LATENT HEAT. 823 



ing water at 100 while the water is becoming steam at 

 100 is in the same way expended in the evaporation of 

 the water. This heat disappears without producing 

 any rise of temperature, and hence is called latent heat. 

 The heat which has become latent may, however, be 

 reproduced, or given out so as to become sensible, when 

 the liquid solidifies, or steam is reconverted into water. 

 If water is to be converted into ice, it is not sufficient 

 merely to cool it to 0. Water in a vessel, which is 

 placed in a larger one containing ice at 0, does not 

 freeze; it is necessary to bring the water in contact with 

 bodies which have a temperature lower than 0, so 

 that, after the water has fallen to 0, still more heat 

 may be withdrawn from it ; in other words, water 

 at must give out heat in order to be converted 

 into ice, and the quantity of the heat thus to be given 

 out is precisely equal to the quantity which disap- 

 pears when ice at is converted into water at 0. 

 A liquid, while solidifying and reproducing the heat 

 which has become latent, does not alter its temperature. 

 A corresponding reproduction of heat, without change 

 of temperature, takes place when a vapour passes into 

 the state of a liquid. 



The reproduction of heat which has been expended 

 in altering the state of aggregation of a body can be 

 most easily observed in the case of liquids whose point 

 of solidification has been retarded. Thus, if melted hypo- 

 sulphite of soda is caused to solidify by adding a small 

 grain of the solid substance, the heat given out at the 

 moment of solidification is sufficient to be felt by hand. 



If l kgr of ice at is placed in l kgr of water at 80, the 

 ice melts and 2 kgr of water at are obtained. The 

 whole of the heat given up by the water at 80 cooling 



