242 EIGHTEENTH CENTURY. rr. ill. 



and hotter till it reaches ioo° Centigrade, when it will boil. 

 Here, again, it will remain at the same temperature, and 

 though you go on boiling it till it has all passed away in steam, 

 the last drop of water will never be hotter than ioo° C. So 

 that here again the heat which is added remains hidden 

 and does not become apparent. This last fact about boiling 

 water had been long known to philosophers, but no one found 

 any explanation of it until Black began his experiments on 

 melting ice ; and he then came to the conclusion that the 

 heat is employed in altering the condition of the water, 

 hat is, in changing it, in the one case from solid ice into 

 water, and in the other from water into a vapour. 



He proved this by some simple experiments which are 

 not difficult to make. He took two glass flasks, and filled 

 one with ice just on the point of melting, and the other with 

 an equal weight of ice-cold water. These he hung in a 

 moderately warm room, which he kept all the time at the 

 same heat (8° -5 C). At the end of half an hour the ice- 

 cold water had risen four degrees (from 0° to 4°), but the 

 melting ice remained at 0°, and it was ten hours and a half 

 before the ice had disappeared and the water had reached 

 the same temperature as that which the water in the other 

 basin had attained in half an hour. Now the melting ice 

 had been receiving heat for twenty-one half-hours, and 

 therefore had taken in 2 1 x 4, or 84° of heat, while it only 

 showed a rise of 4°. It was clear, therefore, that the re- 

 maining 80° must have been spent in turning the ice into 

 water. 



Black now tried the same thing in another way. He 

 found that a pound of water at 79° C. would exactly melt a 

 pound of ice. So he again took two vessels, in one of which 

 he put a pound of ice-cold water at 0° and a pound of hot 



