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, 4). 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 
c 
Fic. 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 
