598 Hon. R. J. Strutt on the Electrical 



This gives an approximation to the critical temperature. 

 In the case of mercury the surface-tension for different 

 temperatures is given by Frankenheim*. His results point 

 to a critical temperature of about 750°. 



Again, in the large majority of cases, the absolute critical 

 temperature bears a fairly constant ratio, about 1*6, to the 

 absolute boiling-point. This holds fairly well through a 

 great range of boiling-points, from liquid hydrogen upwards. 

 This would make the critical temperature about 724°, in fair 

 agreement with the estimate from surface-tension. 



If these estimates were anywhere near the mark, there 

 would be no great difficulty in determining the electrical 

 resistance up to the critical temperature. But other methods 

 of estimating this critical temperature unfortunately lead to 

 a very different conclusion. Thorpe and Riicker f have found 

 that in many cases the absolute critical temperature (6) can 

 be calculated from the formula 



0+273)^-273 

 " 1-9951V,-1) 

 where V* is the volume at some temperature t, the volume at 

 0° being taken as unity. If this be applied to mercury, 

 taking t as 100° C, the critical temperature indicated is no 

 less than 2700° 0. 



Let us now consider the matter from the standpoint of 

 density. 



It is usually found that the critical density of a substance 

 is about one-third the density of the liquid at low temperatures, 

 also that it is about 4*4 times the density which the same 

 substance would have at that temperature and pressure if it 

 behaved like a perfect gas J. (This may be alluded to as the 

 theoretical gas density.) 



Combining these tw T o generalizations, we see that the 

 theoretical gas density under the critical conditions would be 

 about j~ of the liquid density at low temperature. This 

 latter for mercury is about 13*6. Thus the theoretical gas 

 density for this substance under critical conditions should be 

 very nearly unity. This does not of course in itself determine 

 the value of the other critical data. But if a value be assumed 

 for one of them, the corresponding value for the other can be 

 found. The following are approximate values : — 



Assumed critical temperature (°C.) 273 546 819 1092 1365 1638 1911 

 Corresponding critical pressure I 222 333 4M 55& 666 m g86 

 in atmospheres | 



* Pogg. Ann. Ixxv. p. 2^9 (1848). 



t Chem. fcoc. Journ. vol. xlv. p. 135 (1884). 



% Young, Phil. Mag. Feb. 1892, p. 185. 



