A. Because the _candle will be extinguished_, if the mine contains carbonic acid gas: but if the candle is _not extinguished_, the mine is _safe_, and the man may fearlessly descend. Q. _Why are PERSONS sometimes KILLED, by leaning over BEER VATS?_ A. Vats (where beer has been made) contain a _large quantity of carbonic acid gas_, produced by the "vinous fermentation" of the beer; and when a man incautiously _leans over a beer vat_, and inhales the carbonic acid, he is immediately _killed_ thereby. Q. _Why are PERSONS often KILLED, who enter BEER VATS to clean them?_ A. Carbonic acid (being _heavier than atmospheric air_) often rests upon the _bottom of a vat_: when, therefore, a person enters the vat, and _stoops to clean the bottom_, he inhales the pernicious gas, which _kills_ him. Q. _Why are the JUNGLES of Jarva and Hindostan so FATAL to life?_ A. Because vast quantities of _carbonic acid_ are thrown off by decaying _vegetables_; and (as the wind cannot penetrate the thick brushwood to blow it away) _it settles there_, and destroys animal life. Q. _Why are PERSONS sometimes KILLED by having a CHARCOAL FIRE in their bed-rooms?_ A. When charcoal is burned, the _carbon of the charcoal_ unites with the _oxygen of the air_, and forms _carbonic acid gas_, which is a narcotic poison. Q. _Why does the carbonic acid gas of a CHARCOAL FIRE RISE and DISPERSE itself about the room; whereas the carbonic acid gas of a BEER VAT SETTLES near the FLOOR?_ A. The carbonic acid gas of a charcoal fire _is heated_ by the _combustion of the fuel_, and rises; but the carbonic acid gas of a beer vat is _not_ heated, and, therefore, rests on the _bottom of the vat_. Q. _Why do persons throw LIME into BINS to PREVENT their offensive SMELL, in summer time?_ A. Bins contain large quantities of _carbonic acid gas_, which readily _combines with lime_, and produces "_carbonate of lime_," which is entirely free from all offensive odour. Q. _Why do persons throw LIME into SEWERS in summer time?_ A. Sewers (like bins) contain large quantities of _carbonic acid_, which readily _combines with lime_, and produces _carbonate of lime_; and thus the offensive gas of the sewer is neutralized. Q. _Can carbonic acid be removed in any way BESIDES by LIME?_ A. Yes; _water_ thrown into a pit will disperse the carbonic acid. Q. _What effect has WATER on CARBONIC ACID GAS?_ A. Water (under _pressure_) _absorbs_ carbonic acid gas; and _parts_ with it (when the _pressure is removed_) in the form of EFFERVESCENCE. Q. _Why does AERATED WATER effervesce, when the CORK is removed?_ A. While the _cork was fastened down_, the water _absorbed_ the carbonic acid; but the moment _the pressure is removed_ (by taking out the cork) the gas is given out with _effervescence_. Q. _Why does SODA WATER effervesce?_ A. Soda water contains 8 times its own bulk of carbonic acid gas, which makes its escape in _effervescence_, the moment that the _cork is removed_.