n6 



GENERAL SCIENCE 



GLASS NECK 



METAL NECK 



RUBBER 



direct rays of the sun may intensify this increase in volume, 

 and there is great risk of bursting the gas envelope. 



By compressing gases sufficiently 

 it is possible to cause a change in 

 them from the gaseous to a liquid 

 state. When this pressure is re- 

 leased the liquid changes back to 

 the gaseous form. In the change 

 from a liquid to a gas a large 

 amount of heat disappears (be- 

 comes " la tent"). It is used in 

 giving increased velocity to the 

 molecules of the gas. A common 

 application of this fact is in the 

 manufacture of artificial ice 1 . An 

 exhaust pump is employed to con- 

 tinuously remove ammonia gas 

 from pipes containing liquid am- 

 monia so that vaporization goes 

 on in them rapidly. This same 

 pump may also serve as a com- 

 pressor to force the ammonia gas 

 much compressed (perhaps lique- 

 fied) back into the pipes of the 

 cooling tank. The same ammonia 

 PIG. 43. A thermos bottle, is thus used repeatedly to cool 



The air was withdrawn from the sa j t so l u tion (see page 99) in 

 between the double glass walls, 



and the opening then sealed which the pipes are submerged. 

 &?&L%&3 The energy required to run the 

 the bottle. The metal case is pump and compressor is furnished 



for protection only. (Tower, 



Smith & Turton.) ' by an engine. 



1 A discussion of the steps involved usually may be found in texts in 

 Physics in connection with the subject of heat, and in Chemistry in connection 

 with the topic ammonia. 



