Chap. 10.] Derbyjhire Spar. 59 



the mod phofphorefcent, but none of them exhibit this 

 quality except when well warmed. They are fcarcely 

 harder than common calcareous fpars, and therefore do 

 not ftrike fire with fteel. They do not melt by them- 

 felves j but very much promote the fufion of other 

 ftones, particularly the calcareous. They do not ef- 

 fervefce with acids, either before or after being fub- 

 mitted to the action of fire. 



Mr. Whitehurfl explains in the following manner 

 the formation of fparry and ftalactitical productions. 

 When water impregnated with the fparry acid proceeds 

 flowly through different ftrata of earths and minerals, 

 it becomes charged with a variety of fhefe fubftances 

 in folution ; and as it exudes gradually on the furface 

 of caverns and filfures, the aqueous particles evaporate, 

 and the fparry matter cryflallizes in various forms, 

 including in its own fubftance the heterogeneous mat- 

 ters with which it is charged. 



If the quantity of water thus impregnated, which 

 exudes through the pores of the earth or ftone, is not 

 more than will eafiJy evaporate in the ordinary heat: 

 of the atmofphere, a fparry cruft is formed. If the 

 quantity of water exuded exceeds the quantity evapo- 

 rated, flalactites are produced in one inftance, and tubes 

 in another. 



If a drop of water hangs from the roof, almoft 

 dropping, the aqueous particles evaporate from the 

 furface fooner than from its interior parts. A cryftal- 

 lization therefore takes place on the furface, while the 

 center remains fluid : the water thus detained is con~ 

 tinually increafmg, and the tube gradually extends 

 downwards. By this procefs, tubes are frequently 

 formed of two feet in length, and one tenth of an 

 inch in diameter. The appearance of caverns prna- 

 mented with thefe Iplcndid productions exceeds that 



of 



