68 



alkali, to which thej improperly give the name 

 of nitre. 



Besides common alum, and that called the 

 plumed,* which are found in many parts of Chili, 

 a semi-crjstallized aluminous stone has been 

 discovered in the Andes. This stone, called by 

 the inhabitants i-)olcura, is brittle and of a pale 

 white, of a very fine grain and a vitriolic taste; 

 its external appearance is like that of white mar- 

 ble, but it contains no calcareous particles, nor 

 is it in reality any thing but a clay saturated with 

 vitriolic acid, analagous to the aluminous stone 

 of Tolfa. It is useful in dying, and the quar- 

 ries from whence it is procured comprehend a 

 space of many leagues among the mountains, 

 which also afford another stone in some measure 

 resembling it, but coarser and of much less 

 value. Its yellow hue, and the quantity of sul- 

 phur and pyrites it contains, distinguish this last 

 from the real polcura, which is very pure, and 

 not combined with any metallic substance. 



The four principal kinds of vitriol, the green 

 or 'won, the blue or copper, the white with a 

 zinc base, aiul the mixed, are found in a stalactite 

 or crystallized state as well as that of efflorescence 

 in the mines, and even isolated in difl'erent earths ; 

 the metallic substances which produce it being, 



* This name is given to a species of talc, coiinsllng of ft hi' 

 yncnts, otherwise called the asbestos i/oe.Dictioniiiiire do 

 1 "Academic. 



