2-5] The Three States of Matter 3 



motion of some kind must persist in order to account for the energy of 

 the original motion. This original motion will, in fact, have become re- 

 placed by a small vibratory motion of the molecules about their positions 

 of equilibrium according to the Kinetic Theory, by heat-motion. 



4. If the body under consideration continues to acquire heat in any 

 way if, that is, the energy of vibration is caused continually to increase 

 a stage will in time be reached in which some of the molecules are 

 possessed of so much kinetic energy that the forces from the other 

 molecules no longer suffice to hold them in position : they will, to borrow 

 an astronomical term, escape from their orbits. When the body has reached 

 a state such that this is true of a great number of molecules, it is clear 

 that the application of even a small force, provided it is continued for 

 a sufficient length of time, will, by taking advantage time after time, as 

 opportunity occurs, of the weakness of the forces tending to retain individual 

 molecules, cause the mass to change its shape. When this stage is reached, 

 the body has assumed a plastic or liquid state. 



When a molecule of a liquid escapes from its orbit it will in general 

 wander about amongst the other molecules until it falls into a new orbit. 

 If, however, it was initially near to the surface of the liquid, it may be possible 

 for it to escape altogether from the attraction of the other molecules, just 

 as it is possible for a projectile, if projected from the earth's surface with 

 sufficient velocity, to escape from the earth altogether. When this happens 

 the molecule will leave the liquid, so that the mass and volume of liquid 

 will continually diminish owing to the loss of such molecules. Here we 

 have the Kinetic Theory interpretation of the process of evaporation, the 

 vapour being formed by the escaped molecules. 



If the liquid is contained in a closed vessel the escaping molecules will 

 impinge on the side of the vessel, and after a certain number of impacts, 

 may fall back again into the liquid. When a state is reached in which 

 the number of molecules which fall back in this way is exactly balanced 

 by the number which escape, we have, according to the Kinetic Theory, 

 a liquid in equilibrium with its own vapour. 



5. If we suppose the whole of the liquid transformed into vapour in 

 this way, we have the Kinetic Theory conception of a gas. The molecules 

 can no longer be said to describe orbits, but describe rectilinear paths with 

 uniform velocity except when they encounter other molecules or the walls 

 of the containing vessel. It is clear that this view of the nature of a gas 

 will sufficiently explain the property which a gas possesses of spreading 

 throughout the whole of any closed space in which it is placed. It is not 

 necessary to suppose, as was at one time done, that this expansive property 

 of a gas is the result of repulsive forces between the molecules. 



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