A text-book on chemistry : for the use of schools and colleges . cid. But the growth of plants has the re-verse action, removing carbonic acid and replacing it byoxygen, so that for many centuries in succession the con-stitution of the atmosphere is unchanged. Of the mechanical properties of the air, the first toFi. 179. which we have to direct our attention is its press-ure, which takes effect equally in all directions, up-ward, downward, and laterally. Thus, if we takea glass tube several feet long, a, Fig. 179, closedat one end and open at the other, and having filledit full of water, place
A text-book on chemistry : for the use of schools and colleges . cid. But the growth of plants has the re-verse action, removing carbonic acid and replacing it byoxygen, so that for many centuries in succession the con-stitution of the atmosphere is unchanged. Of the mechanical properties of the air, the first toFi. 179. which we have to direct our attention is its press-ure, which takes effect equally in all directions, up-ward, downward, and laterally. Thus, if we takea glass tube several feet long, a, Fig. 179, closedat one end and open at the other, and having filledit full of water, place over the mouth of it a pieceof card, b, and turn it upside down, the card willnot fall off, nor the water flow out; they remain,^7as it were, suspended on nothing, but in reality sus- What are its relations to animals and plants ? Why does its densitydecrease with the altitude ? How does its temperature vary ? Whichare the fixed, and which the variable constituents of the air? Give someillustrations of the upward pressure of the air. PKES5U11E OF THE AIR. 193. tained by the upward pressure of the air. Or if we takea bottle, a, Fig. 180, with a hole, b, half an inch Fig. diameter in the bottom of it, and having filledit with water, close the mouth of it with the fin-ger, it may be held up in the air without the wa-ter flowing out, although the aperture b is wideopen. In this instance, again, it is the upwardpressure of the air which sustains the the glass globe a, Fig. 181, with its neck b, be inverted in some water contained in ajar, c, and the whole covered by an air-pump re-ceiver. As the receiver is exhausted, bubblesof air pass through the water and escape away,but as soon as the pressure is restored thewater is forced out of the jar upward into theglobe. The air-pump enables us to exhibit in a very strikingmanner many of the chief mechanical properties of the at-mosphere. Thus, if upon the plate of it there FjV lg2be placed a glass receiver, a, Fig. 182,
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