134 PHYSIOLOGY AND HEALTH. 



closely united, yet the oxygen has a stronger affinity for car- 

 bon and hydrogen than for nitrogen ; and whenever, under 

 appropriate circumstances, the carbon or hydrogen is pre- 

 sented to the air, the oxygen leaves the nitrogen and unites 

 with the carbon, and forms carbonic acid, or with the hydro- 

 gen, and forms water. In other words, the air is decom- 

 posed, its simple elements are separated from each other, and 

 a new compound is formed by the union of carbon and 

 oxygen, or by the union of hydrogen and oxygen. 



295. Carbonic acid is a composition of oxygen and car- 

 bon. This is a gas heavier than air, and lighter than water. 

 If it be in a vessel with water, it rises to the top ; and if in 

 a vessel with air, it sinks to the bottom. It is so much heavier 

 than air that it can be poured from one tumbler to another, 

 like water. It is found in some caves, and at the bottom of 

 some wells. It is the fixed air which is the product of fer- 

 menting bread, beer, wine, and cider, and fills the bubbles 

 that rise to the top of these liquids at the time of their fer- 

 mentation. When the beer is drawn out from the vats, in 

 the great breweries, this gas often falls to the bottom, and 

 partially fills these reservoirs. It is also the product of com- 

 bustion of charcoal ; and often, where this fuel is burning 

 without any outlet near the floor for this gas to run off, or a 

 chimney of sufficient draft to carry it upward, it partially or 

 entirely fills the room. Wherever this gas is, there can be 

 no pure air, for this is excluded by it as certainly as it would 

 be by water ; and it is as unsafe for a man to enter a cavern, 

 well, vat, or a room containing it, and carry his head below the 

 surface of this gas, as it would be if these contained water. 



296. When the chest expands, the air rushes in and fills 

 all the air-tubes and the air-cells throughout the lungs. 

 There it comes almost in contact with the venous blood, 

 which is distributed in the numberless little vessels, and 

 separated from the air-cells only by a thin film of memlrane, 

 through which the gases can pass. There an interchange 

 takes place between the fluid and the gas. The blood ab- 

 sorbs from the air some of its oxygen, and the air takes from 



