6 DIFFUSION AND OSMOTIC PRESSURE 



energy of a particle varies with the square of its average 

 velocity. We neglect here, as comparatively unimpor- 

 tant, all other forms of motion which a particle may possess, 

 such as that of rotation, and consider only its transla- 

 tory motion. Therefore, whenever the temperature of a 

 quantity of matter is raised by any means, the average 

 translatory velocity of its particles is increased. Now, the 

 force of impact of a moving body is proportional to its 

 momentum, which is equal to the product of its mass and 

 velocity at the time of impact. But since one particle may 

 strike another particle at any point in its free path, here 

 again the average velocity must be considered. Therefore, 

 since the mass of a particle is a constant quantity, any 

 increase in the average velocity will cause a corresponding 

 increase in momentum, and also in the force of impact. But 

 the force of recoil is practically equal to the force of impact, 

 and this latter force is the repellent force which tends to 

 separate the particles. Thus, with rising temperature the 

 repellent force is increased, the force of cohesion is more 

 and more nearly overcome, and the particles become more 

 and more widely separated. Also, with the rapid decrease 

 in the cohesive force incident upon the increase in its acting 

 distance, a limit is soon reached beyond which the force 

 tending to cause separation is greater than the other, and 

 the particles fly apart indefinitely. In this condition we 

 say the substance is a gas. If it was a liquid or solid at 

 the lower temperature, it has now been vaporized by heat. 



II. THE THREE STATES OF MATTER 



Matter exists in three states the gaseous, the liquid, and 

 the solid. In gases the kinetic energy of the particles is so 

 great that the cohesive force is entirely overcome and the 

 particles tend ever to increase their distance apart. From 

 this it necessarily follows that a mass of gas in a closed 



