CHAPTER VII 



PROPERTIES OF GASES; THE ATMOSPHERE; ATMOSPHERIC 



PRESSURE 



99. Properties of Gases. Since gases are a form of 

 matter, they have weight and occupy space. Some gases 

 are elements, such as oxygen, hydrogen, and nitrogen. Some 

 are compounds, as ammonia and carbon dioxide. Some are 

 mixtures, as illuminating gas and air. 



Gases have some of the same physical properties as solids 

 and liquids, but in different degrees. The special proper- 

 ties of expansibility and compressibility belong to gases in 

 a greater degree than to solids or liquids. A cubic foot of 

 air on being heated becomes more than a cubic foot, although 

 its weight remains the same. By pressure it can be made to 

 occupy much less than a cubic foot of space, but its weight is 

 still the same. If some gas is removed from a vessel con- 

 taining one cubic foot, the weight of the remaining gas is 

 diminished, but it expands to fill the space and its volume is 

 still one cubic foot. 



100. Molecular Motion. The expansibility of gases is 

 a result of the action of molecules. The molecules of gases, 

 as well as of other bodies, are in very rapid vibration. As 

 they strike one another, they rebound and thus set other 

 particles in more rapid vibration and are separated farther 

 and farther. They strike the walls of the vessel that con- 

 tains the gas, and if the space is enlarged, or an opening 

 is made, the gas expands to fill the larger space or finds its 

 way out. 



101. Solubility of Gases. Many gases will dissolve in 

 water and remain more or less permanently dissolved, ac- 



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