PIPETS 19 



Tubing having a caliber of 6 mm., with thin walls, that does not 

 become opaque, brittle, or "run" on heating, and that does not con- 

 tain lead, may be used. The question of alkalinity is also of importance 

 in connection with the tubing. Many of the cheaper grades undergo 

 disintegrative changes, which are accompanied by the setting free of al- 

 kali, especially when the glass is heated. Glass of this kind should be 

 discarded, as it may introduce an element of error into our experiments 

 and observations. 



2. A convenient length of tubing about 10 to 12 inches is chosen; 

 this will make two pipets. If a sufficient length of tubing for both 

 sides is not available, one end may be heated and drawn out with 

 forceps, or a handle may be added by fusing to this short end an odd 

 piece of glass. 



It is convenient to have on hand a supply of tubes cut to correct 

 lengths, plugged at each end with a ball of cotton, and sterilized in a 

 hot-air sterilizer. They are then ready to be drawn out as needed, 

 thus furnishing sterile pipets with cotton plugs that tend to prevent 

 contamination. 



3. The flame must be so regulated as to play upon only so much of 

 the tube as will suffice to furnish the glass required for drawing out the 

 tubing. If a Bunsen flame is used, the tip of the inner greenish flame 

 should be applied. The margins of the flame are the hottest, and for 

 this reason the tube must be shifted from side to side and be constantly 

 rotated. 



4. In order to secure uniform heating and satisfactory pipets the 

 tube must be kept constantly rotated from the moment it enters 

 until it leaves the flame. The two ends of the tube are to rest upon the 

 middle finger of each hand while the thumb and forefinger hold the 

 tube in position at either side and impart the rotatory movement. It 

 is also necessary that the tube be displaced laterally from time to time, 

 so as to bring each portion of the middle segment of the tube in turn into 

 the edge of the flame (Fig. 2). If the latter precaution is omitted, we 

 shall obtain a pipet with a central bulb or thicker segment and with 

 thinner segments on each side corresponding to the portions of the tube 

 which lie in the edges or hottest portion of the flame. 



5. The tube is heated in this manner until the glass is quite plastic. 

 No attempt is made to draw out the tube until it has been entirely 

 withdrawn from the flame, as otherwise a portion becomes unduly thin 

 and plastic and divides, leaving a small, bent, and very poor pipet in 

 each hand. 



