MEASUREMENT OF QUANTITY OF MATTER 3 



degree of accuracy should depend upon the object of the com- 

 parison. 



We see that two operations take place in weighing any 

 substance. The first consists in finding out a quantity of 

 matter which will counterpoise the substance, or which will 

 take its place on the pan and counterpoise equally a third 

 body on the other pan. The second operation needed is to find 

 out how many times this quantity of matter contains the 

 standard quantity. When a set of weights is used the second 

 operation consists in reading the marks or numbers on the 

 several weights required for counterpoise. 



When a spring-balance is used the pointer indicates how 

 many times the standard quantity of matter would be required 

 to produce the same elongation of the spring. In this case 

 the counterpoise, or what corresponds, has been made once for 

 all by the maker, and the results marked on the scale. Com- 

 pare a spring-balance with an ordinary one. 



3. To Test the Accuracy of a Set of Weights by the 

 Balance. Counterpoise a 2-gram weight with shot and paper, 

 then replace it by another. If they contain equal quantities 

 of matter the counterpoise will be maintained. Test the two 

 10-gram weights similarly, and other equivalents, such as 

 the two 10-gram weights with the 20-gram. Any excess or 

 deficit may be marked, if the comparison has been made with 

 an exact standard, and the balance is reliable. 



4. To Investigate the Construction of an Accurate Balance. 

 It is advisable at this stage to learn the necessity of great 

 care in the use of such balances as are used in a laboratory. 

 This may be done by taking to pieces very carefully such a 

 balance s the one illustrated (fig. 1). 



1. Note the rest or catch which prevents the knife-edges 

 being worn, by saving them from unnecessary jarring. 



2. Compare the balancing of the beam upon its knife-edge, 

 when the pans have been removed, with that of a strip of wood 

 upon a blunt point. 



3. Note that the knife-edges supporting the pans enable 

 the matter wherever it may happen to be placed in the pan to 



B2 



