The air in a room may be said to resemble two climates: as it is lighter than the external air, a current of colder or heavier air is continually pouring in from the crevices of the windows and doors; and the light air must find some vent, to make way for the heavy air. If the door be set a-jar, and a candle held near the upper part of it, the flame will be blown outwards, showing that there is a current of air flowing out from the upper part of the room; and, if the candle be placed on the floor, close by the door, the flame will bend inwards, showing that there is also a current of air setting into the lower part of the room. The upper current is the warm, light air, which is driven out to make way for the stream of cold, dense air, which enters below.
BUBBLES ON CHAMPAGNE.
Pour out a glass of champagne, or bottled ale, and wait till the effervescence has ceased; you may then renew it by throwing into the liquor a bit of paper, a crumb of bread, or even by violently shaking the glass. The bubbles of carbonic acid chiefly rise from where the liquor is in contact with the glass, and is in greatest abundance at those parts where there are asperities. The bubbles setting out from the surface of the glass are at first very small; but they enlarge in passing through the liquor. It seems as if they proceeded more abundantly from the bottom of the glass than from its sides; but this is an ocular deception.
PROOFS THAT AIR IS A HEAVY FLUID.
Expel the air out of a pair of bellows, then close the nozzle and valve-hole beneath, and considerable force will be requisite to separate the boards from each other. This is caused by the pressure or weight of the atmosphere, which, acting equally upon the upper and lower boards externally, without any air inside, operates like a dead weight in keeping the boards together. In like manner, if you stop the end of a syringe, after its piston-rod has been pressed down to the bottom, and then attempt to draw it up again, considerable force will be requisite to raise it, depending upon the size of the syringe, being about fourteen or fifteen pounds to every square inch of the piston-rod. When the rod is drawn up, unless it be held, it will fall to the bottom, from the weight of the air pressing it in.
Or, fill a glass tumbler to the brim with water, cover it with a piece of thin wet leather, invert it on a table, and try to pull it straight up, when it will be found to require considerable force. In this manner do snails, periwinkles, limpets, and other shells adhere to rocks, &c. Flies are enabled to walk on the ceiling of a room, up a looking-glass, or window-pane, by the air pressing on the outside of their peculiarly-constructed feet, and thus supporting them.
To the same cause must be attributed the firmness with which the oyster closes itself; for, if you grind off a part of the shell, so as to make a hole in it, though without at all injuring the fish, it may be opened with great ease.
TO SUPPORT A PEA ON AIR.
This experiment may be dexterously performed by placing a pea upon a quill, or the stem of a tobacco-pipe, and blowing upwards through it.