APPENDIX 151 



PROPERTIES OF GASES 



The most striking characteristic of all gases is their 

 tendency to expand indefinitely in. all directions, espe- 

 cially when heated, and to mix with other gases by a 

 process similar to the diffusion of liquids. The opposite 

 character, extreme compressibility, is equally striking. 

 Indeed, under sufficient pressure accompanied by a very 

 lo\y temperature, gases may be compressed until they 

 become liquid. This tendency to expansion permits 

 ventilation by the simple process of opening the window. 

 The warmer gas tends to expand and rise to the window 

 while the colder gas (air) from the outside tends to 

 sink to the floor. There are thus two currents of gas 

 at all times, one entering and one leaving through the 

 same opening. The atmospheric air is composed of 

 gases which, however light, still have weight sufficient 

 to exert a pressure of about fifteen pounds to the square 

 inch, or to sustain a column of mercury of about thirty 

 inches in height. It is upon this atmospheric pressure 

 that the barometer is based. 



HEAT, LIGHT, AND ELECTRICITY 



Heat is a form of m.otion. Not only is this true, but 

 heat may be converted into motion or motion into heat. 

 All matter, even the densest, is supposed to contain 

 minute spaces between the molecules of which it is com- 

 posed, which spaces are filled with an elastic and im- 

 ponderable ether which permits absolute freedom of 

 vibratory movement of the particles. If the movement 

 is at a rapid rate, the heat will be great; if at a lower 

 velocity, the heat will be less, but heat is always present, 

 even in ice. This is called the undulatory theory of heat. 



The most striking constant action of heat is to cause 



