Density, Temperature, and Pressure of Substances. 333 



the critical density of a liquid from any available density 

 data. The critical densities of a number of liquids have 

 been calculated by means of this equation and are contained 

 in Table IV., which contains also the data used. The agree- 

 ment between these values and the critical densities given in 

 Landolt and Bornstein's Tables, 5th edition, which are also 

 given in Table IV., is quite good. The formula should 

 always give a fairly accurate value of p c , since its form is 

 such that the percentage error in the value of p c caused by 

 errors in the values of p x or p 2 is not in most cases larger 

 than the percentage error in the latter quantities. 



3T 



Equaiion (1) gives the value of B or -^|, which enables 



Pc 



us to calculate T c if p c is known. The values of T c or 



Bp 1/3 



— _ have been calculated in this way for a number of sub- 

 3 J 



stances, and are given in Table IV., using the values of p 

 previously found, and calculating B by means of the data 

 used for calculating p c . The agreement of these values of 

 T c with those taken from Landolt and Bornstein's Tables is 

 as good as can be expected. It will be observed that since 

 the cube root of p c occurs in the expression for T^ the 

 percentage error in the value of p c introduces a much smaller 

 percentage error in the value of T c . 



Further relations of interest can be deduced. We have 

 seen that 



L = l-75^1og£, 



m & p 2 



and we therefore have from equation (1) that the value of D 

 in the equation 



may be written 5*25RT C 



m pl/3 



Substituting the value of L from the latter equation in 

 Clapeyron's equation we have 



where P denotes the pressure of the saturated vapour. 



