328 Mr Haidinger , s Notice respecting Trona, 



in the mineralogical works of the present clay, that the natrum 

 from the lakes in Egypt, is sufficiently hard and compact to 

 allow walls to be constructed of it, as in a fort, near the Na- 

 trum Lakes, called Qasrr or Cassr, which is now abandoned.* 

 But, as this is ascribed to an. admixture of muriate of soda, 

 and may be ascribed to a similar admixture in Pliny, it does 

 not form an undoubted synonym of the species. Yet the co- 

 incidence of the accounts of this author, of houses being built 

 of salt by the Hammanientes,-f- the Amantes of Solinus,;J; a 

 nation carrying on trade with the Troglodytes, with the exr 

 istence of a fort built of soda, is remarkable enough. Be- 

 sides, Pliny comprises many substances under the name of 

 nitrum, which are essentially different ; Dr Kidd has already 

 observed, § that some of the Egyptian nitrum, which cake as- 

 persum redd'it odorem vehementem, must be sal ammoniac, and 

 that often it means also our nitre. It appears that all the 

 efflorescent salts were called nitrum, comprehending sulphate 

 of soda, sulphate of magnesia, and others ; nay, the passage 

 in Pliny, nam quercu cremata nunquam- multum Jhctituliim 

 est, ct jampridem in totum omisswm, seems also to include pot- 

 ash, though this is likewise enumerated among the methods 

 of obtaining salt, querctis optima, ut qua? per se cinere syncero 

 vim salis reddat. 



Among the modern authors, the earliest, and at the same 

 time one of the most detailed accounts was given by Dr 

 Donald Monro, || the first who pointed out, that the pure 

 native crystallised natron occurred in some of the inland 

 parts of Tripoli in Barbary. The salt is there stated to 

 " run in thin veins, of about half an inch, or a little more, 

 thick in a bed of sea-salt ; for all of it that has hitherto been 

 imported into this country, is covered with sea-salt on each 

 side. The one side is always smoother than the other, and 

 appears as if it had been the basis on which it rested ; the 

 other, which should seem to be the upper side, is rougher, by 

 the shooting of the crystals. The pieces of the thin veins ap- 

 pear almost as if the salt had been dissolved in water, and af- 



• Klaproth's Essays, vol. ii. p. 62. f Cap. xxx. 



+ Libr. v. cap. v. vol. i. p. 251. 



§ Outline* of Mineralogy, vol. ii. p. 6. || Phi/. Trans- 1773, p- :>G1. 



