( 123 ) 



a 

 Undoubtedly p may be iief«;lecte(l by llic side df -^ , Exen if y; 



amounts to one Atmosphere, its vahic is certainly si ill sinallci; lliaii 



a v{v-2b) 



0.0001*^' part of — . In the same way p — may undoubtedly 



üi'-" b 



a 

 be neglected by the side of — or 2)i\ {o^ — 2/>) by the side of tr — 



b 



a 



and this for the same reason, for — — is a nuantitv of the same 



r^[jb—v^) 



a 

 order as — . 



So the equation may be simplilied to: 



a 



p b V, — b 



a Kl b 



For the limiting case, when v^ may be equated to b, we get: 



a 



lo ^^ — ^ 

 ^^ a ~ RT' 



If we introduce the critical data, namel}' : 



\ a S a 



pv zz:z and B.Thz=-~- — , 



^ 27 b' 21b 



then we ^q\ the following equation for the calculation of p: 



— loq — =: lo<i 27 



or, as lo[/ 27 is equal to 3,3 and may therefore l)e neai'Iy equated 



27 

 to — we get with a high degree of approximation: 

 8 



P Ik — 1' 



— log - = 3,375 — —-. 

 Pk J 



This last equation is nearly equal to that deri\c(l l)y j)rof. 

 Kamerlincui 0-NNes by means of a graphical method from the equcition 

 of state with a and b constant, namely : 



Kamerlingh Onnes found this equation to hold in apj)roximatioii \\\\ 

 to the critical temi)eraturc, here we could ordy derive it (oi- low 



1) Arch. Neérl. Livre Jub. dédié a H. A. LurEiNTZ. p. 070. 



