40 THE EEASON WHY. 



' While the earth remaineth, seed-time and harvest, and cold and heat, and 

 summer and winter, and day and night, shall not cease." GEN. viu. 



73. What is the source of caloric ? 



The sun is its chief source. But caloric, in some degree, exists in 

 every known substance. 



74. What are the effects of caloric ? 



Heat which, in proportion to its intensity, acts variously upon all 

 bodies, causing expansion, fusion, evaporation, decomposition, 8fc. 



75. Why is caloric called a repulsive agent ? 



Because its chief effects are to expand, fuse, evaporate, or de- 

 compose the substances upon which it acts. 



76. What is an attractive agent, in contradistinction to a 

 repulsive agent ? 



Chemical attraction, or affinity, is an attractive agent as when 

 bodies seek of their own natures to unite and form some new body. 



77. When is a body said to be hot ? 



When it holds so much caloric that it diffuses heat to surround- 

 ing objects. 



78. When is a body said to be cold? 



When it holds less caloric than surrounding objects, and absorbs 

 heat from them. 



79. How may caloric be excited to develope heat ? 



By any means which cause agitation, or produce an active change 

 in the condition of bodies. Thus friction, percussion, sudden con- 

 densation or expansion, chemical combination, and electrical dis- 

 charges, all develope heat. 



80. Why do " burning glasses" appear to set fire to com- 

 bustible substances ? 



Because they gather into one point, or focus, several rays of 

 caloric as they are travelling from the sun, and the accumulation of 

 caloric developes that intensity of heat which constitutes jre. 



81. IVhat is a focus ? ' 



In optics, it is the point or centre at which, .or around which, 

 divergent rays are brought into the closest possible union. 



