Change of State : Solid-Liquid, 33 



supported the theory, and I believe it is now generally aban- 

 doned. 



Since, in the ice-water form of change of state, fusion only 

 takes place at the surface, it seems much more probable that 

 it is an exchange phenomenon analogous to the change which 

 takes place when water is evaporating, according to the kinetic 

 theory. Just as in the case of water-steam, a steady state is 

 reached when the number of molecules escaping from the sur- 

 face of the water into the gas is equal to the number passing 

 from the gas into the water, so in the case of water-ice a steady 

 state (that is to say, the melting-point of ice) is probably 

 reached when the number of molecules passing from the ice 

 into the water is equal to the number passing from the water 

 to the ice. For the analogue of the sealing-wax type of melt- 

 ing we must probably take the change of state which takes 

 place in a liquid-gas above its critical point, where it changes 

 gradually from a state rather liquid than gaseous to a state 

 certainly gaseous. 



In this paper I shall attempt to support this view of solid- 

 liquid change of state. The following is a summary of the 

 argument and the conclusions arrived at. 



It is assumed that the maximum vapour- tension of a sub- 

 stance at any temperature is an indication of the number of 

 molecules crossing its surface in a condition to escape. Now 

 Regnault's experiments show that at 0° ice and water have 

 the same vapour-tension ; that is, the number of molecules 

 crossing the surface of the ice ready to escape is equal to the 

 number crossing the surface of the water in the same condition. 

 Hence, when the two are in contact at 0°, the interchange of 

 molecules is equal. For temperatures below 0°, Kirchhoff has 

 shown that the vapour-tension of water is greater than that of 

 ice, and above 0° it is less than that of ice — if ice can exist. 

 (Another proof of this theorem is here given.) It is, then, easy 

 to give a general explanation of the phenomena of melting 

 and freezing by supposing that, if the temperature is not at 

 the melting-point, the substance in the state with the greater 

 vapour-tension will lose at the expense of the state with the 

 less vapour-tension. 



To explain the alteration of the melting-point by pressure, 

 we must suppose that pressure alters the vapour-tension, and 

 therefore the rate of escape of molecules, and that this altera- 

 tion is different for the two states. Sir William Thomson has 

 shown that a liquid in a capillary tube is in equilibrium with 

 its vapour at a greater or less tension than at the plane sur- 

 face according as the surface is convex or concave, Upwards, 

 and has given a formula for the difference. Accompanying 



Phil Mag, S. 5. Vol. 12. No. 72. July 1881. D* 



