PIPETTES. 41 



The pipette consists of a bulb about half an inch long, 

 each end of which is drawn out into a narrow tube 

 at least six inches long, tapering gradually to the 

 extremities (fig. 12, a). To make such a pipette take a 

 piece of glass tubing about six inches long and a quarter 

 of an inch wide, and heat it in a luminous gas flame at a 

 point half an inch or so from the centre. Continue the 

 heat until the glass is thoroughly softened over at least 

 half an inch of its length, turning the tube round all the 

 time ; then remove it from the flame and draw the two 

 ends apart with a steady uniform pull, so that the 

 heated portion draws out into a capillary tube several 



o 



FIG. 12. Pipettes. 



inches in length. Repeat the process at a point about 

 half an inch from the tapering end of the larger por- 

 tion of the tube ; heat the bulb and then seal off both 

 ends of the capillary portion before the bulb cools. 



The pipettes are necessarily sterile, having been 

 drawn out of partially melted glass, and they will 

 remain sterile indefinitely. Of course the exterior of 

 the glass will become contaminated, and it should be 

 passed through the flame before use. 



The ends of the tube being sealed up while the bulb 

 contains heated air, it follows that the bulb will contain 

 a partial vacuum on cooling. This fact is made use of 



