Heat 83 



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These examples illustrate the transformation of energy. In 

 each case the energy of motion is changed into heat energy. 



In early times heat was called "caloric" and was regarded as 

 a fluid. No one knew just what caloric was, but it formed a 

 hypothesis to explain a change in temperature. Scientists be- 

 lieved that when caloric flowed into a body it became warmer, 

 although it did not gain in weight, and when it flowed out the 

 body became cooler. This hypothesis that heat is a fluid has 

 been replaced by the theory that heat is energy resulting from the 

 motion of molecules. 



The sun gives off an enormous amount of heat of which the 

 earth receives about one two-billionth part. Scientists have dis- 

 covered that the exterior of the sun is made up of extremely 

 hot gases, and they believe that in its interior these gases are very 

 highly condensed. They are, however, unable to account for the 

 origin of this heat. 



Fire. Everyone knows that fire will produce heat. No one 

 knows when fire was discovered. The Indians produced it just 

 as the boy scouts do today, by rubbing together two sticks of 

 dry wood, thus raising the temperature of the wood to the tem- 

 perature at which it takes fire. Our forefathers produced fire 

 by striking a piece of flint with steel and then causing light 

 substances to catch the spark and burst into flame. When the 

 oxygen of the air unites with substances it causes oxidation, or 

 burning. This oxidation is sometimes very slow, as in the rust- 

 ing of iron. If, however, the oxidation is rapid enough to give off 

 heat and light, as when paper burns, it is known as combustion. 



It is less than one hundred years ago that the friction 

 match was invented. This method of starting a fire was an 

 advance over the use of flint and steel. A short, slender piece 

 of pine wood was dipped in oil and then covered on one end 

 with a mixture of sulphur, phosphorus and glue. When this 

 end was scratched on a piece of sandpaper or on some other rough 

 object, the friction caused sufficient heat to make the phosphorus 

 ignite. The phosphorus, as it burned, set fire to the sulphur and 

 then to the oily wood part of the match. It then was very easy to 

 start a fire in other material by applying the burning match. 



