AND MODERN PHYSICS. Ill 



by a gas on the sides of a vessel which contains it 

 may bo accounted for on this assumption. Each 

 molecule as it strikes the side has its momentum 

 reversed the molecules are here assumed to be 

 perfectly elastic. 



Thus each molecule of the gas is continually 

 gaining momentum from the sides of the vessel, while 

 it gives up to the vessel the momentum which it 

 possessed before the impact The rate at which this 

 change of momentum proceeds across a given area 

 measures the force exerted on that area ; the pressure 

 of the gas is the rate of change of momentum }>er 

 unit of area of the surface. 



Again, it can be shown that this pressure is pro- 

 portional to the product of the mass of each molecule, 

 the number of molecules in a unit of volume, and 

 the square of the velocity of the molecules. 



Let us consider in the first instance the case of a 

 jet of sand or water of unit cross section which is 

 playing against a surface. Suppose for the present 

 that all the molecules which strike the surface have 

 the same velocity. 



Then the number of molecules which strike the 

 surface per second, will bo proportional to this velocity. 

 If the particles are moving quickly they can reach the 

 surface in one second from a greater distance than is 

 possible if they be moving slowly. Again, the number 

 reaching the surface will be proportional to the 

 number of molecules per unit of volume. Hence, if 

 wo call v the velocity of each particle, and N the 

 number of particles per unit of volume, the number 

 which strike the surface in one second will be X v; 



