THE HEASON WHY. 75 



" O Lord how manifold are thy works, in wisdom hast thou made them all : tha 

 earth is full of thy riches." PSALM civ. % 



285. Why does wood which is "green" hiss and steam 

 when it is burnt ? 



Because it contains a large amount of water, which must be 

 evaporated before combustion can proceed. 



286. What is the effect of this evaporation T 



A great deal of heat is unprofitably expended in driving off the 

 water of the fuel. 



287. Why does poking a fire cause it to ~burn more 

 brightly? 



Because it opens avenues through which the air may enter to 

 supply oxygen. 



288. Why do " blowers" improve the draft of air through, 

 afire ? 



Because, by obstructing the passage of the current of air over 

 the fire, they cause additional air to pass through it, and therefore 

 a greater amount of oxygen is carried to the coals. 



289. What is smoke ? 



Unconsumed particles of coal, rendered volatile by heat, and 

 driven off. 



290. What is soot? 



Carbon in minute particles, driven off with other volatile matters 

 and deposited on the walls of chimnies. 



291. Why do fresh coals increase the quantity of smoke ? 

 Because they contain volatile matters which are easily driven off; 



and because, also, they reduce momentarily the heat, so that those 

 matters that first escape cannot be consumed. 



292. Why do charcoal and coke fires burn clearly and 

 without flame ? 



Because the hydrogen has oeen previously driven off from those 

 substances. 



293. Why is it difficult to light charcoal and coke fires? 



