APPENDIX A 



PIPETTES 



Preparation of pipettes. The quantitative bacteriological analyses 

 of milk and other dairy products involves the use of a large amount 

 of glassware, especially pipettes. One-cc. pipettes, suitable for use 

 in quantitative work, can be easily prepared. Glass tubing of 

 4 mm. internal diameter is cut into pieces 20 inches long. The piece 

 is heated in the center and drawn out 

 so as to form the tips of the pipettes. 

 The pieces are then cut in two, and 

 both ends slightly fused in the flame 

 to remove the sharp edges. The pi- 

 pettes are rendered perfectly free from 

 grease and dirt by boiling them in a 

 solution of washing powder, rinsing 

 well with distilled water, and heating 

 in a hot-air sterilizer for an hour at 

 175-200 C., or by washing in alcohol 

 and then in ether. 



A Schuster dropping bottle is thor- 

 oughly cleaned and dried. The top of 

 the bottle should be drawn out so it 

 can be easily introduced within the 

 lumen of the pipette, and the hole in 

 the bottle made very small. In the 

 bottle are placed 13.6 grams of perfectly clean*, dry mercury. The 

 upper part of each pipette is coated with a thin coat of paraffin by 

 warming the glass and rubbing it on a piece of paraffin. The pipette 

 is held between the first two fingers, the tip resting on the ball of 

 the thumb. The mercury is poured into the pipette slowly so as to 

 allow the air to escape. A very small bubble of air will usually be 

 retained in the tip, but this may be neglected. The upper level of 

 the mercury which indicates a volume of one cc. is marked on the 

 paraffin by a pin scratch. The mercury is returned to the dropping 



125 



FIG. 19. SCHUSTER DROPPING 

 BOTTLE 



The top of the bottle is drawn so 



as to form a capillary tube from 



which the mercury will run 



slowly 



