Chap, xxxvii.j FUSION. 471 



is its point of ebullition, or boiling point. It also is 

 a fixed temperature for each substance. 



Water illustrates very well what takes place in 

 these occurrences. If a mass of ice at C. be placed 

 in a vessel, and heat applied, in a short time part 

 of the ice will become converted into water, part 

 will still remain undissolvecl. If the ice and water 

 be well stirred, the temperature throughout will be 

 found uniform, and still C. If the heat be again 

 applied just till the ice is all converted into water, 

 the temperature will still be C. In spite of the 

 application of heat, and the solution of the ice, there 

 is no increase of temperature. If the heat continue to 

 be applied, it is only after all the ice has been converted 

 into water that the continued application of the heat 

 becomes manifest in an increased temperature of the 

 water. A considerable amount of heat, that is to say, 

 has become absorbed, has apparently disappeared in 

 the conversion of the solid into the liquid state. 

 Work has been done in the conversion ; and the 

 equivalent of the work done is given by the amount 

 of heat spent in the process. In order that the water 

 may pass back into the solid state a similar amount of 

 heat must be given off. So the heat consumed in the 

 process of melting was said to become latent in the 

 water ; and the amount of heat necessary to convert a 

 unit mass of ice into water without raising the tem- 

 perature is called the latent lieat of water. Since the 

 same phenomenon is seen in the liquefaction of any 

 solid, though the amount of heat spent varies with 

 the substance, the phrase latent heat of fusion is 

 employed. 



The latent heat of water may be estimated by 

 determining the temperature which a pound of water 

 must have to convert a pound of ice at C. into 

 water, so that the mixture has a temperature of C. 

 That temperature, it is found, must be 79 to 80 C. t 



