SECT, xxv.] STEAM. 261 



thaws, is a consequence 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 

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

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

 ing to the same principle, vapour is a combination of caloric 

 with a liquid. By the continued application of heat, liquids 

 are converted into vapour 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 tempera- 

 ture 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 

 proportion to the temperature at which the water boils ; it 

 may therefore be increased to a degree that is only limited 

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

 that has been made subservient to th wants of man. 



It is found that the absolute quantity of heat consumed in 

 the process of converting water into steam is the same at 

 whatever temperature water may boil, but that the latent 

 heat of steam is always greater exactly in the same propor- 

 tion as its sensible heat is less. Steam raised at 212 under 

 the ordinary pressure of the atmosphere, and steam raised at 



