[CHAPTER IV]
MECHANICS OF GASES
(1) Weight and Pressure of the Air
51. Weight of Air.—It is said that savages are unaware of the presence of air. They feel the wind and hear and see it moving the leaves and branches of the trees, but of air itself they have little conception.
To ordinary observers, it seems to have no weight, and to offer little resistance to bodies passing through it. That it has weight may be readily shown as follows: (See Fig. 29.) If a hollow metal sphere, or a glass flask, provided with tube and stopcock, be weighed when the stopcock is open, and then after the air has been exhausted from it by an air pump, a definite loss of weight is noticeable.
Fig. 29.—Proof that air has weight.
If the volume of the sphere is known and it is well exhausted of air, a fair approximation of the weight of air may be obtained. Under "standard conditions," which means at the freezing temperature and a barometric pressure of 76 cm., a liter of air weighs 1.293 g. while 12 cu. ft. of air weigh approximately 1 lb.
52. Pressure of Air.—Since air has weight it may be supposed to exert pressure like a liquid. That it does so may be shown in a variety of ways.