60 A MANUAL OF BACTERIOLOGY 



first mark. The limit of the upper end of the mercury 

 column is again marked, giving a second volume, and by 

 repeating the process the stem of the pipette may be 

 graduated into any number of equal volumes. With such 

 a pipette it is not easy without considerable practice to 

 regulate the flow of the contained fluid with a rubber 

 teat. By adopting the method of " throttling," however, 

 any degree of control may be obtained. In order to 

 throttle, a short length of glass tubing is taken of a 

 diameter small enough to slip into the end of the undrawn 

 extremity of the pipette. This piece of tubing is then 

 drawn out in the flame and cut off. The fine extremity 



FIG. 8. " Throttled " pipette. The upper figure shows the fine point 

 of the " throttle." 



is then again drawn out in a by -pass flame and cut off so as 

 to get a short length of very fine bore. This piece is 

 then sealed into the pipette by means of sealing-wax. 

 Fig. 8 shows such a throttled pipette and explains its 

 construction. 



A less fragile " throttle " may be made as follows : 

 After the glass tubing has been drawn out and cut off, 

 the small aperture of the throttle piece is gradually 

 reduced in size by careful heating in a small flame until 

 only the merest pore remains. A little practice is neces- 

 sary in order to obtain just the right throttle. 



The pipette is used with a rubber teat as shown in the 

 figure. In using, care must be taken to avoid wetting 

 the fine point of the throttle, for this prevents it from 



