EXPERIMENTAL KNOWLEDaE OF THE PROPERTIES OF MATTER. 113 



* the passage of a body from the solid to the liquid state produces no 

 appreciable change in the curve of the elastic forces of its vapour ' (its 

 vapour pressure) ; ' this curve keeps a perfect regularity before and after 

 the transformation.' 



Ethylene dibromide melts at 9'53° (Regnault, loc. cit.) 



„ „ 8-2° to 8 4° (J. C. S. xlv. p. 520) 

 Benzene ,, „ 4'4° „ 4"5° (Regnault, loc. cit.) 



Acetic acid „ „ 16° (Regnault, loc. cit.) 



„ ,,16-55° (Pettersson.J. C.S.42, 3) 



It is seen from the convenient position of the melting-points in refer- 

 ence to ordinary temperatures that these bodies are v^ell chosen for this 

 purpose. 



As an example we give some results of Regnault for acetic acid : — 



Liquid acid Solid acid 



Temp. Vap.-pressure Temp. Vap.-pressure 



Thus the curve of vapour-pressures for acetic acid (abscissEe temp., 

 and ordinates vap. -pressures) seems to show that there is a difference 

 of vapour-pressure due to state, and that the solid acid has a greater 

 vapour-pressure than the liquid ; but when this acid has been thoroughly 

 dried by distilling over phosphoric anhydride, the results obtained .showed 

 <;he vapour-pressure for the solid acid less than that from the liquid ; but 

 here acetone was recognised as having been developed by the action of 

 the phosphoric anhydride ; and although most of this was removed by 

 distillation some, no doubt, remained ; the two specimens of acetic acid 

 were thus impure, one with water, the other with acetone, and they gave 

 contrary results. And no trustworthy results were obtained with the 

 other substances. 



Thus two interesting questions are raised by Regnault's investigations 

 on vapour-pressures : — 



(1) Whether static and dynamic methods give, when carefully per- 

 formed, identical results. 



(2) "Whether when at the same temperature a body can exist eitl:er in 

 the solid or in the liquid state the vapour-pressure in both states is the 

 same ; i.e. whether the pressure is the same from the solid as from the liquid. 



Regnault decided both these questions in the affirmative ; subsequent 

 investigations have confirmed, as I think, Regnault's answer to the first 

 question, as they have undoubtedly reversed his answer to the second. 



Application of Theory to the Second Question. 



It should be mentioned, in reference to the second of these questions, 

 that in 1858 Kirchhoff, from theoretical considerations, showed that if the 

 vapour- pressure of ice and of water were the same at any the same tem- 

 perature, then -^, where j5 is the vapour-pressure, must be different.' 

 This was, from the theoretical point of view, an important step. But in 



' Poggendorff's Annalen, ciii. 

 1 886. I 



