EXPANSION OF AIE 



115 



exhausted has been removed, close the rubber tube tightly with the 

 Hoffman's screw and weigh again. Unclarnp the Hoffman's screw 

 and allow the air to enter the bottle. The weight should be now the 

 same as at first. Or, instead of weighing a bottle of air, weigh an 

 incandescent light bulb. Make a hole in it with a blowpipe and 

 weigh again. Is the weight now the same as before? 



We have found by the previous experiment that air has 

 weight. With the apparatus used it was impossible to 

 tell exactly the weight of the air extracted or to determine 

 the weight of a definite volume of 

 the air. If we had been 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. 

 12 cu. ft. weigh about 1 Ib. The 

 air extends to so great a height 

 that although very light, the weight 

 of so great a mass of it is consider- 

 able. 



Now that air has been found to 

 have weight, it can be seen why a 

 light body like a balloon will float 

 in it in the same way that a stick 

 will float in water. The weight of 



the air varies with the pressure and temperature, as we 

 shall find later. 



53. Expansion of Air when Heated. Air expands very 

 much when heated, as was seen in Experiment 17. It is 

 found that if air at freezing is heated to the temperature 

 of boiling water, it will expand about -^ of its volume. 

 The force with which air expands is so great that some- 

 times when buildings are on fire and there is no opening 

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



BALLOON. 



The gas in the balloon 

 is lighter than air, so 

 the balloon floats in air 

 as a piece of wood does 

 in water. 



