CHAPTER XVIII 

 SOME PROPERTIES OF GASES 



108. Gas Pressure. We have already learned that 

 substances are composed of very minute particles called 

 molecules and that these molecules are in motion. The 

 molecules of water are so far apart that molecules of 

 salt or sugar can occupy the spaces between them when 

 salt and sugar are dissolved in the water. The mole- 

 cules of gases are farther apart than those of water, and 

 they move about faster than the molecules of water. 

 When two gases are mixed, the molecules of the one 

 occupy the spaces between the molecules of the other. 

 Under ordinary conditions the two gases will remain 

 mixed. Air is an example of such a mixture of gases. 

 Since molecules cannot be seen with the best microscopes, 

 it is evident that they must be very minute, and the 

 number of them contained in a cubic centimeter of any 

 substance is enormous. It may be that a thousand 

 molecules laid side by side would not make a speck long 

 enough to be seen with a good microscope. 



That molecules, even in a quiet room, are in contin- 

 uous and quite rapid motion can be proved by recalling 

 some of our experiences. If an ammonia bottle is opened 

 or the gas turned on without lighting it, the odor in a 

 very short time will have become perceptible in all parts 

 of 'the room. This shows that enough of the molecules of 

 the gas to affect the nerves of smell have moved across 

 the room. These molecules, being in motion and travel- 

 ing at high speed, strike against one another and against 



