212 WELLS'S NATURAL THILOSOPHT. 



HoTf do we Independently of the sun, however, the earth is a source of 

 know that the heat. The proof of this is to be found in the fact, that as we 

 of'h^'tr^'""^'^ descend into the earth, and pass beyond the hmitsof the mfiu- 

 ence of the solar heat, the temperature constantly rises. 



At what rate 477. TliG increasG of temperature observed as 

 peratureoA'he wG (lesceiid iiito the earth, is about one degree 

 earth increase f. ^^ ^j^^ thermometerfor every fifty feet of descent. 



Supposing the temperature to increase according to this ratio, at the depth 

 of two miles water would bo converted into steam ; at four miles, tin would 

 be melted ; at five miles, lead ; ana at thirty miles, almost every earthy sub- 

 stance would be reduced to a fluid state. 



The internal heat of the earth does not appear to have any sensible effect 

 upon the temperature at the surface, being estimated at less than 1-3 0th of 

 a degree. The reason why such an amount of heat as is supposed to exist in 

 the interior of the earth does not more sensibly affect the surface is because 

 the materials of which the exterior strata or crust of the earth is composed, 

 do not conduct it to the surface from the interior. 



Under what ^78. When electricity passes from one sub- 



ii de^Hcity^a stancG to anothcr, the medium which serves to 

 source of heat? conduct it is vGry frequently heated ; but in 

 what manner the heat is produced we have no positive in- 

 formation. 



The greatest known heat with which wo are acquainted, is thus produced 

 by the agency of the electric or galvanic current. All kno\vn substances can 

 be melted or volatilized by it. 



Heat so developed has not been employed for practical or economical pur- 

 poses to any great extent ; but for philosophical experiments and investiga. 

 tions it has been made quite useful. 



How is chem- 479. Many bodies, when their original con- 

 '^Vceof hLit^? stitution is altered, either by the abstraction 

 of some of their component parts, or by tlie 

 addition of other substances not before in combination 

 with them, evolve heat while the change is taking place. 



In such cases, the heat is said to be due to chemical ac- 

 tion. 



What is chem- 480. We apply the term chemical action to 

 icai action? thosc Operations, whatever they may be, by 

 which the form, solidity, color, taste, smell, and action of 

 substances become changed ; so that new bodies, with 

 quite different properties, are formed from the old. 



A familiar illustration of the maimer in which heat is evolved by chemical 



