74 GASEOUS BODIES. 



at the bottom of a deep vessel, and a small quantity of 

 this gas be poured upon it, it will become extinguished, 

 although from the transparency of the gas the eye will 

 not perceive it. This gas is given out in large quantities 

 during the process of fermentation ; the bubbles which 

 are thrown up from beer recently brewed contain this 

 gas. In several instances, men that have been employed 

 to clean out large vats in breweries, when emptied of 

 their contents, have been suffocated, by entering them 

 without having taken the proper precaution. It is usual 

 to let down a candle, when, if the candle burns, it is 

 considered respirable ; the same precaution is generally 

 taken before descending a well, which, if carried through 

 a chalky soil, is very likely to be charged with carbonic 

 acid gas. Champagne and cider owe their sparkling 

 qualities to this gas, and so do all fermented liquors. 

 Bottled porter contains it in large quantities. Water 

 contains a small quantity of it naturally, but by means 

 of a forcing pump it may be made to absorb two or three 

 times its bulk. Soda-water is water impregnated with 

 it, and that very grateful beverage, ginger-beer, is in- 

 debted to it for its briskness and pleasant taste. Carbonic 

 acid gas is considered to possess much utility in pro- 

 moting vegetation. It is probably decomposed by the 

 organs of plants, and its base furnishes the carbon which 

 they retain while they give out the oxygen. Carbonic 

 acid gas abounds in the mineral world : chalk, or car- 

 bonate of lime, marble, spars, and other calcareous 

 substances, contain nearly one -half of their weight of it. 

 It may be extricated from either of these bodies by heat, 

 heat being the grand antagonist to affinities of all kinds. 

 In the process of making lime, for instance, the carbonic 

 acid gas is driven off from the limestone (carbonate of 

 lime) in the kiln, leaving the lime uncombined. Carbonic 

 acid gas may be best obtained for experiment from car- 

 bonate of ammonia, by putting a small quantity of it 

 into a glass retort, and applying the heat of a lamp ; the 

 carbonate of ammonia then becomes decomposed, the 

 water imbibes the ammonia,* and allows the carbonic 

 acid gas to escape. 



* This is called the Volatile alkali, and exists, when uncombined, 



