AIR AS AFFECTED BY HEAT AND COLD 109 



able to do this, we should have found that on an average day, 

 at sea level, the weight of a liter, a little more than a quart, 

 of air, is about 1.2 grams. Twelve cu. ft. weigh about one 

 pound. The air extends to so great a height that although 

 very light, the weight of so great a mass of it is enormous. 



Expansion of Air when Heated. Air expands very 

 much when heated, as was seen in Experiment 18. It is 

 found that if air at freezing is heated to the temperature 

 of boiling water, it will expand about T V of its volume. The 

 force with which air expands is so great that sometimes 

 when buildings are on fire and there is no opening for the 

 confined air to escape, the walls are blown out or the roof 

 blown off by the expansion of the hot air, and great injury 

 is done to those fighting the fire. That air expands upon 

 being heated is readily seen when an air-filled toy balloon 

 is brought frorn the cold outer air 

 into a hot room, the covering 

 begins at once to tighten and the 

 balloon to swell. 



Weight of Air as Affected by 

 Heat and Cold. Experiment 42. FIGURE 55 



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 currents that are generated 

 will have no upward push on the flask. (Figure 55.) Do not 

 allow the hot air to get under the 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, hot air is lighter than cold air. A liter of 

 air at freezing under ordinary pressure weighs about 1.293 



