2 t ELEMENTS OF LABORATORY WORK 



of matter /however unequal in size or different in appearance 



" 06\mterf)6isin brie quantity of matter by another will 

 indicate equal quantities of matter only when the instrument 

 used is correct. But by performing two operations as in (2) 

 and (3) we find two quantities of matter which are counter- 

 poised under precisely similar circumstances with a third un- 

 changed quantity of matter (the piece of lead kept in the 

 same pan). There is nothing changed in these two operations 

 except the pieces of wood. Although the balance used may 

 be inaccurate, the same inaccuracy holds for each case, and 

 thus we can make sure that two bodies are equal quantities of 

 matter even with an inaccurate balance. 



2. To Compare Two Quantities of Matter. 1. Take several 

 equal quantities of matter, and find how many of thm counter- 

 poise with a given piece of wood, cutting away the wood if 

 necessary. It is convenient to use a set of weights that is, 

 a number of bodies so arranged and measured that we can 

 readily make up from them a quantity of matter which shall 

 contain the smallest quantity any required number of times. 

 The need of a large number of equal quantities is thus avoided. 

 Use grams. One gram will be the standard. 



2. See how many of the same standard quantities of matter 

 are equal to a larger piece of wood, cutting away as before if 

 necessary. 



3. Compare similarly two other pieces of wood, but use 

 much smaller standards. Notice that less cutting away, if 

 any, is needed. Use centigrams. A centigram is one-hun- 

 dredth of a gram. With milligrams or thousandths of a gram 

 the comparison becomes still more accurate. 



The above exercises show : 



1. That two quantities of matter can be compared, by 

 seeing how many times each contains a standard quantity. 



2. That the standard quantity, if large, does not enable 

 us to measure exactly ; but the smaller the standard, the more 

 exactly can we measure and compare. 



3. That the limit of exactness can never be attained, as 

 inequalities will be shown by more delicate balances. The 



