376 CARBONATES. 



(e.) They contain either one equivalent of acid to one of base, 

 and are then called carbonates ; or two of acid to one of base, and 

 are then called bi-carbonates. 



(/*.) Most of the carbonates exist native, and all may be formed 

 by passing carbonic acid gas through the base, suspended or dissolv- 

 ed in water. 



1. NAME AND HISTORY. 



(a.) In the shops called salt of tartar, salt of wormwood and pearl- 

 ashes, 



(b.) The carbonate of potassa was always considered as the pure 

 alkali, till Dr. Black discovered the error.f 



(c.) The alkalies as found in the shops, under the names of pearl- 

 ashes, sal soda and volatile alkali, have been called sub-carbonates, 

 and when saturated with carbonic acid they were called carbonates. 

 As it is ascertained that in the former state they consist of one equiv- 

 alent of alkali and one of base, and in the latter of two equivalents 

 of acid and one of base, the last is now called bi-carbonate and the 

 first simply carbonate. 



2. PREPARATION. For common purposes there is no occasion to 

 prepare this salt artificially, but for instruction or to attain greater 

 purity, it may be done, 



!a.) By deflagrating tartar with one eighth of pure nitre, 

 b.) Tartar may be calcined in a crucible, which destroys the tar- 

 taric acid ; lixiviation and evaporation give about one third part of 

 dry carbonate. 



(c.) Nitre being mixed with one fourth of dry powdered charcoal, 

 find thrown into a red hot crucible, both acids are destroyed, and the 

 alkali obtained amounts to rather less than one half of the nitre em- 

 ployed. J The alkali obtained from tartar may be made to crystal- 

 lize, and the crystals contain carbonic acid 22, 1 proportion ; potassa 

 48, 1 proportion; water 18, 2 proportions, =88, the equivalent. 



(d.) Caustic potash absorbs carbonic acid gas with avidity, and 

 when saturated, and evaporated to dryness, it forms the carbonate of 

 potash, containing, according to an average of three analyses, car- 



* For the natural history of the carbonate, see potassa. In vegetables, it is prob- 

 ably combined, for the greater part, with acids, which being destroyed by the 

 fire, carbonic acid is thus formed and unites to the alkali. 



t It has been already mentioned that the old chemical books describe efferves- 

 cence with acids, as a test of alkalies ; whereas this property belongs to their 

 carbonates. Dr. Black first proved that this is their common state, that the carbonic 

 acid greatly allays their acrimony, and that they are caustic only when deprived of it. 



t A little sulphate and muriate of potassa, and a little silica, are apt to remain in 

 the alkali thus prepared, and it is difficult to remove them. 



