PROPERTIES OF MATTER: GRAVITATION 21 



will be the action of gravity upon it. Thus by measur- 

 ing the force with which gravity pulls a body we can 

 judge of the amount of matter in it. 



The weight of any body is the measure of the force 

 with which gravity pulls it. This may be found by 

 holding the body suspended by some A E 



known force. Fig. 8 shows a common 

 spring balance ; anything hung upon the 

 hook will be pulled downward until the 

 stretched spring exerts as much force 

 upon it as does gravity; there it will 

 stop, and the pointer will show this force 

 in pounds or ounces, etc. 



31. Specific Gravity. Of two pieces 

 of lead, the larger weighs more ; but a 

 piece of lead may weigh more than a 

 much larger piece of wood. That is, some 

 forms of matter are naturally heavier than others. In 

 order to compare the weights of different forms of matter, 

 we must weigh equal amounts of volume (size) of the dif- 

 ferent substances. To express these comparisons easily, 

 the weight of each substance is referred to that of water 

 as a standard. For example, a piece of iron is found to 

 weigh seven times as much as an equal volume of water, 

 a piece of lead eleven times as much, a piece of gold 

 nineteen times as much, and so on. A list is then made, 

 each substance being named and followed by the num- 

 ber showing how many times the substance is heavier 

 than water, and the number is called the specific gravity 

 of the substance. 



FIG. 8 



