242 EIGHTEENTH CENTURY. PT. m. 



mixture still remained at o, 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 7 9 

 and 80. 



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

 see (Chapter XXXV.) in the history of the science of the 

 nineteenth century ; but the first step was to prove its dis- 

 appearance 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 the outward pressure of steam increases 

 very rapidly as the temperature rises, 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 100 C., for all the rest of the heat had 

 been used in forming more steam the moment the pressure 

 was removed. 



James Watt, 1736-1819. Black had now completed 

 his discovery, . and from that time he taught in all his 

 lectures that heat becomes latent or absorbed when a solid 



