It exists in various natural bodies in which carbon and oxygen are combined; it is evolved by the decomposition of numerous bodies called carbonates, in which carbon is united with a particular base, such as the carbonate of lime, the carbonate of iron, the carbonate of copper, &c. It is also evolved by the processes of fermentation, by the breathing of animals, the combustion of fuel, and the functions of plants. Carbonic acid also exists in various waters.
Carbonic acid is found most largely in solid combinations with other bodies: it forms 44-100ths of all limestones and marbles, and it exists in smaller quantity, combined with other earths, and with metallic oxides.
800. What are the states in which pure carbonic acid exists?
Pure carbonic acid may exist in the solid, the liquid, or the æriform state. In the solid state it is produced only by artificial means, and it is then a white crystallised body, in appearance like snow; in the liquid state it is a heavy colourless fluid; in the æriform state it is a pungent, heavy, colourless gas, and is known as carbonic acid gas.
801. Why does bottled porter produce large volumes of froth, much more than the bottle could contain?
Because, by the fermentive process, carbonic acid has been developed in the porter, and is held in liquid solution; but it always has a strong tendency to escape, and directly the pressure is removed, it evolves into gas, by which it occupies much greater space, and forces the porter in millions of small bubbles out of the bottle.
802. Why does soda-water effervesce?
Because carbonic acid gas is forced into the water by pressure. Pressure alters the gas into a liquid, and directly the pressure ceases, the liquid again evolves into gas.
803. Why does spring water taste fresh and invigorating?
Because it contains carbonic acid.