MATERIALS WHICH PLANTS DERIVE FROM THE AIR. 31 



24. Carbonic acid gas constitutes but a small part of the 

 atmosphere, 5,000 gallons of air containing only about two 

 gallons of it ; but it is produced in enormous quantities by 

 various operations in nature : thus, it is formed by the burn- 

 ing of the coal consumed in our fires, the process of burning 

 consisting merely in the union of the combustible matter of 

 the vegetable substance, whether it be the remains of ancient 

 forests as coal, or the wood which is at present growing on 

 the earth, with the oxygen of the atmosphere, and its con- 

 version into this in\dsible gas. In some volcanic countries it 

 is also evolved from the earth. It is considerably heavier 

 than common air, and consequently accumulates in caverns 

 and deep wells; and, beiug incapable of supporting life, it 

 has frequently occasioned the death of persons incautiously 

 descending into them. Like nitrogen it is incapable of sup- 

 porting flame, and this quality has been the means of warn- 

 ing workmen of its presence in suspected places : thus, if a 

 candle bums brightly in a well or newly opened cave, it is 

 safe to descend ; but if the candle be extinguished, or even 

 bum feebly, we should endeavour to remove the deleterious 

 gas before we enter. The most effectual means of doing this 

 is to pour into the well or cave a few gallons of a mixture of 

 quick lime and water; the carbonic acid unites with the lime 

 in the same manner as when it meets with it in the open 

 field, and the poisonous gas is locked up in the same state in 

 which it exists in the limestone mountain, and is rendered 

 incapable of doing injury. 



25. It has already been shown (23) that carbonic acid 

 exists in limestone. In Ireland it forms a large portion of the 

 rocks of that formation which occupy so much of the centre of 

 the kingdom; and the "hard chalk" cliffs of Antrim, the 

 red limestone of Strangford Lough, and the black of Dublin, 

 contain nearly 44 per cent, of their weight of this gas. When 

 limestone is heated in the limekiln, the carbonic acid gas is 



liquid, procured by boiling the leaves of the red cabbage in water, the 

 colour of the liquid is changed to a bright red. 



h. It is heavier than air, and extinguishes flame — A bottle filled 

 with it may be inverted over a lighted taper, when the heavy gas will 

 descend and put out the light. 



c. The tube conducting the gas may be allowed to descend into a 

 wine glass containing some lime water, when the water will imme- 

 diately be rendered milky from the formation of a compound of the car- 

 bonic acid and lime {carbonate of lime.) 







