234 
Geology of Sydney. 
It may be well to state here that pure water is 
not an active agent in dissolving limestones, but water 
charged with carbonic acid gas 1 has a powerful effect 
on these rocks. Now, it will be asked, — Where does the 
water procure the supply of carbonic acid that renders 
it so potent a solvent for solid rocks ? The reply is : 
Mostly from the air. But, to make the matter clear, 
it should be stated that carbonic acid, or (what amounts 
to the same thing) carbon dioxide is produced — 
(1.) By the breathing of all animals; man exhales about 
one pound of carbon dioxide per day. The pro- 
duction of carbon dioxide in breathing is commonly 
proved in this way : Some quicklime is shaken up 
with water, and the clear solution poured off. 
This is known as lime-water. If the breath is 
now made to bubble through, by blowing gently 
through a glass tube or a straw, the clear solution 
becomes milky, from the formation of carbonate 
of lime. The carbon is provided from the carbon 
dioxide exhaled by the lungs. If a saucer of lime- 
water is left standing, a scum of carbonate of 
lime soon forms, owing to the presence of carbon 
dioxide in the air. 
(2.) In burning coal, the carbon of the fuel is converted 
into carbon dioxide, every ton of coal burned 
producing quite two tons of the gas. 
(3.) In all processes of animal and vegetable decay the 
carbon of the organic matter is oxidised to 
carbon dioxide. 
1 Strictly speaking, carbonic acid is not a correct name for this gas. Acids 
always contain hydrogen, which this gas does not. Carbon dioxide is a more correct 
term. Carbon dioxide, however, when dissolved in water, reddens litmus, thus show- 
ing one of the special characters of acids. 
