74 ELEMENTARY AGRICULTURAL CHEMISTRY 



forms slowly on the surface of water, and that snow and ice 

 melt so slowly when a thaw sets in. 



The latent heat of ice, or of the fusion of ice, is thus said 

 to be 80. 



Another change of state occurs when liquid water becomes 

 gaseous. If a quantity of water at the ordinary temperature 

 be gradually and regularly supplied with heat, a thermometer 

 placed in the liquid will indicate a steady rise of temperature. 

 It is evident that the heat supplied is being used in raising 

 the temperature of the water. This will go on almost regu- 

 larly until a temperature of about 100° C. is reached. No 

 further rise will occur, and even if the rate of supply of heat 

 be doubled or quadrupled no effect upon the thermometer will 

 be observed ; the water, however, will now be slowly changing 

 into steam, which, as it leaves the liquid, has the same tem- 

 perature (100° C). Evidently the heat supplied is now being 

 used up in converting the liquid into vapour without producing 

 any rise in temperature. The quantity of heat required to 

 change the unit weight of water at 100° into steam at 100° is 

 called the latent heat of steam, or of the vaporisation of water. 

 It is very large, being 536 times as great as the amount required 

 to raise the same weight of water through 1° in temperature. 



The quantity of heat necessary to raise 1 gramme of water 

 from 0° to 1° is called the thermal unit, unit of heat, or 

 calorie. 



Now water evaporates into dry air (or into any gas or space 

 not already saturated with aqueous vapour) at any temperature, 

 and since that which is converted into vapour requires its latent 

 heat, a considerable absorption of heat ensues. This is the reason 

 why wetted things are cooled when exposed to air; it is not 

 because the water is cooler than other substances, but because 

 it evaporates and absorbs the heat necessary for its conversion 

 into vapour. The more rapid the evaporation, the greater is 

 the reduction of temperature. The rapidity is increased by 

 higher temperature and by quicker renewal of the atmosphere in 

 contact with the wet surface, e»g., by a draught of air or wind. 



