116 FIRST TEAR SCIENCE 



the roof blown off by the expansion of the hot air, and 

 great injury done to those fighting the fire. That air ex- 

 pands upon being heated is readily seen when a toy balloon 

 is brought from the cold outer air into a hot room, the 

 covering begins at once to tighten and the balloon to swell. 



54. Weight of Air as Affected by Heat and Cold. Experi- 

 ment 61. Take two open flasks of nearly the same weight and 

 capacity and balance in as nearly a vertical position as possible at the 



ends of the arms of a beam balance. 

 Bring the flame of a Bunsen burner 

 to the upper side of the bulb of one 

 of the flasks so that the hot air cur- 

 rents that are generated will have no 

 upward push on the flask. Do not 

 allow the hot air to get under the 

 Fig. 52. flask. What is the effect? 



As the previous experiment shows, and as we should 

 expect from the fact that air has been found to expand 

 when heated, it follows that hot air is lighter than cold 

 air. A liter of air at freezing under ordinary pressure 

 weighs about 1.293 grams, but at the temperature of boil- 

 ing water it weighs only about .946 grams. So a mass of 

 cold air, being heavier, will exert more pressure at the 

 surface of the earth than a mass of hot air. 



As air is a gas whose particles can move freely among 

 themselves we should expect that a heavier column of cold 

 air would sink down and distribute itself along the surface 

 under surrounding lighter air just as a column of water 

 falls when its supports are withdrawn and forces up the 

 lighter air which surrounds it. 



A similar action is seen when water is poured upon oil; 

 the water sinks to the bottom and forces the oil to rise. 

 Thus if air is heated at an} r place, we should expect that 

 there would be a rising current of hot air and a current 



