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_. [Verse: "Whereupon are the foundations thereof fastened? or who laid the cornerstone thereof."--JOB XXXVIII.] 804. _Why does boiled water taste flat and insipid?_ Because the _carbonic acid_ has been _driven off_ by boiling. 805. _Why does beer which has been standing in a glass taste flat?_ Because its _carbonic acid_ has escaped as _carbonic acid gas_. 806. _Why, when we look into a glass of champagne, do we see bubbles spontaneously appear at the bottom, and then rise to the top?_ Because, in the places where the bubbles are formed, the _liquid carbonic acid_ is evolving into _carbonic acid gas_. 807. _Why do the bubbles arise from two or three points in columns, rapidly succeeding each other?_ Because, when the formation of gas once begins, and bubbles ascend, there is _less pressure_ in the line of the _column of bubbles_; the carbonic acid, therefore, draws towards those points as the _easiest channel of escape_. These explanations equally apply to the "working" of beer, by which yeast is produced; to the effervescence of various waters, acidulated drinks, ginger beer, &c., and also to the "sponging" of bread, &c. 808. _Why does gunpowder explode?_ Gunpowder is made of a very intimate _mechanical mixture_ of _nitrate of potash_, _charcoal_, and _sulphur_. When these substances are heated to a certain degree, the nitrate of potash is decomposed, and its _oxygen_ combines with the _charcoal_ and _sulphur_, instantaneously forming _large volumes of carbonic acid gas_ and _nitrogen_, which, seeking an escape, produce an explosion. 809. _Why does charcoal act as a powerful disinfectant?_ Because the _carbon_ readily absorbs, and combines with _various gases_, neutralising their _offensive odours_, and destroying their _unhealthy properties_.