236 CONVECTION. 



A. If you look at the spout of a 

 boiling kettle, you will find that the 

 steam (which issues from the spout) is 

 always invisible for about half an inch ; 

 after which, it becomes visible. 



Q. WHY is the steam INVISIBLE for only 

 HALF AN INCH, and not either all INVISIBLE or all 

 VISIBLE ? 



A. The air is not able to condense 

 the steam as it first issues from the 

 spout, but when it spreads and comes in 

 contact with a larger volume of air, the 

 invisible steam is readily condensed into 

 visible drops. 



Q. Why is our BREATH YISIBLE in winter- 

 time? 



A. Because it is condensed by the 

 cold air into small drops, which are 

 visible to the eye. 



Q,. Why do STEAM-ENGINES sometimes BUEST? 



A. Steam is very elastic ; and this 

 elasticity increases in a greater propor- 

 tion than the heat which produces it; 

 unless, therefore, some vent be 

 freely allowed, the steam heaves and 

 swells, till it bursts the vessel which con- 

 fined it. 



