82 PHYSIOGRAPHY. [cHAP. 



wood-charcoal, animal-charcoal, &.c. During all the processes 

 of combustion, respiration, and decay, this carbon combines 

 with the oxygen of the air to form carbonic acid, and hence 

 this gas is constantly being poured into the atmosphere. 

 Breathe through a straw into a glass of clear lime-water, and 

 you will see that the liquid becomes milky as the carbonic 

 acid gas expired or breathed out from your lungs bubbles 

 through the previously limpid liquid. If you then pour 

 a little vinegar into the cloudy liquid, the milkiness imme- 

 diately clears up, because the acid dissolves the solid white 

 carbonate of lime which had been formed by your breath. 

 Carbonic acid gas is set free by the action of the vinegar ; 

 and, if there is enough of the solid carbonate in the lime- 

 water, you may actually see the gas escaping in little bubbles. 

 This bubbling, or effervescence, is likewise produced when 

 vinegar, or almost any other acid, is poured upon an egg- 

 shell or an oyster-shell, upon a piece of chalk or limestone 

 or marble. All these substances consist, in truth, of 

 carbonate of lime, and are decomposed by the acid with 

 evolution of carbonic acid gas. If Cleopatra ever dissolved 

 the pearl, as the story tells, or Hannibal softened the rocks 

 of the Alps with vinegar, a chemical decomposition was 

 effected exactly like that just described. In consequence of 

 the gas being thus, as it were, bound in various solid sub- 

 stances, its discoverer. Dr. Black of Edinburgh, bestowed 

 upon it the name oi fixed air. A taper plunged into this 

 air is at once extinguished, and an animal is suffocated. 

 Hence the great necessity of duly renewing the air in 

 dwelling rooms. And it is obvious that, the greater the 

 number of people in the room, and the greater the number 

 of gas-burners, lamps, or candles alight, the more need is 

 there of efficient ventilation. 



As carbonic acid gas is being constantly produced by 



