148 GENERAL SCIENCE 



its weight to the tenth of a gram. Now pump as much 

 air into the bladder as it will hold without bursting and 

 weigh it again. When the bladder is filled almost to 

 bursting capacity, it will weigh about half a gram more 

 than when it is merely full. The increase in weight is 

 due to the weight of the extra air forced into the bladder. 

 A basket-ball with enough air in it to be used in a game 

 weighs about three grams more than it does when it is 

 empty. The air in the ball is more dense than the air 

 outside. The air that is forced into the ball to make 

 it more solid adds to the weight of the ball. 



To find the weight of a given volume of air, try the 

 following experiment. Take a glass bottle or other 

 vessel that can be closed air tight, determine its volume 

 and weigh it to a hundredth of a gram. Now attach it 

 to the exhaust air pump and remove as much air as 

 possible and then weigh it again. The difference in the 

 two weights is the weight of the air pumped out. Divide 

 the weight by the number of liters the vessel holds and 

 you will have the weight of the air per liter. A liter of 

 air at standard pressure and at o C. weighs 1.29 grams. 

 Twelve cubic feet of air weigh a pound. Schoolrooms 

 contain from 600 to 1000 pounds of air. Find how 

 much your schoolroom holds. 



It is evident from these experiments that air can be 

 put into and taken out of a vessel. It can be weighed 

 and handled like a liquid or solid. The air is a mixture 

 of gases, it is matter, it has weight, and it is held on the 

 earth like any other substance. It is drawn toward the 

 center of the earth by the force of gravity. 



102. Air Pressure. Pressure is force per unit area. 

 The areas most commonly used are the square inch and 

 the square centimeter. 



