PIPETS 



21 



microbes that have been mixed with the serum have or have not been 

 killed by it; (c) the glass loop, which acts as a trap, preventing ex- 

 traneous contamination; and (d) the handle, upon which the rubber 

 teat can be fitted. With a little practice these pipets are readily made. 



1. Select glass tubing about 6 inches in length. 



2. Heat one portion about two inches from the end, and when 

 sufficiently plastic, draw it out for about two or three inches or until it 

 is long enough to give a spiral loop of the desired dimensions (Fig. 3). 

 While the glass is still plastic hold the left hand steady, and with the 

 right hand lower the tubing and make a spiral loop in such manner 

 that the loop is closely applied, but does not touch the sides of the upper 

 and lower segments of the tube. Actual contact with the sides must be 



FIG. 3. METHOD OF MAKING A LOOPED PIPET. 



avoided, for this would produce strain and predispose the tube to 

 fracture. 



3. The longer portion of tubing is now heated and drawn out to a 

 capillary pipet and broken through at the desired point. 



4. Instead of this method, the capillary portion may be drawn be- 

 fore the loop is made. 



3. Graduated Pipets. 1. In this work 1 c.c. pipets graduated into 

 riir c.c. ; 5 and 10 c.c. pipets graduated into yVc.c. will render satisfactory 

 service. The pipets should be calibrated close to the tip, and should 

 preferably be long, with a narrow lumen, rather than short, with a wide 

 lumen, as the latter renders the markings too close to one another. For 

 pipeting small amounts, as in certain complement-fixation tests, a 0.2 c.c. 

 pipet graduated to TTJF c.c. will be found quite serviceable, permitting 



