66 METHODS OF CULTIVATION OF BACTERIA 



dish is then laid on the card, the number of colonies in a 

 few of the sectors counted, and an average struck as before. 

 In counting colonies it is always best to aid the eye with a small 

 hand-lens. 



Method of counting Living Bacteria in a Culture. This 

 is accomplished by putting into practice a dilution method 



such as that described on p. 52. 

 Measured amounts of high dilutions 

 are plated, and the numbers of 

 colonies which subsequently develop 

 are counted. In applying such a 

 method it is necessary to have pipettes capable 

 of measuring small quantities of fluid. Those 

 discharging -05 and '1 c.c. will be found con- 

 venient, and such pipettes can have sub- 

 divisions which enable them to be used for 

 measuring still smaller fractions of a cubic 

 centimetre. Pipettes of this kind can be 

 obtained at the instrument makers. Wright 

 has described a method by which a pipette 

 (Fig. 30) for measuring small quantities of 

 fluid can be made from ordinary quill tubing. 

 The method is as follows : A piece of quill 

 tubing about 15 cm. long is drawn out to a 

 capillary stem. A standard 5 c.mm. pipette 

 (such as that of the Gower's hsemocytometer), 

 or the pipette described later on p. 108, is 

 filled with mercury and the metal transferred 

 to the capillary stem and run down to near 

 its extremity ; the upper and lower limits of 

 the mercury are marked with an oil pencil ; 



Fl o^ 3 '~ Wrigh A' s the mercury is then displaced up the tube 

 250 c.mm. pipette ,. ., i T , i i i 



fitted with nipple. " lts previously distal end is at the proximal 

 of the two marks, and a third mark is made 

 at the new position of the upper end of the droplet ; the mani- 

 pulation is repeated three more times, and finally the tip of 

 the tube beyond the lowest mark is broken off. Thus on the 

 capillary part of the pipette we have five divisions, each cap- 

 able of holding 5 c.mm. of fluid. The rest of the pipette is now 

 calibrated- so as to determine that part capable of containing 

 225 c.mm. and 250 c.mm. This is done by placing a rubber 

 nipple on the wide end of the pipette and sucking up some 

 water tinted with, say, methylene-blue till the 25 c.mm. mark 

 is reached ; a small air-bubble is then allowed to enter the 



250 c. mm, 



225 



20 



15 



10 



2-5 



