488 HEALTH AND DISEASE 



Air is 773 times lighter than an equal volume of water, and 14 '47 

 times heavier than the same volume of hydrogen. For each degree centi- 

 grade of increased temperature it expands -^js of its volume. The pressure 

 of the atmosphere on each square yard of surface is about 20,000 lbs., 

 and as the horse presents between 5 and 6 square yards of surface, it 

 supports a pressure of 100,000 lbs., or 50 tons. 



The air is rarely or never saturated with aqueous vapour, but the 

 quantity rises with the temperature, and is greater therefore in hot climates 

 than in cold, and in summer than in winter. It is greater also on plains 

 than in mountainous regions, by day than by night, and where there is 

 much vegetation than in arid districts. In moist tropical regions it may 

 rise to 3 volumes in 100 of air, whilst in middle Europe it does not usually 

 exceed 1'3 per cent, and may perhaps in this country be regarded as con- 

 stituting between 1 and 2 per cent by volume. 



Air after Respiration. — When air that has been once inspired is 

 examined, it is found to have undergone considerable alteration. The 

 proportion of nitrogen, indeed, is not materially altered — it still remains at 

 ai)out 79 in 100 parts, — but about 5 parts of oxygen have been abstracted, 

 leaving the percentage of oxygen at about 16 per cent, while the quantity 

 of carbon dioxide is increased from 1 part in 2500 to 112'5 parts in 2500, 

 or to 4'5 per cent. The air is, in addition, rendered warmer, providing, 

 of course, that when it was inspired it was not higher in temperature than 

 that of the body. When this occurs, as it sometimes does in hot climates, 

 the air inspired may actually be cb led by passing through the lungs. As 

 a rule, the temperature of the expired air is raised to nearly 98°. More- 

 over, it is saturated with watery vapour which has evaporated from the 

 moist mucous tracts of the nose, trachea, and alveoli of the lungs. It is 

 also freed from all particles of dust and from all living or dead germs, 

 which adhere to the moist surface of the respiratory tract, to be tossed 

 outwards by the wave-like movements of the cilia of the ciliated cells 

 lining the air-passages, and ultimately to be coughed up. Finally, some 

 volatile gases or oils are added to it, as is proved by the peculiar odour 

 of the breath characteristic of each animal, and especially observable after 

 the use of certain articles of diet, such as garlic. 



Putting the changes in the air in a tabular form, they may be thus 



represented : Before inspiration. After expiration. 



Oxygen "21 16 



Nitrogen 79 79 



Carbon dioxide 0-04 45 



Aqueous vapour ... ... 1 or more per cent Saturated 



Temperature Variable About 98° 



More or le.ss germ-laden Germ-free 



