COLLECTION OF MINERALS. 287 



colour ; and, next to them, a blue crystal of the 

 quindecioc tonal variety, sixteen lignes thick, and 

 weighing nearly 4 oz. 2 drams ; on the shelf im- 

 mediately underneath, a reddish yellow crystal, 

 longitudinally striated ; another red, vulgarly 

 called Brazil ruby ; and a white topaz from the 

 same country, cut like a brilliant, and given to 

 the Museum by professor Geoffroy Saint-Hilaire. 

 This last variety is that called by the Portuguese 

 the water-drop and mina nova. Above and below, 

 on the same stage, are various groups of pale 

 yellow crystals, remarkable for their size. 



The greater number of stones, sold under the 

 name of Brazil rubies, and those which are called 

 by dealers burnt topazes, are merely topazes of 

 a reddish yellow, which have been exposed to 

 the action of fire to give them a fine rose-coloured 

 tint. 



The nitrate of potash, which we see in the 

 same case, is the salt called nitre, or saltpetre, 

 employed in the manufacturing of gunpowder; 

 which is a mixture of about six parts of nitre, 

 one of charcoal, and one of saltpetre. Aqua- 

 fortis is extracted from this substance, which is 

 the reason why it is also called nitric acid. 



After the nitrate of potash follow the species 

 which have soda for their basis ; and the most 

 important of which is the muriate of soda, or 



