274 THEORY of tie EARTH. 



THE general tendency of heat is to produce fluidity and 

 foftnefs ; as that of cold is, on the contrary, to harden foft 

 and fluid bodies. But this foftening power of heat is not uni- 

 form in its. nature) it is made to ac"l with very different effect, 

 according to the nature of the fubftance to which it is applied. 

 We are but' limited in the art of increafing the heat or the cold 

 of bodies ', we find, however, extreme difference in their fub- 

 ftances with refpecl: to fufibility. 



A FUSIBLE fubftance, or mineral compofition in a fluid ftate, 

 is emitted from thofe places of the earth at which fubterrane- 

 ous fire and expanfive force are manifefled in thofe eruptive 

 operations. In examining thefe emitted bodies, men of fcience 

 find a character for fuch productions, in generalizing the fub- 

 ftance, and underftanding the natural conftitution of thofe bo- 

 dies. It is in this manner, that fuch a perfon, finding a piece 

 of lava in any place of the earth, fays with certainty, Here is a 

 ftone which had congealed from a melted ftate. 



HAVING thus found a diftinguifhing character for thofe 

 fufed fubftances called, in general, lavas, and having the moft 

 vifible marks for that which had been actually a volcano, natu- 

 ralifts, in examining different countries, have difcovered the 

 moft undoubted proofs of many ancient volcanos, which had 

 not been before fufpedled. Thus, volcanos will appear to be 

 not a matter of accident,- or as only happening in a particular 

 place, they are generaHo the globe, fo far as there is no^ place 

 upon the earth that may not have an eruption of this kind ; al- 

 though it is by no means necefiary for every place to have had 

 thofe eruptions. 



VOLCANOS are natural to the globe, as general operations ; 

 but we are not to confider nature as having a burning moun- 

 tain for an end in her intention, or as a principal purpofe in 

 the general fyftem of this world. The end of nature in placing 

 an internal fire or power of heat, and a force of irrefiftible ex- 

 panfion, in the body of this earth, is to confolidate the fediment 



collected 



