52 PHILOSOPHICAL TRANSACTIONS. [aNNO i684. 



the fields, insomuch that the Egyptians must take as much pains to lessen the 

 fatness of their land, as other nations do to encrease the fatness of theirs.* 



In mechanics we have this account of it: it is said by Pliny, cap. de Vitri In- 

 ventione, that a company of merchants being thrown upon a shore where there 

 were not any stones to be found, were forced to take great pieces of Egyptian 

 nitre [natron] out of their ships, and make walls, upon which they hung their 

 boiling kettle ; the nitre being heated by the fire, mixed with the sand, and ran 

 into several streams of glass, which afterwards hinted the way of making glass. 

 It is likewise of use in dyeing; for Pliny and Vitruvius affirm, that by the help 

 of this, the true azure is made, and that without this, there cannot be a true 

 shadow. 



In the last place I come to consider wherein it differs from saltpetre, and sal 

 ammoniac. It may be distinguished from saltpetre, first by its fermenting : it 

 will ferment with any acid ; but saltpetre will not : I found that it would fer- 

 ment with vinegar, as the old commentators observe in their comments upon 

 Jeremiah and the Proverbs, but saltpetre will not: which gave occasion to some, 

 in those texts, to alter the word nitre. 2dly, It may be distinguished from 

 saltpetre-f- in its taste; for natron has a lixivial taste; but the other not. 3dly, 

 By the volatile spirit which it affords : for from the one comes over a volatile 

 alkali ;| but from the other a corrosive acid. 4thly, The natron affords a red 

 clammy substance, insipid; but the other not. This clammy substance, if I 

 mistake not, is that which by Pliny is called aerugo salis. This it has from the 

 earth ; and therefore it is again said by Pliny, sunt ibi nitrarias in quibus et rufum 

 exit a colore terras.^ 5tlily, Like saltpetre it will not crystallize.|| 6thly, In 

 the fire it makes no detonation. 



From sal ammoniac it may be distinguished, 1st by its colour; for the natron 

 is reddish, the other not; 2dly, by the texture of its parts; in sal ammoniac 

 the parts seem close and firmly knit together, but the natron is spongy and per- 

 forated ; 3dly, if mixed with sal ammoniac, this emits the same spirit as when 

 it is mixed with quicklime.^ 



* It is not easy to see what proof this observation affords of the use of natron in agriculture, 



f Saltpetre is formed of nitric acid and potash ; this salt (natron) of carbonic acid and soda. 



+ The volatile alkali here mentioned is an extraneous additament. 



§ What this red clammy substance is, has been mentioned in one of the preceding notes. 



U It crystallizes, but the crystals differ in their form from those of nitre. 



f Sal anrmioniac is formed of muriatic acid and ammonia ; natron is formed of carbonic acid and 

 soda. By trituration with this carbonate of soda (natron) a decomposition of the muriate of ammonia 

 (sal ammoniac) is effected ; the muriatic acid leaving the ammonia to combine with the soda, to 

 which it has a greater affinity. The ammonia (vol. alkali) thus disengaged rises up in the form of 



