306 . CARBON. 



Potassium heated in a small glass bulb blown upon a tube, 

 through which gaseous carbonic acid is transmitted, undergoes 

 oxidation, and liberates carbon, the existence of which in the 

 gas may thus be shown. But burning phosphorus, sulphur and 

 other combustibles are immediately extinguished by carbonic 

 acid, and the combustion does not cease from the absence of 

 oxygen only, but from a positive influence in checking combus- 

 tion which this gas exerts, for a lighted candle is extinguished 

 in air containing no more than a fourth of its volume of carbonic 

 acid. It is generally believed that any mixture of carbonic acid 

 and air will support the respiration of man, which will main- 

 tain the flame of a candle, and therefore a lighted candle is often 

 let down into wells or pits suspected to contain this gas, to as- 

 certain whether they are safe or not. But although air in which 

 a candle can burn may not occasion immediate insensibility, 

 still the continued respiration for several hours of air containing 

 not more than 1 or 2 per cent of carbonic acid, has been found 

 to produce alarming effects (Broughton). The accidents from 

 burning a pan of charcoal in close rooms are occasioned by this 

 gas. It acts as a narcotic poison upon the system. A small 

 animal thrown into convulsions from the respiration of this gas, 

 may be recovered by sudden immersion in cold water. 



Carbonic acid is thrown off from the lungs in respiration, as 

 may be proved by directing a few expirations through lime 

 water. The air of an ordinary expiration contains on an average, 

 as observed by Dr. Prout, 3.45 per cent of the gas, and the pro- 

 portion varies from 3.3 to 4.1 per cent, being greatest at noon, 

 and least during the night. Carbonic acid is also a product of 

 the vinous fermentation, and is the cause of the agreeable pun- 

 gency of beer, ale and other fermented liquors, which become 

 stale when exposed to the air from the loss of this gas. It also 

 exists in all kinds of well and spring water, and contributes to 

 their pleasant flavour, for water which has been deprived of its 

 gases by boiling is insipid and disagreeable. Carbonic acid is 

 also largely produced by the combustion of carbonaceous fuel, 

 and appears to exist in considerable quantity in the earth, being 

 discharged by active volcanoes, and from fissures in their 

 neighbourhood long after the volcanoes are extinct. The Grotto 

 del Cane in Italy owes its mysterious properties to this gas, and 

 many mineral springs, such as those of Tunbridge, Pyrmontand 

 Carlsbad are highly charged with it. It comes thus to be always 



