228 



Mr. William Sutherland on the 

 Table XIV. 



Critical temperature, T C =120Z/409R£ ; critical pressure, 

 p=36l/mk 2 . 





(C 2 H 5 ) 2 0. 



C0 2 . 



S0 2 . 



NHg. 



N 2 0. 



Critical f exper. ... 

 Temperature. \ cale 



Critical f exper. . . . 

 Pressure. \ calc 



194 

 199 



27-1 

 29-3 



32 



52 



59 



78-6 



155 



125 



60 

 56 



130 



96 



87 

 84 



35 

 36 



57 



57 



The want of accuracy in the agreement in parts of this 

 table is to he ascribed partly to inaccuracy in the ordinary 

 determinations of the critical point, as I have already pointed 

 out that capillary action must sometimes largely affect the 

 numerics of the critical state when these are determined in 

 capillary tubes (Phil. Mag. August 1887). Regnault, in 

 his account of his experiments on the saturation-pressures of 

 C0 2 , expressly declares that be had liquid C0 2 at 42° 5 which 

 is 10° above the apparent critical temperature in capillary 

 tubes ; and Cailletet and Colardeau (Compt. Rend, cviii.) 

 have shown that although the meniscus between gas and 

 liquid C0 2 disappears to the eye about 31° or 32°, yet cha- 

 racteristic differences between liquid and gas can be proved 

 to exist several degrees higher than this. Hence an error of 

 at least 10° is possible in ordinary determinations of critical 

 temperatures. On the other hand, an error of 5 per cent, in 

 the value of an absolute temperature of about 400° as given 

 by our equation would amount to 20°. Table XIY. is to be 

 taken in the light of these facts. 



We have now ascertained a second property that our 

 equation for volumes below the critical is to possess : it must 

 begin to apply when v=7k/6, as the other form cannot apply 

 below this volume at the critical temperature. At this volume 

 the kinetic-energy term in our form above the critical region 

 becomes 



RT(1 + 12/13), or 25RT/13; 



so that 25R/13 is the lower limit of the term which in the new 

 equation is to take the same place as ~Rvf(v) hitherto. Hence 

 for this term the form 



25R(1 + F(v))/13 



naturally suggests itself, and as F(i') is to vanish when 

 v = 7k/6, we get (7k/6 — v)/yjr(v') as a suggestion for its form ; 

 and it only lemains from the data obtained, as I have said, by 



