CARBONATES. 375 



it fixed air.* Its composition was first demonstrated in 1772, by 

 Lavoisier, who, as already stated, proved that the diamond, by being 

 burned, becomes carbonic acid gas. 



12. NATURAL ORIGIN. 



Carbonic acid gas is formed abundantly by the respiration of ani- 

 mals; from our candles and lamps, and from our fire-places, and 

 from furnaces, from fermentation and putrefaction it is perpetually 

 rising into the air. It forms nearly half, T 4 /o j of the beds and moun- 

 tains of marble and limestone, and exists in various other natural car- 

 bonates, and abundantly in shells. Its fatal prevalence appears to be 

 prevented by the fact that vegetables during their growth decompose 

 this gas, absorbing its carbon for food, and liberating the oxygen to 

 recruit the waste of the atmosphere. 



The late Dr. Woodhouse, proved by many experiments, that when- 

 ever vegetables emit oxygen gas, it is from the decomposition of car- 

 bonic acid present in the air, and dissolved in the waters which they 

 imbibe. He justly rejected the idea that they give out oxygen gas 

 of themselves, or from the decomposition of water. f 



13. MEDICAL AND ECONOMICAL USES. 



It is highly salutary in the brisk and acidulous natural mineral wa- 

 ters, such as those of Saratoga and Ballston, and in imitations of them 

 by art, either with or without saline substances ; in fermented li- 

 quors, to which this agent imparts life and pungency, and in a de- 

 gree to all natural waters. It operates as a tonic, diuretic and an- 

 tiseptic remedy. It is said to be very useful in the hemorrhoids or 

 piles ; it is a reagent in the laboratory. 



CARBONATES. 



General facts and characters. 



Some of them have been long known, and were used before the 

 discovery of the power of carbonic acid to neutralize the alkalies. 



The carbonates effervesce with acids, and emit carbonic acid. 

 They are decomposed by heat, more or less violent ; the gas 

 being expelled, and the base remains.} Potassa, soda and lithia, 

 are exceptions. 



(c.) Alkaline carbonates turn the vegetable blues green, and have 

 an alkaline taste. 



(<?.) They are soluble in water, and the carbonates of the alkaline 

 earths become so by an excess of carbonic acid. 



* The miners, alluding to its effect on respiration, call it choke damp. 



t See 2d volume of Nicholson's Journal, 8vo. and an abstract in Mease's Domestic 

 Encyclopedia. 



t Charcoal is added to some of the carbonates before ignition, and aids in produ- 

 cing the effect ; sometimes by decomposing the carbonic acid itself. Baryta and 

 strontia are usually managed in this manner. 



it! 



