SKCT. XXV. LATENT HEAT. 227 



is the principal cause of its fluidity. Ice remains at the 

 temperature of 32 of Fahrenheit till it has combined 

 with or absorbed 140 of caloric, and then it_melts, but 

 without raising the temperature of the water above 32 ; 

 so that water is a compound of ice and caloric. On 

 the contrary, when a liquid is converted into a solid, a 

 quantity of caloric leaves it without any diminution of 

 temperature. Water at the temperature of 32 must 

 part with 140 of caloric before it freezes. The slow- 

 ness with which water freezes, or ice thaws, is a con- 

 sequence of the time required to give out or absorb 140 

 of latent heat. A considerable degree of cold is often felt 

 during a thaw, because the ice, in its transition from a 

 solid to a liquid state, absorbs sensible heat from the atmos- 

 phere and other bodies, and by rendering it latent main- 

 tains them at the temperature of 32 while melting. Ac- 

 cording to the same principle, vapor is a combination of 

 caloric with a liquid. By the continued application of 

 heat, liquids are converted into vapor or steam, which 

 is invisible and elastic like common air. Under the 

 ordinary pressure of the atmosphere, that is, when the 

 barometer stands at 30 inches, water acquires a constant 

 accession of heat till its temperature rises to 212 of 

 Fahrenheit ; after that it ceases to show any increase 

 in heat, but when it has absorbed an additional 1000 of 

 caloric it is converted into steam. Consequently, about 

 1000 of latent heat exists in steam without raising its 

 temperature, and steam at 212 must part with the same 

 quantity of latent caloric when condensed into water. 

 Water boils at different temperatures under different 

 degrees of pressure. It boils at a lower temperature 

 on the top of a mountain than in the plain below, 

 because the weight of the atmosphere is less at the 

 higher station. There is no limit to the temperature 

 to which water might be raised ; it might even be made 

 red-hot, could a vessel be found strong enough to resist 

 the pressure. The expansive force of steam is in pro- 

 portion to the temperature at which the water boils ; it 

 may therefore be increased to a degree that is only lim- 

 ited by our inability to restrain it, and is the greatest 

 power that has been made subservient to the wants of 

 man. 



