CH. xxvin. LATENT HEAT. 241 



that here again the heat which is added remains hidden 

 and does not became apparent. This last fact about boil- 

 ing water had been long known to philosophers, but no 

 one found any explanation of it until Black began his ex- 

 periments on melting ice ; and he then came to the conclu- 

 sion that the heat is employed in altering the condition of 

 the water, that 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 o to 4), but the 

 melting ice remained at o, 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 21 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 



o 



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 o, and a pound 

 of hot water at 79, and when they were properly mixed he 

 found, as he expected, that the heat of the mixture was 

 half-way between the two, that is 395. In the other 

 vessel he put a pound of ice at o, and a .pound of hot 

 water at 79, and here, when the ice had disappeared, the 



