CHAPTER XIL 

 CHANGE OF STATE. SOLID LIQUID. 



Melting of Ice and Melting of Wax Melting of Ice at a Definite Point and on the 

 Surface only Latent Heat Supercooling Regelation Effect of Pressure on 

 Melting Point Melting Points of Solids Explanation of Melting on the Kinetic 

 Theory Resemblance of Solution to Fusion Evaporation from Solids. 



Melting Of Ice and Melting Of Wax. The change from the solid 

 to the liquid state may be illustrated by the familiar examples of the 

 melting of ice and the melting of fatty or waxy substances. The latter 

 are probably mixtures, and the process may be complicated by the melt- 

 ing of one constituent before another. But, however this may be, we at 

 once observe a difference between the two cases. A piece of sealing- 

 wax, as its temperature rises, gradually softens from a solid into a 

 viscous liquid without any discontinuity, and on cooling it gradually 

 hardens again into a solid. 



Melting of Ice at a Definite Point and on the Surface only. 



A piece of ice, however, remains solid to the moment of melting it 

 is never really soft, and in the converse process of freezing, the water 

 does not gradually get thick, but crystals of quite solid ice form within 

 it. If we take a quantity of ice below the melting point, and gradually 

 supply heat, keeping the ice at a uniform temperature throughout, the 

 ice rises up to 0, still continuing solid. The surface of the ice then begins 

 to melt, and if care is taken to keep the temperature still uniform through- 

 out, no further rise takes place till the whole of the ice is melted, the 

 temperature remaining at a perfectly definite point. Indeed, on the 

 constancy of this melting point depends our system of graduating ther- 

 mometers. The ice melts at its surface alone, and there is no softening 

 into a plastic condition, as in the case of wax. 



Latent Heat. The heat taken up by each gramme ot ice, in 

 turning from ice at to water at 0, is constant in amount, and is 

 termed the "latent heat" of water. 



Freezing. Taking the converse process, if a quantity of water be 

 cooled down gradually, the temperature being kept uniform meanwhile 

 by stirring, the water remains liquid without any sudden or remarkable 

 increase in its viscosity. At 0, crystals of solid ice begin to appear, and 

 grow at the expense of the water. The temperature is arrested at 

 throughout the mixture, if the stirring is kept on while the ice is grow- 

 ing. The same amount of heat per gramme is given up to the surround- 

 ings as was absorbed in the change from ice to water. 



Supercooling. If we begin again and, instead of stirring the water, 

 we keep it as quiet as possible during the cooling, then, in general, the 

 temperature falls continuously past to perhaps - 3 or - 4. Next, 



