304 CARBON. 



as gas, this body is more suitable than any other substance to 

 effect the reduction of metallic oxides, that is, to deprive them 

 of their oxygen, and to produce from them the metal with the 

 properties which characterize it. 



CARBONIC ACID. 

 Eg. 276, or 22.13; CO 2 ; density 1524,1 : [~J~]. 



This gas was first discovered to exist in lime-stone and the 

 mild alkalies, and to be expelled from them by heat and the 

 action of acids by Dr. Black, and was named by him Fixed Air. 

 He also remarked that the same gas is formed in respiration, 

 fermentation and combustion; it was afterwards proved to 

 contain carbon by Lavoisier. 



Preparation. Carbonic acid is readily procured by pouring 

 hydrochloric acid of sp. gr. 1.1, upon fragments of marble con- 

 tained in a gas bottle, or by the action of diluted sulphuric acid 

 upon chalk. A gas comes off with effervescence, which may 

 be collected at the water trough, but cannot be retained long 

 over water without considerable loss, owing to its solubility. 

 When generated in the close apparatus of M. Thilorier for the 

 purpose of liquefying it, this gas is evolved from bicarbonate of 

 soda and sulphuric acid. 



Properties. This gas extinguishes flame, does not support 

 animal life, and renders lime-water turbid. Its density is con- 

 siderable, being 1524, or a half more than that of air, the gas 

 containing 2 volumes of the hypothetical carbon vapour and 

 2 volumes of oxygen, condensed 'into 2 volumes, which form the 

 combining measure. Cold water dissolves rather more than an 

 equal volume of this gas ; the solution has an agreeable acidu- 

 lous taste, and sparkles when poured from one vessel into 

 another. It communicates a wine-red tint to litmus paper, 

 which disappears again when the paper dries ; when poured 

 into lime-water it first throws down a white flaky precipitate 

 of carbonate of lime or chalk, which it afterwards redissolves 

 if the solution be added in excess. The quantity of this gas 

 which water takes up is found to be exactly proportional to the 

 pressure ; a very large volume of the gas is forced into soda, 

 magnesia and other aerated waters, much of which escapes on 

 removing the pressure from these liquids. 



This gas was liquefied by Mr. Faraday, whose method has 



