206 



HEAT. 



are each equal to 4*6 mm., and each will be in equilibrium with the vapour, 

 neither growing nor diminishing. If the temperature is - 1 0., the 

 pressure of water vapour exceeds that of ice vapour by nearly , mm. 

 Hence, when the water vapour has saturated the space, the pressure in 

 A will exceed the maximum for the ice, and the ice will grow. Distilla- 

 tion will go on into the bulb A till all the water has gone from B. 



If the temperature of the bath rises ever so little above 0, the 

 vapour-pressure of ice exceeds that of water, and the vapour from the 

 ice tends to supersaturate the space above the water, and condensation 

 goes on in B. But the condensation is not now confined to B ; it occurs 

 on the surface of the ice, and melting takes place there. 



Suppose now that we have a mixture of ice and water at ; imagine 

 each block of ice to be separated from the surrounding water by an 

 indefinitely thin vacuous layer ; evaporation will take place into this till 

 the common vapour-pressure is reached, and then a steady state of equal 

 interchanges is arrived at. If the temperature falls below 0, the ice 



. 120, 



receives more than it gives up ; if the temperature rises above O 8 , the ice 

 gives up more than it receives. 



Now let us do away with this vacuous space, and replace the evapora- 

 tion by the " mobility " of the molecules, using this term to describe the 

 tendency of the molecules to escape from their position in this case 

 from the surface. This mobility is to some extent proved to exist by the 

 evaporation. Then, at the mobilities are equal. Above the 

 mobility of the ice molecules is the greater. Below the mobility of the 

 water molecules is greater. We may remark that probably the passage of 

 molecules across the bounding surface of ice and water is much greater 

 than the passage across the surface of ice or of water into a space 

 only containing gas. For in the one case, we have the pull of the 

 solid or liquid molecules in front assisting the escape, while in the other 

 we have only the feeble pull of the gas in the space. 



There is one fact which remains to be explained on this theory, that 

 is, the melting of ice at 0. No attempt to raise ice above has as 

 yet succeeded melting always takes place at the surface. 



The effect of pressure in lowering the melting point is explained by the 

 greater mobility due to the pressure. We know that in general pressure 

 increases mobility. Thus, in viscous solids there is a transfer from 

 points of greater to points of less pressure, or there is more mobility 

 at points of greater pressure ; and we know, too, that the vapour-pressure 



