Change of State: Solid-Liquid. 35 



In conclusion, it is pointed out that the sealing-wax type of 

 melting is probably similar to the change of ice into water 

 below the lower, or above the upper, critical points, if these 

 exist. 



Melting and Freezing of the Ice-water Type at ordinary 

 Temperatures and Pressures. 



It seems to have been conclusively proved by experiment that, 

 in bodies of the ice-water type, change of state, either from solid 

 to liquid, or the reverse, takes place only at the surface, or at a 

 surface separating dissimilar portions. This would also seem to 

 follow from the fact that the change of state always requires a 

 certain finite amount of energy to be abstracted from, or sup- 

 plied to, the mass without alteration of temperature. In the 

 middle of a homogeneous body, where the temperature varies 

 gradually, we must have the energy per unit of volume a con- 

 tinuous quantity as we pass from point to point. Hence, when 

 at any point there is sufficient energy per unit of volume to 

 change the state, either the surrounding temperature must be 

 far above the ordinary temperature for change of state, or the 

 surrounding substance must occupy an intermediate condition 

 between the two states. On the former supposition we should 

 certainly not have the ordinary change of state, though some- 

 thing of the sort may[occur in the case of Dr. Carnelley's " hot 

 ice ;" and in the latter we should have the sealing-wax type, 

 and no signs of this have been observed. 



Since, then, change of state is a surface phenomenon, we are 

 led at once to connect it with the escape of molecules which we 

 know to be always taking place from the surface, as indicated 

 by the definite vapour-tension which the body possesses, 

 whether solid or liquid. Now Begnault's experiments have 

 shown that at 0° ice and water have the same vapour-tension, 

 and at the same time a mixture of ice and water at that tem- 

 perature maintains the same proportion between the two con- 

 stituents as long as no heat is allowed to pass into or out of it; 

 that is, as many molecules escape from the water into the ice 

 as pass in the opposite direction from the ice into the water. 

 We seem, then, to be justified in assuming that the number of 

 molecules coming up to a given surface with a sufficient velocity 

 to escape is indicated by the maximum vapour-pressure at that 

 temperature. 



Now suppose that we have a mixture of ice and water 

 below 0°. Kirchhoff has shown (Pogg. Ann. ciii. p. 206j 

 that below 0° the vapour-tension of water exceeds that of ice 

 by *044 millim. of mercury per degree ; and his reasoning 

 will equally prove that it falls below it bv the same amount 



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