360 The Isometrics of Liquid Matter. 



and the liquid isometric of compound matter is exceedingly 

 striking. Solid metallic isometrics are quite different from 

 the above (Table XI.). If 1/k be the compressibility and ft 

 the coefficient of thermal expansion, the quantity 10 6 /k/3 is an 

 estimate of the initial slope of the isometric, in degrees per 

 atmosphere. Taking k and /3 from Everett's tables (/. c), 

 the slopes for steel, iron, and copper respectively are only 

 •014°, '016°, and "011° per atmosphere. They thus preserve 

 an order of magnitude smaller than that of glass, or the above 

 organic liquids. 



44. For the time being the above results admit of the fol- 

 lowing interpretation : — Whenever a substance passes from 

 the liquid to the gaseous state, no matter whether this takes 

 place continuously above the critical temperature, or discon- 

 tinuously below it, the underlying cause is a change of mole- 

 cule from a more complex to a less complex type. As long 

 as the molecule remains unchanged the isometrics are straight. 

 When the change of molecule takes place so as to begin with 

 the liquid molecule and pass continuously into the gaseous 

 molecule, the isometrics curve continuously for the linear 

 isometric of the true liquid to that of the true gas. Such an 

 explanation is of course tentative. It rests on evidence purely 

 experimental and therefore of uncertain interpretation ; and 

 it is suggested by a controversy which I have summed up 

 elsewhere * as follows : — " The linear relation was predicted 

 from theoretical considerations by Dupre (1869) and by Levy 

 (1884) — considerations soon proved to be inadequate by 

 Massieu, H. F. Weber, Boltzmann, and Clausius. Ramsay 

 and Young (1887) established! the relation in question expe- 

 rimentally for vapours, but not, I think, very fully for liquids 

 decidedly below their critical points. Reasoning from these 

 data, Fitzgerald (1887) investigated the consequences of the 

 law; viz.: — (1) specific heat under constant volume is a 

 temperature-function only ; (2) internal energy and entropy 

 can be expressed as a sum of two terms, one of which is a 

 volume-function only and the other a temperature-function 

 only. Thus Ramsay, Young, and Fitzgerald arrive substan- 

 tially at the same position from which Dupre and Levy 

 originally started." 



However, too much care cannot be taken in keeping clearly 

 in mind that pressures which, in relation to the usual labora- 

 tory facilities, are exceptionally large, may yet be mere 



* Am. Journal [3] xxxviii. p. 407 (1889). 



t For a pressure-interval not exceeding about 80 atmospheres for a 

 group of isometrics, nor about 30 atmospheres for a single isometric. See 

 Kamsay and Young, Phil. Mag. [5] xxiii. p. 435 (1887). 



