CHAPTER VII 

 DENSITY AND SPECIFIC GRAVITY 



41. Weight. It is a matter of common observation that all 

 objects of the same size are not equal in weight. A piece of 

 iron is much heavier than a piece of cork of the same volume. 

 We explain this by saying that iron is denser than cork; that 

 is, that a given volume of iron has more matter in it than an 

 equal volume of cork. Since heating a substance causes its 

 molecules to move farther apart, we infer that a given volume 

 of a substance in the gaseous state will weigh less than the 

 same volume of it in the solid form, and this is found to be 

 exactly the case. It weighs less because there are fewer mole- 

 cules of it in a given volume. This is the reason warm air 

 rises. Heating it causes it to be less dense and therefore 

 lighter, and it is then pushed up by the surrounding cooler 

 and heavier air. We must not confuse density and weight, 

 however. The mass of a body is the amount of matter in it 

 and this depends upon its density, but the weight of a body is 

 simply the pull of the earth upon it, which, owing to the shape 

 of the earth, may differ in different places. For instance, an 

 object weighing 589 pounds at the equator would weigh 590 

 pounds at the poles. A body weighing 100 pounds on the 

 earth would weigh nearly 1)^ tons on the sun. 



42. How Density is Measured. In order to measure a 

 thing, we must have a standard with which to compare it. 

 The standard for measuring the density of gases is usually the 

 air at the temperature of melting ice. For measuring the 

 density of solids and liquids, water is taken as a standard. 

 This latter makes a most convenient standard since it has been 



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