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CHAPTER III 



OBSERVATIONS OF CHANGE OP TEMPERATURE 



29, Change of Temperature Causes Change of Density. 

 1. A hot body is placed in contact with a cold one. The cold 

 body becomes warmer and the hot one colder. Generally, 

 when bodies of different temperatures are placed together they 

 will be found to assume the same temperature gradually. 



2. Observe the increase in size of a piece of iron, or better, 

 of platinum, when in contact with a hotter body, for example, 

 a gas flame. Counterpoise when cold, and after it has been in 

 contact with the hot body, replace it on the balance. Observe 

 that the balance does not indicate any change in the quantity 

 of matter. Generally, a rise in temperature is accompanied 

 by a decrease in density, i.e., the same quantity of matter fills 

 a larger volume. 



3. Fill a small flask with water, mark on the flask the 

 level of the water, and fill a similar flask with the same 

 volume of mercury, and mark its level likewise. Place the 

 flasks in contact with a warmer body, e.g., place them within 

 a vessel containing hot water heated by a gas flame. Observe 

 that the level of each liquid changes, but to a different extent ; 

 while the quantity of matter in each case may be shown, by 

 the balance, to be unchanged. Both the mercury and the 

 water are raised to the same temperature ; but their densities 

 are not equally diminished. The glass of the flasks will also 

 change in density, but to the same extent for each, so that the 

 apparent expansions give the actual relative expansions. 



The thermal condition or temperature of a body changes 

 by thermal conduction or radiation. Conduction is dependent 

 on material contact, that is, it only takes place when bodies 



D 2 



