58 



Domestic Science 



Fig. 21. 



forefinger over the open end (see Fig. 21). Leaving 

 the jet of the pipette just below the 

 surface of the water, slightly release 

 the pressure of the finger, thus allowing 

 the water-level to sink slowly to the 

 graduation mark. When the lowest 

 point of the meniscus touches the mark, 

 press the finger firmly on the opening 

 at the upper end of the pipette, raise 

 the latter from the water, and insert its 

 lower end in the counterpoised beaker. 

 Remove the finger and the water will 

 run into the beaker. Do not blow out 

 the last drop but allow the pipette to 

 drain in a vertical position with its jet 

 against the wet side of the beaker for 15 seconds. 

 Place the pipette on the bench and transfer the beaker 

 and its contents to the left-hand scale -pan. Put a 10 g. 

 weight on the right-hand pan and raise the arrestment. 

 This weight should be found to bring about an almost 

 exact balance. Should it not do so, find the weight 

 required to effect a balance. If it be found to vary 

 from 10 g. by more than *03 g., the pipette is probably 

 improperly graduated. Measuring- vessels for liquids 

 are usually graduated to deliver the volume marked 

 on them when the contained liquid is at a temperature 

 of either 15 C. or 60 F. 10 c.c. of water at this 

 temperature weigh slightly less than 10 g. to be exact 

 9*99125 g. The type of balance supposed to be used 

 will not show, such small differences of weight as '00125 g. 

 so that, for our purposes, the weight of the water should 

 be 9*99 g. Since, however, we are not using a very 

 expensive balance nor very accurately adjusted weights, 

 a pipette which delivers a quantity of water at 15 C., 



