54 THE WONDERFUL CENTURY. 



The molecular theory of gases. 



The very remarkable properties of gases, their ap- 

 parently unlimited elasticity and indefinite powers of 

 expansion, were very difficult to explain on any theory 

 of their molecules being subject to such attractive and 

 repulsive forces as seem to exist in other states of matter. 

 A consideration of these properties, together with, the 

 power of diffusion, by which gases of very different den- 

 sities form a perfect mixture when in contact, and the 

 fact that by the application of heat almost all liquids and 

 many solids can be changed into gases, led to the con- 

 ception that they owed their peculiar properties to their 

 molecules being in a state of intensely rapid motion in 

 all directions. On this theory the molecules are very 

 far apart in proportion to their size, and are continually 

 coming in contact with each other. Owing to their per- 

 fect elasticity, they rebound without loss of motion or 

 energy, and their continual impact against the sides of 

 the vessel containing them is what gives to gases their 

 great expansibility. From a study of these various 

 properties it has been calculated that, at ordinary tem- 

 peratures, there are some hundreds of trillions of mole- 

 cules in a cubic inch of gas, and that these collide with 

 each other eight thousand millions of times in a second. 

 The average length of the path between two collisions 

 of a molecule is less than the two hundred thousandth of 

 an inch; yet this small length is supposed to be at 

 least a hundred times as great as the diameter of each 

 molecule. 



From the fact that all gases expand with heat and 

 contract with cold, it is concluded that the ether-vibra- 



