ALKALIES. 251 



SEC. III. SODA. 



1. NAMES. The caustic soda was always, and is still unknown 

 to commerce ; anciently, the carbonate was called natron, natrum 

 and nitrum, whence the nitre of the Scriptures. It is mentioned in 

 the Bible, as a detergent, and as disagreeing (effervescing ?) with vin- 

 egar ; both of which qualities belong to the carbonate of soda, but 

 neither of them to nitre. In Africa, they call it trona; on the shores 

 of the Mediterranean, soda and barilla. It has been called marine 

 and mineral alkali. The term soda is now universally used. 



2. HISTORY. Indicated by Geber, an Arabian chemist, in the 

 ninth century, but confounded with potash till after the middle of the 

 last century; and unknown in its pure state until the discovery of the 

 carbonic acid. Effervescence with acids was formerly considered 

 as characteristic of soda as well as of the other alkalies, but it be- 

 longs to them in the state of carbonate only, and not in the pure state. 



3. POINTS OF SIMILARITY BETWEEN IT AND POTASSA. 



(a.) Their history is so nearly the same, that it is necessary only 

 to indicate the difference. 



(b.) All that respects the preparation is identical, and their prop- 

 erties are very similar. 



4. SODA ORIGINATES FROM *MARATIME AND MARINE PLANTS, the 



algae fuci, salsola soda, &LC. : the plants are dried, burned and lixivi- 

 ated, and the lixivium evaporated to dryness. The crude soda of 

 commerce, called barilla, is the incinerated salsola soda: kelp, a 

 coarser variety, is the incinerated sea weed, and often contains only 

 from 2 to 5 per cent of alkali ; white good barilla contains 20 per 

 cent. The crystallized carbonate of soda of commerce is obtained 

 either from the calcination of the sulphate with charcoal and chalk in 

 a reverberatory furnace, or by decomposing the muriate of soda by 

 carbonate of potash. Ure. 



5. PROPERTIES. 



(a.) Caustic soda is at first deliquescent in the air, like potassa, 

 but unlike that alkali it never runs into the consistency of an oily fluid ; 

 for it soon becomes efflorescent, from combination with the carbonic 

 acid contained in the atmosphere : a change which potash never un- 

 dergoes. 



(b.) Caustic soda is in the form of gray sub-crystalline masses, 

 which can scarcely be distinguished from potassa, by the eye or by 

 any sensible properties. 



6. The force of attraction in soda for the acids, is inferior to that 

 of potassa: the soda salts are decomposed by potassa. 



* Salsola is a maratime plant, (i. e. it grows on the sea shore,) but the algae are 

 marine; the carbonate of soda of truly marine plants only, yields iodine. J. T. 



