treated according to Van der Waals's Equation. 337 



from e towards c, but the part ah c cannot be experimentally 

 realized, as the fluid in that region is in an unstable state. 



Fig. 1. 









\ 



\ 





V 



C 





i 

 i 



i 



i 





"\ 







i 



i 

 l 



\ 1 





i 







I 



i 



1 

 i 



|a 









i 



i 



— (? 







—Pressure 



/L^^~ 



*r 







+ Pressure 



Pressure and density of a fluid. 



For isothermals corresponding to higher temperatures 

 than the one traced, the part between e and e' diminishes^ 

 and finally disappears at the critical temperature, at which 

 point there is a critical density and a critical pressure. 

 For lower isothermals than the one drawn, it is possible for 

 the point c to touch the axis of no pressure and even to pass 

 to the other side as at c 1} and the liquid then exists under a 

 negative pressure. 



If the fluid were to follow the path e a, it would reach the 

 limit of stability at a, and would then pass suddenly to/; 

 and in the same way, if it were made to traverse the path 

 fe c, it would reach another limit of stability at c and would 

 suddenly change to f. In such a cyclic change the loop 

 f afcf would be traced out, and there would be a loss of 

 energy as ihe fluid was made to pass from vapour to liquid 

 and back to vapour, giving rise to what may be called fluid 

 hysteresis loss ; and, on the other hand, when no precautions 

 are taken to preserve the fluid from disturbance, the path 

 /' e' efh followed to and fro, and there is no dissipation of 

 energy. The path is ihen anhysteretic. Van der Waals's 

 equation, which is a cubic in p (the density), represents with 

 considerable success the shape of the fluid isothermals, and 

 gives a satisfactory account of the critical constants. 



Now the ferromagnetic equation is a repetition of Van der 

 Waals's equation, and it must give to the magnetic iso- 

 thermals similar forms to those which have been discussed.. 



