SOURCES OF HEAT. 213 



action 'm to be found in the experiment of pouring cold water upon quick- 

 lime. The water and tlie lime combme together, and in so doing liberate a 

 great amount of heat, sufficient to set lire to combustible substances. 



How is heat 481. Hcat is always evolved when a fluid is 

 ciwng'ifof form transformed into a solid, and is always ab- 

 ia matter? sorbed wben a solid is made to assume a fluid 

 condition. As water is changed from its liquid form when 

 it is taken up by quicklime, therefore heat is given ofi". 



The heat produced by the various forms of combustion, is the result of chem- 

 ical action. 



, , , 482. Heat exists in two verv difi'ercnt con- 



In what two 



conditions dots ditious, as I'ree, or Sensible Heat, and as 



heat enst ? ' ' 



Latent Heat.* 

 whatissensi- 483. Whcu thc hcat retained or lost by a 

 bieheat? Lody is attended with a sense of increased or 



diminished warmth, it is called sensible heat. 

 ■What is latent 484. Wheii the heat retained or lost by a 

 ^^^'^ body is not perceptible to our sense, it is called 



latent heat 



„ , Every substance contains more or less of latent heat. Al- 



How do we ■' 



know heat to though our senses give us no direct information of its pres- 

 exist in^a body^ ence, we may, by a variety of experiments, prove that it ex- 

 percelye it? ists. Thus, the temperature of ice is 32° by the thermometer, 



but if ice be melted over a fire and converted into water, the 

 water will be no hotter than the ice was before, although in the operation 

 140 degrees of heat have been absorbed by the water. "VThen, on the contrary, 

 water passes into ice, a large amount of hcat which was before latent in the 

 water, passes out of it, and becomes sensible.^ 



485. Another important source of heat is 



How is me- i • i • i i • 



chanir-ii action mechanical action, heat being produced by 



a source of heat ? p.. I'l ^ . 



iriction and Dy the condensation, or compres- 

 sion of matter. 



What are illus- Savage nations kindle a fire by the friction of two pieces 



trations of the of dry wood ; the axles of wheels revolving rapidly frequent'y 

 hoat"by°"fric- ^e'^O'"© ignited ; and in the boring and turning of metal tha 

 tiou? chisels often become intensely hot. In all these cases the 



friction of the surfaces of wood or of metal in contact, dis- 

 turbs the latent heat of these substances, and renders it sensible. 



The following interesting experiment was made by Count Rumford, to il- 



• Latent, from the Lntin word lltfo, to be hid. 



t The phenomena of latent heat are further considered under the head of liquefaction. 



