10 DIFFUSION AND OSMOTIC PRESSURE 



For a given amount of gas the pressure is constant at a 

 constant temperature. But change in temperature means 

 simply change in the kinetic energy of the particles. There- 

 fore a rise in temperature must cause a corresponding increase 

 in gas pressure, and a fall a corresponding decrease. Keep- 

 ing the pressure constant, a rise in temperature produces an 

 increase in volume, and vice versa. It has been found 

 experimentally that the volume of a given mass of gas 

 under constant external pressure varies with its absolute 

 temperature (273 -j- the given temperature Centigrade). 

 This is the principle of Gay-Lussac, sometimes called 

 that of Charles. 



But if, in the elastic chamber mentioned above, the tem- 

 perature be kept constant and the resiliency of the walls 

 be increased, thus increasing the external pressure on the 

 gas, the volume will be decreased. As this occurs, how- 

 ever, the gas will increase in density, and continually more 

 particles will strike unit area of the wall in unit time. Thus 

 the internal pressure upon the bag will also be increased, 

 until at length another state of equilibrium will be reached, 

 wherein the external and internal pressures are again equal. 

 But during the readjustment the volume of the gas has 

 decreased. As long as the temperature (i. e., the kinetic 

 energy of the particles) is constant, an increase in external 

 pressure produces a decrease in volume, and a decrease in 

 external pressure an increase in volume. Experimentally 

 it is demonstrated that the volume of a given mass of gas 

 at a constant temperature varies inversely as the external 

 pressure to which it is subjected. This is the principle of 

 Boyle. 



Still another principle has been discovered for gases. 

 If the volume and temperature both remain constant, and 

 if the number of particles is increased (i. e., the concen- 

 tration), the pressure will be correspondingly increased. It 



