54 REPRESENTATION BY SUEFACES, ETC. 



this plane figure and the surface of dissipated energy will represent 

 all the states into which the body can be brought by any kind of 

 processes, without producing permanent changes in external bodies 

 (except in the medium).* 



* The body under discussion has been supposed throughout this paper to be homo- 

 geneous in substance. But if we imagine any material system whatever, and suppose 

 the position of a point to be determined for every possible state of the system, by 

 making the co-ordinates of the point equal to the total volume, entropy, and energy 

 of the system, the points thus determined will evidently form a solid figure bounded 

 in certain directions by the surface representing the states of dissipated energy. In 

 these states, the temperature is necessarily uniform throughout the system ; the 

 pressure may vary (e.g., in the case of a very large mass like a planet), but it will always 

 be possible to maintain the equilibrium of the system (in a state of dissipated energy) 

 by a uniform normal pressure applied to its surface. This pressure and the uniform 

 temperature of the system will be represented by the inclination of the surface of 

 dissipated energy according to the rule on page 34. And in regard to such problems as 

 have been discussed in the last five pages, this surface will possess, relatively to the 

 system which it represents, properties entirely similar to those of the surface of 

 dissipated energy of a homogeneous body. 



