CHAPTER XIV. 



SPECIAL METHODS OF WORK. 



Pipettes. 



Sterile, drawn-out tube pipettes, as employed by the 

 Pasteur school, are invaluable in bacteriological work. 

 They can be readily prepared and a good supply should be 

 kept on hand. The method of preparing pipettes is as 

 follows : 



Glass-tubing having a diameter of about 8 mm. is cut 

 up into lengths of about 35 cm. (14 in.). A slight constric- 

 tion is made at a distance of about 6 cm. (2^ in.) from e'ach 

 end. This is not necessary, but it serves to prevent the 

 cotton plug from descending. Moreover, the tube can be 

 readily sealed at this place whenever it is desirable to do 

 so. The ends are then rounded in the blast-lamp. A piece 

 of cotton is pushed into each end of the tube by means of a 

 drawn-out piece of glass tubing. The tubes, thus equipped 

 (Fig. 61 a) are placed in a horizontal position in a dry-heat 

 oven and sterilized (p. 160). The sterilized tubes should be 

 kept in stock and from these the pipettes can be made in a 

 few minutes whenever desired. 



To make the pipette proper, the plugged sterile tube 

 (a) is heated at the middle in a blast-lamp. The broad 

 flame should be used in order to soften as long a piece of 

 the tubing as possible (3-5 cm.). When the middle is thor- 

 oughly softened the two halves should be slowly drawn 

 apart. A relatively wide, thick-walled capillary is thus 

 obtained; whereas, if the tube is drawn out rapidly the re- 

 sulting capillary will be very narrow and thin walled. The 



