COMPOSITION OF CARBONIC ACID. 29 



there be no strong draft to sweep it away. It may in 

 this manner be transferred from one vessel to another. 



3. A third important property of this gas is, that 

 all animals compelled to breathe it instantly fall, and 

 in a very few moments die. This may be shown by- 

 placing a mouse or other small animal in an atmo- 

 sphere of it. Owing to its weight, it sometimes ac- 

 cumulates in sheltered hollows, and is the cause of 

 fatal accidents. In brewers' vats when fermentation 

 takes place, and in some wells, it is apt to collect, and 

 persons lowered incautiously to clean them suddenly 

 fall insensible. All danger maybe avoided by simply 

 lowering a lighted candle before any one goes down: 

 if the candle burns freely at the bottom, there is no 

 risk in descending. 



This gas consists of carbon and oxygen ; 6 lbs. of 

 carbon and 16 lbs. of oxygen forming 22 lbs. of car- 

 bonic acid. Chemists call it carbon 1 and oxygen 2. 

 It is easy to prove this fact by burning charcoal, which 

 it will be remembered is one form of carbon, in a jar 

 of pure oxygen gas. When the charcoal has ceased 

 to burn, the air remaining in the jar will be carbonic 

 acid; as carbon and oxygen w T ere the only two sub- 

 stances present, the carbonic acid must plainly have 

 been formed by their union in certain proportions. 



This is another instance of those strange chemical 

 changes in the properties of bodies, with which all 

 who study this subject soon become familiar. Carbon, 

 a hard inflammable solid, unites with oxygen, a light 

 gas, supporting combustion and animal life in a most 

 remarkable degree; to form another kind of gas, having 

 a much greater weight, entirely incombustible, and, 

 when unmixed with air, destructive to almost every 

 form of life. 



Carbonic acid exists naturally in verv large quantity 

 It is invariably present in the atmosphere. For a 

 long time this was thought to be accidental, but later 

 3* 



