THE MICROSCOPE AND MICROSCOPIC METHODS 33 



and at autopsy. Each pipette is sterilized and discarded after 



use. 



These pipettes are made by cutting glass tubing of a suitable 

 size, diameter 3 mm. to 9 mm., into pieces from 20 to 40 cm. in 

 length. The cut ends are smoothed in the flame. In the tubes 

 of larger caliber it is well to make a constriction about 5 cm. from 

 each end. Each end is plugged with cotton. The tubes are then 

 sterilized by dry heat. By heating the middle of the tube in 

 a blast lamp or over a large Bunsen flame, the glass may be 

 softened and then drawn out into a capillary of any desired 



FIG. 21. Drawn-out tube pipettes of Pasteur, a, Plugged, sterile tube as 

 kept in stock; b, the same heated at x in blast-lamp and drawn out; then sealed at 

 x; c and d, completed pipettes; e, the same with bulb. (After Novy.) 



length and caliber. This is melted in the middle and severed 

 by the flame, giving two pipettes. When a large capacity is 

 desired a bulb may be blown in the tube between the capillary 

 and the cotton plug. This requires a little practice. The tip 

 of the pipette is finally broken off with aid of a file, sterilized by 

 the flame and the pipette is ready for use. The various steps in 

 the preparation of pipettes are illustrated in the figures (Fig. 21). 



The Hanging-drop. Living bacteria may be studied with 

 the microscope while suspended in some fluid substance. The 

 platinum loop having been heated to a red heat in the flame and 

 having been allowed to cool, a small portion of the culture or 



3 



