditions as above. 



Or if a dilution of ^xjtjtt is thought desirable, then i c.c. of the 

 primary dilution is transferred to a second tube of 9 c.c. of water, 

 and I c.c. of this admixture to a flask containing 49 c.c. of water. 

 The flask will contain the final dilution of i in 5000. A dilution 

 of I in lO.OCX) will require four tubes of 9 c.c. of water, etc., etc., etc. 



Whatever the dilution may be, the subsequent procedure is 

 the same up to the enumeration of the colonies. For each milk 

 three or four tubes of liquefied gelatine and an equal number of 

 Petri dishes will be required. The tubes of gelatine should be 

 clearly marked i, 2, 3, etc., to correspond with numbers on the 

 covers of the Petris, and each of these latter should be clearly 

 labelled both with the amount of the final dilution in each case, and 

 the exact fraction of this dilution which has been used for plating 

 purposes. Otherwise confusion is likely to occur in the subse- 

 quent estimation of the number of organisms. 



The most convenient fractions will be found to be -05 c.c. (one 

 drop from the calibrated pipette), -i c.c. (two drops), -2 c.c. (four 

 drops), -5 c.c. (ten drops). 



The procedure is as follows: — 



1. Remove the plug of the flask or tube containing the final 

 dilution and aspire into the calibrated dropping pipette a sufficient, 

 quantity of the liquid. 



2. Take the tube of gelatine marked i, from which the attendant 

 will have withdrawn the plug, and drop carefully, without allowing , 

 it to touch the tube walls, one drop from the pipette into the lique- 

 fied medium. 



3. Replace the plug and place the tube in a beaker of warm 

 water. Proceed in the same manner with the tube marked 2, but 

 add two drops of the medium. In the third tube four drops will 

 be required, and in the fourth ten drops. 



I 



