COLLECTION OF MINERALS. 289 



under the name of natron > and was obtained 

 by the evaporation of the waters of certain rivers 

 or lakes, especially in Egypt, where it is in great 

 abundance. In Europe it is found in an efflo- 

 rescent form on the surface of the ground, or on 

 the sides of old walls ; it is also found in great 

 quantities in the ashes of several vegetables. 

 It is employed in manufacturing • glass, in the 

 composition of hard soap, and is also used in 

 medicine. 



The muriate of ammonia, which we see in the 

 seventeenth case, is better known by the name 

 of sal ammoniac. It is found amongst volcanic 

 productions. There is a specimen in this case of 

 a concrete form, from Vesuvius. It is now ex- 

 tracted from putrefied animal matter, and is em- 

 ployed in the arts for plating and soldering 

 metals. 



Under the name of alkaline sulfate ofalumine, 

 we here find alum, that substance so useful to 

 dyers, who make use of it to fix and strengthen 

 colours. It is only found in nature in small 

 quantities in the form of filaments, to which the 

 name of plumose alum has been given. We re- 

 mark a fine specimen of this fibrous variety, 

 which the celebrated Tournefort brought from 

 the island of Miio, in iyo3. The alum, winch 

 is sold in commerce, is obtained by washing sub- 



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