2 Tlie Kinetic Theory conception of Matter [CH. i 



so brings into play a system of repulsive forces tending to keep the two 

 bodies apart, and this system of forces can only be interpreted as the 

 aggregate of the forces from individual molecules. It follows that molecules 

 exert forces on one another, and that these forces are, in general, repulsive 

 when the molecules are sufficiently close to one another. On the other 

 hand, the phenomenon of cohesion shews that the force between two mole- 

 cules may, under certain conditions, be one of attraction. 



The Three States of Matter. 



3. The fact that a solid body, when in its natural state, resists both 

 compression and dilatation, indicates that the force between molecules 

 changes from one of repulsion at small distances to one of attraction at 

 greater distances. This change from a repulsive to an attractive force 

 suggests a position of stable equilibrium in which a pair of molecules 

 can rest in proximity to one another. If we imagine a great number of 

 molecules placed in proximity and at rest in an equilibrium configuration, 

 we have on the Kinetic Theory conception of matter, a mass of matter in 

 the solid state, and, as there is no motion, this matter must be supposed, 

 in accordance with the fundamental hypothesis of the theory, to be entirely 

 devoid of heat. 



The molecules of which the substance is formed will be capable of 

 vibration about their positions of equilibrium, and when these vibrations 

 occur, we say that the body possesses heat. As the vibrations become more 

 vigorous we say that the temperature of the body increases. 



For example, we may imagine the vibratory motion of the molecules 

 to be set up in the first instance by rubbing the surface of the body 

 against a surface of a similar body : here we have a case of heat generated 

 by friction. The act of rubbing will consist firstly in placing the surfaces 

 of the two bodies so near to one another that the molecules near the 

 surface of one exert a perceptible action on the molecules near the surface 

 of the other, and then in moving the surfaces over one another so as to 

 disturb these surface molecules from their positions of equilibrium. At 

 first the motion will be confined to the neighbourhood of the parts actually 

 rubbed, but the motion of these parts will gradually induce motion in the 

 adjoining regions, until ultimately the motion will have spread over the 

 whole mass. In this latter phenomenon we have an instance of conduction 

 of heat. 



As a second example, we may imagine two masses, both devoid of 

 internal motion, to impinge one upon the other. The impact will at first 

 cause systems of waves to be set up in the masses, but after a sufficient 

 time the wave character of the motion will have become obliterated, whilst 



