JAMKS CLKItK MAXWKI.I. 



if ttt bo the mass of each molcrule, the mass which 

 strikes the surface |>er second is X m r ; the velocity 

 of eacli particle of this muss is /', therefore tho 

 momentum destroyed per second by the impact is 

 X m v x r, or N m r% and this measures the pressure. 

 Hence in this case if /> be the pressure 



p = N in i*. 



In the above we assume that all the molecules in 

 the jet are moving with velocity v perpendicular to 

 the surface. In the case of a crowd of molecules 

 flying about in a closed space this is clearly not true. 

 The molecules may strike the surface in any direction ; 

 they will not all be moving normal to the surface. 

 To simplify the case, consider a cubical box tilled 

 with gas. Tho box has three pairs of equal faces at 

 right angles. \Ve may suppose one third of tho 

 particles to be moving at right angles to each face, 

 and in this case the number per unit volume which 

 we have to consider is not N, but \ X. Hence tho 

 formula becomes /> = \ X m >-. 



Moreover, if p be the density of the gas that is, 

 the mass of unit volume then Xm is equal to p t 

 for m is the mass of each particle, and there arc X 

 particles in a unit of volume. 



Hence, finally, p = \ p K 



Or, again, if V be the volume of unit mass of tho 

 gas, then p V is unity, or p is equal to I/ V. 



Hence y>V = 1?*. 



Formula* equivalent to these appear first ta have 

 been obtained by Herapath about the yeaf 1816 

 (Thomson's "Annals of Philosophy," 1810). The 



