CHANGE OF STATE SOLID LIQUID. 205 



an experiment resembling the determination of the specific heat by the 

 method of mixtures. If the specific heat of a substance in the liquid 

 state is o-, and in the solid or', while its latent heat is L, then on cooling 

 w grammes of it from the liquid state, above the melting point, to the 

 solid state, & below the melting point, the total heat given up is 



10(0- + L + o-' ff), 



and knowing the capacity of the calorimeter in which the cooling takes 

 place, and the rise of temperature of the calorimeter, this heat may be 

 measured, o- and a-' may be found beforehand and then one observation 

 will give L. If not, separate experiments in which 6 and & are varied 

 will enable us to determine all three quantities. 



This method was employed by Person, who warmed the substance in 

 a small copper vessel, and then immersed the vessel and its contents in 

 the calorimeter. 



For the latent heat of ice, Bunsen made use of his calorimeter, de- 

 termining the quantity of ice melted by pouring into the calorimeter a 

 known quantity of water at a known temperature. 



The following are a few values of latent heat : 



Person. Bunsen. Smith. 



Water, . . . 79'2 80'2 79*818 



Phosphorus, . . 5'0 



Lead, . . . 5'36 



The Explanation of Melting on the Kinetic Theory. 



We have already pointed out that, in cases of fusion resembling that 

 of ice, there is no gradual change from solid to liquid no softening 

 throughout the mass and that the melting always occurs at the surface. 



Again, in the converse process, ice is not spontaneously formed within 

 the body of the water till at least many degrees below zero, even if then, 

 and, in general, solidification requires the presence of ice or some dis- 

 turbance to start it. 



We cannot, therefore, regard the two states as in any way continuous 

 one with the other. At melting, there is an abrupt change. If we have 

 a piece of ice in water at 0, on supplying heat the water grows at the 

 expense of the surface ice, and on withdrawing heat the ice grows at its 

 surface at the expense of the water. This closely resembles the change 

 from liquid to gas by surface evaporation, which we explained by suppos- 

 ing interchanges of molecules between the gas and liquid, and the 

 resemblance suggests an explanation of a similar kind for the change 

 between solid and liquid, in which this also is regarded as a matter of 

 exchange of molecules. 



Perhaps we may make this explanation clearer by considering an 

 ideal experiment. 



Let A and B (Fig. 120) be two bulbs containing ice and water 

 respectively, and connected by a tube C, the space above the ice and 

 water in A and B and in the tube C containing water vapour only. Let 

 the bulbs be surrounded by a constant temperature bath, so that both 

 are at exactly the same temperature. 



If this temperature is C., the vapour-pressures of ice and water 



