in COUNTING COLONIES IN PLATES 95 



In order to keep the pipette sterile, after it has been 

 used, push it, through a loosely fitting cotton-wool 

 plug, into a flask containing sterile water, which is 

 kept at boiling point over a gas flame (Fig. 3). 



Each time before using the pipette, of course, it must 

 be allowed to cool. 



Examine the plates from day to day, and count the 

 colonies which appear, in the following manner : 



Cut out a piece of paper, fitting exactly the under 

 surface of the Petri's capsule, and divide it into 

 sixteen equal segments, of 

 which one is painted black 

 (vide Fig. 4). Fix this to 

 the under surface of the 

 capsule, and with a lens 

 or dissecting microscope 

 count the colonies over 

 the black segment. Mul- 

 tiply this number by six- 

 teen, and the result will 



give the number of organisms in *25, '1 cc., or 

 whatever quantity of water was used. 



(b) Roll tubes. 



(1) Prepare gelatine tubes as before. 



(2) Shake them gently, so as to distribute the water and 

 its organisms as uniformly as possible. 



(3) With a tube containing boiling water melt a groove 

 in a block of ice. 



