CH. XXVIII, DK. BLACK. 241 



CHAPTER XXVIII. 



SCIENCE OF THE EIGHTEENTH CENTURY (CONTINUED). 



"Ooctrine of Latent Heat, taught by Dr. Black in 1 760 — Water con- 

 taining Ice remains always at 0° C, and Boiling Water at 100° C, 

 however much Heat is added — Black showed that the lost Heat is 

 absorbed in altering the condition of the Water — Watt's Applica- 

 tion of the Theory of Latent Heat to the Steam-engine — Early His- 

 tory of Steam-engines — Newcomen's Engine — Watt invents the 

 Separate Condenser — Diagram of Watt's Engine — Difficulties of 

 Watt and Boulton in introducing Steam-engines. 



Discovery of Latent Heat by Dr. Black in 1760. — ^We 



must now go back a few years, to the time when Dr. Black 

 was lecturing at Glasgow in 1760; for he then made a 

 remarkable discovery about heat, which belongs to the 

 history of physics rather than of chemistry. This was 

 the discovery of latent heat, or of heat which becomes lost 

 or hidden whenever ice is turned into water, or water into 

 steam. 



If you put a lump of ice in a saucepan on a stove, and 

 when it begins to melt stir it gently so as to keep the water 

 well mixed, you will find that so long as the smallest piece of 

 ice is left in the water, a thermometer standing in the sauce- 

 pan will not rise higher than 0° Centigrade, or the melting- 

 point of ice. Now the heat from the stove must be con- 

 tinually entering the water, otherwise the ice would not melt. 

 What then becomes of this heat ? Again, if you keep the 

 water on the stove after the ice is melted, it will grow hotter 

 12 



