272 FORMATION OF BASALTES. 



caloric that are flying off, and those of the substance itce}?, 

 \\ inch tend 10 unite. 

 Met 1 ad - ^ t ' ,ls bUOStailCe oe & good conductor of heat, and the 

 allycoul-jd attractive sower of its particles equal the expansive power 

 of the caloric, it will return to its natural state wilhont 

 ige of fornl. Such is a metal in fusion^ which is left to 

 cool graduallj T . 

 Sd frlni cast ^' lllK ' er tne saine circumstances, the caloric, rapidly ab*. 

 ma. sorbed, is separated in succeftive strata, this substance will 



separate into planes, which will be perpendicular to the di- 

 rection the caloric takes to escape. Such is the case witli 

 cast iron in fusion, the surface of which is wetted to separate 

 thin cakes from it: a cause analogous to what may have di- 

 vided basaltes into leaves, or thin strata, either perpendi- 

 cular to the axes of the prisms, or around a globular uw\ss. 

 Tempering of If, the motion of the caloric being uniform, the attrai> 

 - tcei - tion of cohesion do not equal the expansile power, the par- 



ticles of the substance will remain dilated; and then, if it 

 be a good conductor of heat, they will maintain their situa- 

 tion without experiencing any division. Such is the effect 

 of tempering steel, where the cohesive power of the parti- 

 cles of iron is broken by the interposition of carbon. But 

 fjuartz broken if the substance.be not a good conductor of heat, it will 

 fall to powder ; as quartz strongly heated and immersed in 

 water. The first of these circumstances has perhaps nevi*r 

 Occurred in volcanic productions, but the other must have 

 been very frequent. 

 Prismatic di- To obtain prismatic divisions, let us suppose a basaltic 

 tisions. mass still in its pasty state covering a considerable plain ; 



and which, yielding to the laws of gravity, adheres strongly 

 to the base that supports it. Then, if the expansive forte 

 of the igneous or aqueous gasses happen to cease in conse- 

 quence of their sudden or accelerated extrication, the par- 

 ticles, losing their fluid state, will tend to approach each 

 other, yielding to the laws of gravitation, and also obeying 

 the attraction of cohesion that they exert toward each other ; 

 and jhey cannot contract, but by following the diagonal di- 

 rection resulting from these two powers. But the extent Of 

 this mass, its gravity, and the inequality of surfaces, op- 

 posing a general contraction like that which is experienced 



by 



