PROPERTIES OF GASES 93 



denser. Here, by means of a cold water sprinkler, the gas is 

 cooled to 45 or 50 F. and is condensed under high pressure to a 

 liquid state. It then passes to the storage tank. 



When ammonia is received ready for use, it is in a liquid state 

 and enclosed in steel drums, which are only partly filled, leaving 

 space enough for expansion so as to prevent an explosion. Am- 

 monia drums have exploded, but always under conditions of over- 

 heating, for in general, with proper care, there is no danger. 



95. Air Pumps. It is often desirable to force air into 

 or remove it from a vessel. Air is forced into a vessel by 

 machines called air pumps, air compressors, condensing 

 pumps, and blowing engines or blowers. The air pump con- 

 sists of a tube or pipe with a rim, ground smooth and flat, 

 extending from the cylinders. 



Notice the tire of an automobile as air is pumped into it. 

 As the air enters, the tube expands, due to the pressure of 

 the gas, until finally the pressure becomes great enough to 

 support the weight of the automobile. To remove air from 

 a vessel, a screw connection is fitted tightly to it. As the 

 piston is drawn up a partial vacuum is caused by the pres- 

 sure of the air underneath, so that the air from the vessel 

 immediately rushes to the cylinder, forcing the valve upward. 

 This continues until the air pressure is reduced to such an 

 extent that it is unable to force the valve of the cylinder 

 open. 



96. Boyle's Law. When the outside temperature is the 

 same as that of the air within a vessel, the product of the pres- 

 sure and volume is constant. This is called Boyle's Law. To 

 illustrate: If the volume of a gas is 2 cu. ft. at a pressure of 1 

 atmosphere (15 Ibs.), then the volume would be decreased 

 one-half as the pressure is increased twofold. Boyle's Law is 



