CHEMISTRY. 43 



water, which would not nourish plants, but on the contrary 

 prove destructive to their tender germs and roots : it absorbs 

 gases from the air as before stated ; prevents putrefaction (and 

 acidity to some extent,) in the soil, and is itself an indispen- 

 sable element in vegetation. 



Carbonic acid is a gas or air, which results from the com- 

 bustion of charcoal, when charcoal is burned, it nearly all 

 disappears in the form of gas, leaving only a small residue of 

 ash behind. Carbonic-acid-gas is heavier than common air, 

 colorless, invisible, having an agreeable pungent taste and 

 odor, but cannot be respired without poisonous effects resulting 

 from it. Carbonic acid may be obtained for experiment from 

 white marble, which is a carbonate of lime, or from common 

 limestone. The combining number of carbon is 6. 



It is neither combustible nor a supporter of combustion, a 

 lighted taper dipt into a jar this gas is instantly extinguished ; 

 it often exists in deep wells, mines, caverns and pits, and 

 proves fatal to those who enter them, the precaution should 

 therefore always be taken to let down a lighted candle, which 

 will determine the presence or absence of the gas. It is this 

 gas also which proves so deleterious in ill ventilated rooms 

 heated by coal fires. It is formed during the combustion of 

 all wood, coal and oil fires, it is generated by the respiration 

 of animals and the growth and decay of vegetation: it is 

 produced also, together with alcohol, during the fermentation 

 of sugar. It is evolved in vast quantities from the ground in 

 volcanic countries, and exists in combination with metallic 

 oxides in the earth : these compounds are called carbonates, 

 the most important of which is carbonate of lime. This gas 

 has an acid reaction : water dissolves its own volume of it, and 

 forms an agreeable sparkling solution: it is this gas which 

 escapes during the effervesence of soda water and various 

 kinds of beer. 



It is apparent that the excessive accumulation of so poi- 



