149-153] 



Isothermals 



133 



Hence this area must vanish, which is the result already stated. Objections 

 can be, and have been, urged against this argument, but to discuss the ques- 

 tion any further would carry us too far into the domain of thermodynamics. 



151. The figure which is obtained from figure 9, upon replacing the 

 curved parts of isothermals such as ZQXRY by the straight line ZXY, is 

 represented in fig. 10. This figure then ought to represent the main features 

 of the observed systems of isothermals of actual substances. There can of 

 course be no attempt at anything of the nature of quantitative agreement. 



P 



V--V 



FIG. 10. 



Comparison with Experiment. 



152. It would in no way be consistent with the plan of this book to 

 give a detailed account of the agreement or disagreement between the theory 

 which has been arrived at and experiment. We shall therefore be content 

 with mentioning two sets of experimental results, for the sake of illustration, 

 rather than confirmation, of the theory. 



153. In tig. 11, the curves are the isothermals of carbon dioxide as 

 found experimentally by Andrews*. The figures on the left-hand denote 



* Phil. Trans. CLIX. p. 575 (1869) and CLXVII. p. 421 (1876). 



