SECT. XXVII. CONVERSION OF WATER INTO STEAM. 277 



of electricity discharged, and is due to its excessive velocity 

 and the violent compression of the air in its transit through 

 the atmosphere. Prodigious quantities of heat are constantly 

 absorbed or disengaged by the changes to which substances are 

 liable in passing from the solid to the liquid and from the 

 liquid to the gaseous form and the contrary, causing endless 

 vicissitudes of temperature over the globe, and endless ex-, 

 pansions and contractions, which are correlative terms for heat 

 and cold, while radiation of heat is merely a transfer of motion 

 from the particles on the surface of bodies to the adjacent 

 particles of the atmosphere. 



By the continual application of heat, that is of the expansive 

 force, liquids are converted into steam or vapour, which is 

 invisible and highly elastic. Under the mean pressure of the 

 atmosphere, that is when the barometer stands at 30 inches, 

 water in a boiler absorbs heat continually till it attains the 

 temperature of the boiling point, which is 212 Fahrenheit. 

 After that it ceases to show any increase of sensible heat ; but 

 when it has absorbed an additional 1000 of heat or expansive 

 energy, that energy converts it into steam, and a condensing 

 force equivalent to 1000 of heat reduces it again to water. 

 Water boils at different temperatures under different degrees of 

 pressure. It boils at a lower temperature on the top of a 

 mountain than on 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, for the intensity of the expansive force prevented 

 from having effect by the extreme pressure of the boiler would 

 be converted into sensible heat which might eventually render 

 the water red hot. Thus, since the force of steam is in pro- 

 portion to the temperature at which the water boils, or to the 

 pressure, it is under control, and, perhaps with the exception of 

 electricity, it is the greatest power that has been made sub- 

 servient to the 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 absolute heat 

 of the steam is greater exactly in proportion as its sensible heat 

 is less. Thus, steam raised at 212 Fahrenheit under the mean 



