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may alfo be produced by the clrcumflances 

 of the atmofphere ; as, fhould a ftrong 

 wind, of a very cold temperature, blow at 

 the time. The melted lavas in our cru- 

 cibles will be found to give greater weight 

 to this latter conje(^ure. If they are 

 taken from the furnace, and caufed to pafs 

 through a heat gradually lefs ; their fur- 

 face, as they cool, will only fplit in a few 

 cracks, of little depth, arid ufually irregu- 

 lar i but, when they are immediately, in 

 the winter time, carried into the cold air, 

 the fiffures, befides being deeper, will fre- 

 quently be difpofed in luch a manner, as 

 to form fmall polyhedrous prifms, which 

 may eafily be detached from the reft of the 

 lava. 



With rcfpe^t to thole lavas which do 

 not affume a prifmatic form though they 

 fall into the fea, it is certain that, to take 

 that conformation, their mafs muft have a 

 ftrong degree of effervefcence and dilatation, 

 and that it muft be deeply penetrated with 

 the igneous fluid, otherwife the contradion 



neceifary 



