26

CARBONIC ACID GAS.

seizes upon tlie base and sets the carbonic acid gas atliberty.

74. We have already spoken of the conversion ofchalk into lime by heat, in which process the chalkis decomposed, certain matters being expelled ordriven off, and the lime left caustic. Chalk is a com-pound of carbonic acid gas and caustic lime, togetherwith a quantity of water, and is called a carbonateof lime.

75. The carbonic acid is combined with the lime byso weak an attraction, that heat alone is sufficient toexpel the acid, together with the water which the chalkcontains. If a piece of chalk is put into some vinegar,or indeed into any sour liquid, the chalk will be decom-posed, and the carbonic acid will bubble through the fluiduntil the vinegar or other acid is fully combined withlime, and its acid powers entirely neutralized.

76. In consequence of this gas being rather heavierthan common air, it frequently collects in caverns,cellars, and other similar situations, and often occasionsserious accidents, suffocating those who unfortunatelyenter the places thus filled with carbonic acid. Itspresence in such places in the air, in any quantity, mayalways be easily ascertained by letting down a lightedcandle into the well or cellar : if the candle continues toburn, we know that there is enough oxygen present tosupport the life of an animal ; and if the candle is ex-tinguished we are certain that the place is full of carbonicacid gas, and therefore that it must not be entered until