240 EIGHTEENTH CENTURY. PT. HL 



CHAPTER XXVIII. 



SCIENCE OF THE EIGHTEENTH CENTURY (CONTINUED). 



Dr. Black on Latent Heat Watt's Application of the Theory to the 

 Steam-engine Early History of Steam-engines Newcomen's 

 Engine Watt's Engine Watt and Houlton. 



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 hcat^ or of heat which becomes lost or 

 hidden whenever a solid is turned into a liquid, or a liquid 

 into a gas. 



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 o 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 ot this heat? Again, if you keep the 

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

 and hotter till it reaches 100 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 100 C. So 



