SOME OF PROPERTIES GASES 171 



ammonia. It requires about 36 hours for the 300 pounds 

 of water to freeze into a solid piece of ice. 



114. Liquid Air. Steam is a gas and can be liquefied 

 by subjecting it to great pressure or by cooling it. Am- 

 monia (NH 3 ) under ordinary conditions is a gas, and it 

 can be liquefied by cooling or by adding pressure. It 

 can be more easily liquefied by both cooling and in- 

 creasing the pressure. By cooling a gas the molecules 

 become less active and by using pressure they can be 

 forced closer together. If the cooling and increase of 

 pressure are continued far enough, the molecules become 

 so inactive and so close together that the force of attrac- 

 tion which they have for one another overpowers their 

 force of free motion. When molecules are in this condi- 

 tion we have what is usually called a liquid. (In some 

 cases it would be a solid.) Then, in order to make a 

 liquid of any gas all that needs to be done is to lower 

 the temperature of the gas and increase the pressure on 

 it. Air, oxygen, and hydrogen can be liquefied in this 

 way. Under average air pressure, liquid air will boil on 

 ice. In 1900 a temperature of -260 C., or -436 F., 

 was produced by Professor James Dewar, by evaporat- 

 ing liquid hydrogen in a partial vacuum. 



115. Natural and Artificial Gas. During the millions 

 of years while the earth's present crust was in process of 

 formation, great quantities of plant and animal matter 

 were buried beneath the surface. We now remove much 

 of this material in the form of coal, petroleum, and gas. 

 This gas is taken out of the earth by drilling wells, from 

 which it is allowed to flow through pipes to the storage 

 tanks of cities. From these tanks the gas is piped to 

 houses for heating, cooking, and lighting. 



Artificial gas is made by heating wood or coal in ovens 



