20 MOLECULAR MOTION AND ITS ENERGY 11 



further hypothesis being necessary ; here, however, I will 

 first investigate the value of the pressure exerted on a 

 solid wall by reason of its greater intelligibility. 



For this it is necessary to introduce a hypothesis as to 

 the magnitude of the forces exerted by impact against the 

 wall. In choosing this we shall have to be guided by the 

 consideration that a gas suffers no loss of energy through 

 exerting pressure on the solid walls of its enclosure ; the gas 

 therefore receives back from the wall the energy it has given 

 to it. If this is true for the gas as a whole we shall have 

 also to assume for each one of its molecules that at every 

 single impact against the wall its stock of kinetic energy 

 remains undiminished. We thus arrive at the hypothesis 

 that each molecule is, like a perfectly elastic ball, thrown 

 back from the wall with the same speed with which it 

 struck it. A molecule that impinges perpendicularly against 

 the wall receives an impulse which is sufficient not only 

 to stop its motion, but also to give it an equal speed in 

 the reverse direction. The magnitude of this impulse is 

 expressed by the product %mG, wherein m denotes the 

 mass of a molecule and G its speed ; and just as great is the 

 impulse exerted on the wall by the molecule during the 

 impact. 



To obtain from this the total force exerted on the wall 

 we have to multiply this expression by the number of 

 impacts in the unit of time. 



Although this calculation can be made for every possible 

 shape of the enclosure containing the gas, we will for 

 simplicity consider the gas to be in a rectangular parallele- 

 piped, whose edges are a, /3, 7 in length, so that its volume is 

 a@y. If now there are N molecules per unit volume, there 

 are Nafiy molecules altogether. According to Joule's 

 representation of the case, which, as was proved in 

 10, may be used in the calculation instead of the real 

 circumstances, one-third part of this number, or j^Na/By 

 molecules, move in the direction of the edge 7 per- 

 pendicularly against the two faces a/3. These faces will 

 be struck alternately by the molecules moving to and fro 

 between them. 



