SOURCES OF CARBONIC ACID. 115 



forms carbonic acid, together with the vapour of water, which 

 is produced by the union of the hydrogen with the oxygen of 

 the air. 



159. Carbonic acid gas is also given off by all Animals, which 

 form it during the process of respiration or breathing ; for a por- 

 tion of the oxygen which is taken into the lungs, is there com- 

 bined with carbon, which the animal system wants to throw off, 

 and then breathed out again in the form of carbonic acid (See 

 ANIM. PHYSIOL. Chap. vi.). The presence of this gas in the 

 air returned from the lungs, is easily shown by t breathing out 

 through a tube, the end of which is immersed in a deep glass 

 containing lime-water, which is water having a small quantity 

 of quick-lime dissolved in it. After the lungs have been a few 

 times emptied through this tube, the water becomes quite turbid, 

 by the union of the carbonic acid with the lime, and the conse- 

 quent formation of a carbonate of lime ; which, not being soluble 

 in water, falls as a white powder, resembling pounded chalk. It 

 will be hereafter shown that Vegetables, like animals, form car- 

 bonic acid during the whole course of their lives ; by causing the 

 oxygen of the air, surrounding them, to combine with the carbon 

 which they have to give off (Chap. vin.). 



160. Now the carbonic acid thus combined with the lime, 

 might be separated again by heat, or in other modes. If we 

 pour a little vinegar upon limestone or chalk, a bubbling or 

 effervescence is produced ; which is caused by the 'acid of the 

 vinegar combining with the lime, for which it has a greater 

 attraction than has the carbonic acid ; and as they cannot both 

 be combined with the lime, the latter is set free. It is liberated, 

 also, in large quantities, during the process of fermentation; and 

 this it is, which renders it dangerous to walk over a vat in which 

 fermentation is going on, and which extinguishes a candle held 

 in such a situation. Putrefaction, too, is a kind of fermentation ; 

 and carbonic acid is given off in this process. In fact, the re- 

 spiration of Plants and Animals may be regarded as designed to 

 carry off the carbonic acid, produced by a kind of slow putrefac- 

 tion or decomposition, which is always going on within the 

 body : for it is a peculiar characteristic of the compound eub- 



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