CH. XXVIII. LATENT HEAT. 243 



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 39^°. In the other vessel he put a 

 pound of ice at 0° and a pound of hot water at 79°, and 

 here, when the ice had disappeared, the mixture still re- 

 mained at 0°, showing that the whole 79° of heat in the 

 boiling water had been absorbed in melting the ice, and 

 remained hidden or latent in the two pounds of water. The 

 latent heat of water is therefore between 79° and 80°. 



We know now what becomes of this heat, as you will see 

 (chapter xxxiv.) in the history of the science of the nine- 

 teenth century ; but the first step was to prove its disappear- 

 ance into the water, and this we owe to Black ; as well as the 

 fact that still more heat is lost in turning water into steam. 



This last fact he proved by filling a glass bottle half full 

 of water, corking it very tightly, and then heating the bottle 

 till the water began to boil. He was obliged to do this very 

 gently, because steam expands with great power, and he did 

 not wish to drive out the cork or break his bottle. After a 

 little time the water ceased boiling, because the other half 

 of the bottle was full of steam, and there was no room for 

 more to form. But now the water began to grow hotter 

 and hotter, and rose above 100** C, showing that when the 

 heat could no longer form steam it did not remain hidden, 

 but increased the temperature of the water. At last, when 

 he was afraid to heat the bottle any more, he loosened the 

 cork, which flew out with great violence, followed by a cloud 

 of steam. And now notice what happened ; directly the 

 rush of steam was over, the heat of the water in the bottle 

 fell again to £00° C, for all the rest of the heat had been 

 used in forming more steam the moment the pressure was 

 removed. 



