46 THE REASON WHY. 



" Sing praises to the Lord, which dwellcth in Zion, declare among the people his' 

 doings." PSALM ix. 



before the fire, the rays of heat fall upon it, and my hand receives 

 the heat through radiation. 



115. What is the Reflection of heat ? 



The reflection of heat is the throwing back of its rays towards 

 the direction whence they came. In a Dutch oven the rays of heat 

 pass from the fire to the oven, and are reflected back again by the 

 bright surf ace of the tin. There is, therefore, considerable economy 

 of heat in ovens, and other cooking utensils constructed upon this 

 plan. 



116. What is the Absorption of heat ? 



The absorption of heat is the taking of it up by the body to which 

 it is transmitted or conducted. Heat was conveyed to my hand by 

 radiation, and taken up by my hand by absorption 



117. What is the Convection of heat ? 



The convection of heat is the transmission of it through a body 

 or a number of bodies, or particles of bodies, by those substances 

 which first received it ; as when hot water rises from the bottom 

 of a kettle and imparts heat to the cold water lying above it. 



CHAPTER VII. 



118. Wliy does not apiece of wood which is burning at one 

 end, feel hot at the other end ? 



Because wood is a bad conductor of heat. 



119. Why is wood a bad conductor of heat ? 



Because the arrangement of the particles of which it is composed 

 does not favour the transmission of caloric. 



120. Why do some articles of clothing feel cold, and others 

 warm ? 



Because some are bad conductors of heat, and do not draw off 

 much of the warmth of our bodies; while others are better con- 

 ductors, and take up a larger portion of our warmth. 



