Measurement of Weight 57 



in sq. in. or sq. cm. may be obtained with a fair degree 

 of exactness by dividing the weight of the figure by 

 the weight of unit area as previously determined. 

 Compare this with the area as found for the same 

 figure by the method of counting the squares described 

 in Experiment 7. 



Exercise for Student. 



What must be assumed with respect to the nature of the millboard 

 or zinc used for carrying out this experiment ? 



32. The accuracy of graduation of the pipettes, 

 burettes, or other measuring-vessels used for the deter- 

 mination of liquid volumes may be tested by means of 

 a good balance. 



EXPERIMENT 17. Place a light glass vessel, such 

 as a beaker, on the left-hand pan of the balance. On 

 the other pan place a similar vessel. If they do not 

 exactly balance one another, gradually add small leaden 

 shot, or clean dry sand, to the pan on which the lighter 

 beaker stands until an exact balance is obtained. This 

 operation is known as " counterpoising " and is 

 generally resorted to when the actual weight of a 

 containing vessel or other body is not required to be 

 definitely known for the purposes of an experiment. 

 The second beaker may be dispensed with, if desired, 

 and dry sand or shot alone used in effecting the counter- 

 poise. Remove the first beaker from the pan to the 

 working-bench. Insert the jet of a 10 c.c. pipette 

 into a quantity of distilled water in another vessel, 

 place the mouth at the upper end of the pipette, 

 and suck gently till the water has risen above the 

 graduation mark on the stem of the instrument. 

 Remove the end from the mouth and quickly place the 



