542 Mr. Richard ThreJfaU [March 19, 



The general relation for a substance undergoing a change of state 

 at absolute temperature T, such change involving a change of volume 

 A V and an absorption or emission of heat at constant pressure Q p, 

 is, reserving the question of sign — 



^ = ^-? 



dp % 



or in words, the change of melting-point produced by unit change of 

 pressure equals the product of the absolute temperature, and the 

 ratio of the change of volume of unit mass on melting to the quan- 

 tity of heat absorbed or emitted by unit mass in the process. 



Now the greater number of substances when they pass from the 

 liquid to the solid state evolve heat and contract in volume. An 

 increase of volume is of course a positive quantity, and if heat is 

 absorbed during this increase it is reckoned positive also. In the 

 case of water, heat is evolved during freezing as in other cases, but 

 the mixture of ice and water has a smaller volume than the soUd ice. 

 Accordingly the change of volume in this case is negative, and the 

 melting-point falls as the pressure rises. 



The first fairly exact confirmation of the theory appears to be 

 due to De Yisser,* who selected acetic acid most carefully purified as 

 a test substance — though valuable experiments up to much higher 

 pressures had been previously made by many others — particularly 

 by Dewar on water,t Ferche on benzol, J and Damien§ on a variety of 

 substances. 



It is necessary to work with a pure substance in order to test the 

 theory, or at aU events with one whose solid phase has the same 

 constitution as its Hquid phase. If the acetic acid had not been 

 pure the probabihty is that the frozen part would have contained 

 more or less of the impurity than the unfrozen, and consequently 

 a state of affairs not contemplated in the theory would have arisen. 

 From the experimental point of view it is obvious that a sharp 

 melting point is a necessary condition for its accurate observation. 



A quantity of acetic acid — rather over 40 c.c. — is confined by 

 mercury in a closed apparatus based on a previous design by Bunsen, 

 which also contains air in a graduated tube. When the acetic acid 

 melts it expands and compresses the air through the intermediary 

 of the mercury — whereby the pressure can be inferred. The part of 

 the apparatus containing the acetic acid is immersed in a bath which 

 can be kept at any desired temperature. As the melting progresses 

 a pressure is set up by the expansion, and finally attains such a value 

 that no further melting can take place. We then have a mixture 



* Eecueil des Travaux Chimiques des Pays-Bas, xiii'. 1893, 101. 

 t Proc. R. S., XXX. 1880, 533. % Wied. Ann., xliv. 1891, 265. 



§ G. R., cxii. 1891, 785. 



