J.V POPULAR PHYSICS. 55 



In a bright one, since a polished surface retards radiation. 



38. JJJiat causes the snapping of wood when laid on 

 the fire? 



The expansion of the air in the cells of the wood. 



39. Why is one's breath visible on a cold day ? 



The vapor in the breath is condensed by the cold air. 



40. What gives tlie blue color to air ? 



The particles floating in it reflect the blue light of the sun- 

 beam. 



4:1. How does the heat at two feet from the fire compare 

 with that at four feet? 



2* ; 4 2 ; : 1 : 4. 

 Hence it is four times greater. 



4=2. Why does the frost remain later in the morning 

 upon some objects than upon others ? 



The best radiators are the best absorbers. They become 

 warmed by the morning sun more quickly than the poorer 

 radiators, and the frost on them is hence more quickly melted. 



43. Is it economy to use green ivood ? 



No. Its sap has to be changed to vapor, thus absorbing a 

 large amount of energy at the expense of the combustion. 



44. Why does not green wood snap ? 



The pores are filled with water instead of air. The water 

 does not expand rapidly enough to burst off the coverings of 

 the cells, and so simply oozes out gradually, and is vaporized. 



45. Wliy will a piece of metal dropped into a glass or 

 porcelain dish of boiling ivater facilitate the ebullition ? 



The rougher surface of the metal aids in the formation 

 and disentanglement of the steam-bubbles. The bubbles cling 

 longer to a smooth than to a rough surface. This is one 

 cause of that bumping sound often noticed when liquids are 

 boiling in glass dishes. 



