68 CULTIVATION OF AEROBIC MICRO-ORGANISMS 



3. Round off the cut ends in the blow-pipe. 



4. Plug the two ends of each piece with cotton-wool, which should pass 

 some distance into the tubing and should also project a few millimetres from 

 the open end (fig. 54). This is conveniently done by gently pressing the 

 wool in with some blunt-pointed instrument (the thin end of a three-cornered 

 file will do very well). 



5. Hold the middle of the tube in a slightly inclined position and heat it 

 in a medium-sized blow-pipe flame, turning it round and round between 

 the thumb and index fingers until the glass is soft. Withdraw the tube from 

 the flame and draw it out quickly into a fine tube about 30 cm. long 

 (fig. 54, A). Divide it into two in the middle by melting it in the tip of 

 the flame. This will give two pipettes with the capillary end of each 

 sealed. 



A certain amount of skill is required to make these pipettes. Care should be 

 taken that the tube is drawn out straight, and this can best be done by the operator 

 resting his elbows on the table. The tube should always be taken out of the flame 

 before attempting to draw it, and it should be held horizontally while being drawn. 

 The tube should not be drawn too fine, otherwise it will be too fragile for use. 



6. Place the pipettes with their plugged ends downwards in a wire basket 





FIG. 54 The method of making pipettes. 



[or copper cylinder, fig. 1], and sterilize them at 180 C. in the hot air 

 sterilizer. They are then ready for use. 



Method of using a Pasteur pipette. 1. Break off the fine sealed end of the 

 pipette with a pair of dissecting forceps or between the thumb nail and the 

 pulp of the index finger [it is better to make a light scratch with a carburundum 

 pencil before breaking off the point]. 



2. Pass the broken end through the flame of a Bunsen burner or spirit 

 lamp to destroy any organisms which may happen to have been deposited 

 on the surface. 



3. Dip the sterile end into the fluid which is to be used for sowing the 

 medium. The fluid will rise in the tube by capillarity, or it can be aspirated 

 by slightly withdrawing the wool in the other end of the tube. [A con- 

 venient practice consists in slipping an india-rubber teat over the wool- 

 plugged end. By pressing on the teat, air is expelled : if the capillary end 

 be now dipped in the fluid and the pressure on the teat lightly relaxed, as 

 much or as little of the fluid as is required can be drawn up into the tube.] 



In doing this, care must be taken that the aspirated liquid does not soil the 

 wool plug ; and it is necessary also to watch that bubbles of air are not drawn in. 



4. Transfer the fine end of the pipette as rapidly as possible to the medium 

 to be sown, and let one or more drops of the fluid fall on to the medium, 

 either by its own weight or by blowing gently through the other end [or 

 by compressing the teat.] 



5. The aspirated fluid can be kept free from contamination for an inde- 

 finite time by sealing the end of the pipette. Thus, tilt the pipette gently 

 so that the fluid runs up the tube ; heat the point in a small flame (the pilot 

 flame of a Bunsen), and when the glass is soft, draw it out with a pair of 

 forceps, and the tube is completely closed. 



