88 CHEMISTRY. 



bonic anhydride be present in the air to the amount of one 

 tenth of the whole, its poisonous influence is very manifest. 

 And even when it is present only in the small quantity of 

 one or two per cent., bad effects show themselves on breath- 

 ing such an air for some little time. This gas is always 

 present in the atmosphere, but in so very small amount, as 

 you will see in the next chapter, that it produces no effect 

 as a poison. 



108. Carbonic Acid in the Stomach. While this gas is thus 

 a poison in the lungs, it is far otherwise in the stomach. 

 It produces an agreeable tonic effect there, as it is intro- 

 duced in effervescing drinks. The chemistry of these two 

 organs is different. The lungs can use oxygen chemically 

 to advantage ; while carbonic anhydride, which is a deadly 

 poison to the blood in the lungs, is beneficial in the stomach, 

 or at least is not injurious there. 



109. Absorption of Carbonic Anhydride by Liquids. Wa- 

 ter readily absorbs or dissolves about its own bulk of car- 

 bonic anhydride. By means of pressure it can be forced to 

 absorb more than this, the amount absorbed being in propor- 

 tion to the amount of pressure. Thus " soda-water " is com- 

 monly only water into which a large quantity of carbonic 

 anhydride has been forced, and the effervescence is owing to 

 the escape of this gas on taking off the pressure. The wa- 

 ters of many natural springs have considerable of this gas, 

 which, being generated under pressure in the earth, is there- 

 fore largely dissolved in the water, and, escaping from this 

 pressure as the water issues forth, causes an effervescence. 

 In beers and sparkling wines the carbonic anhydride is 

 made by the fermentation of the liquid as it is confined. 

 The bursting of bottles, which sometimes occurs, is pro- 

 duced by the generation of too large an amount of this gas, 

 or by its undue expansion by exposure to heat. 



110. No True Carbonic Acid. Carbonic anhydride is one 



