a Fluid passing through a Porous Plug, 889 



The criterion of an inversion-point is the satisfaction of 

 the equation 



3T 



0. 



where T is the absolute temperature and v the specific volume. 

 The implication of this equation is that if isopiestic values of 

 v be plotted against T, any point of contact of the tangent 

 through the origin (T = 0) corresponds to an inversion 

 temperature. 



Fig. 1. 



V- 



This statement and diagram should replace those on the 

 upper half of Part T. p. 560. By sufficiently accurate 

 draughtmanship therefore the real inversion-points could be 

 determined from the experimental values of Amagat, Sidney 

 Young, and others. But such curves would be unsuitable 

 for accurate work. Sufficient accuracy is obtainable, how- 

 ever, if a difference curve be drawn as follows: — 



v T 



Let A=v — 7^, where v is the specific volume at any 



standard temperature T . We shall call A the discrepancy 

 because it represents the difference between the actual volume 

 and the volume which the gas would have had if it had 

 changed from that at the standard temperature according to 

 the law for perfect gases. It follows easily that 



T d'' 



T^-A; 



dT dT 



Phil. Mag. S. 6. Vol. 19. No. 114. June 19.1.0. 



3 M 



