This property of losing its elastic force as its volume or bulk is increased, is not peculiar to air. It is common to all elastic fluids, and we accordingly find it in steam; and it is absolutely necessary to take account of it in estimating the effects of that agent.
(7.) There are numerous instances of the effects of these properties of atmospheric air which continually fall under our observation. If the nozzle and valve-hole of a pair of bellows be stopped, it will require a very considerable force to separate the boards. This effect is produced by the diminished elastic force of the air remaining between the boards upon the least increase of the space within the bellows, while the atmosphere presses, with undiminished force, on the external surfaces of the boards. If the boards be separated so as to double the space within, the elastic force of the included air will be about 7-1/2lb. on every square inch, while the pressure on the external surfaces will be 15lb. on every square inch; consequently, it will require as great a force to sustain the boards in such a position, as it would to separate them if each board were forced against the other, with a pressure of 7-1/2lb. per square inch on their external surfaces.
When boys apply a piece of moistened leather to a stone, so as to exclude the air from between them, the stone, though it be of considerable weight, may be lifted by a string attached to the leather: the cause of which is the atmospheric pressure, which keeps the leather and the stone in close contact.
(8.) The next class of physical effects which it is necessary to explain, are those which are produced when heat is imparted or abstracted from bodies.
In general, when heat is imparted to a body, an enlargement of bulk will be the immediate consequence, and at the same time the body will become warmer to the touch. These two effects of expansion and increase of warmth going on always together, the one has been taken as a measure of the other; and upon this principle the common thermometer is constructed. That instrument consists of a tube of glass, terminated in a bulb, the magnitude of which is considerable, compared with the bore of the tube. The bulb and part of the tube are filled with mercury, or some other liquid. When the bulb is exposed to any source of heat, the mercury contained in it, being warmed or increased in temperature, is at the same time increased in bulk, or expanded or dilated, as it is called. The bulb not having sufficient capacity to contain the increased bulk of mercury, the liquid is forced up in the tube, and the quantity of expansion is determined by observing the ascent of the column in the tube.