CHANGES OCCASIONED BY HEAT. 555 



that a large quantity of heat is absorbed by the ice 

 in becoming water, and by the water in becoming 

 steam. He reckoned from the above experiments, 

 that ice, in melting, absorbs as much heat as 

 would raise ice-cold water through 140 of tem- 

 perature : and that water, in evaporating, absorbs 

 as much heat as would raise it through 940. 



That snow requires a great quantity of heat to 

 melt it; that water requires a great quantity of 

 heat to convert it into steam ; and that this heat 

 is not indicated by a rise in the thermometer, are 

 facts which it is not difficult to observe ; but to 

 separate these from all extraneous conditions, to 

 group the cases together, and to seize upon the 

 general law by which they are connected, was an 

 effort of inductive insight, which has been con- 

 sidered, and deservedly, as one of the most striking 

 events in the modern history of physics. Of this 

 step the principal merit appears to belong to 

 Black (RA). 



The consequences of this principle are very im- 

 portant, for upon it is founded the whole doctrine 

 of evaporation ; besides which, the principle of 

 latent heat has other applications. But the rela- 

 tions of aqueous vapour to air are so important, 

 and have been so long a subject of speculation, that 

 we may with advantage dwell a little upon them. 

 The part of science in which this is done may be 

 call CM], as we have said, Atmology ; and to that divi- 

 sion of Thermotics the following- chapters belong. 



