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. PYROPHORUS, OR AIR-TINDER. Mix three parts of alum with one of wheat flour, and put them into a common phial; set it in a crucible, up to the neck in sand; then surround the crucible with red-hot coals, when first a black smoke, and next a blue sulphureous flame, will issue from the mouth of the phial; when this flame disappears, remove the crucible from the fire, and when cold, stop the phial with a good cork. If a portion of this powder be exposed to the air, it will take fire. Or, a very perfect and beautiful pyrophorus may be obtained by heating tartrate of lead in a glass tube, over a lamp. When some of the dark brown mass thus formed is shaken out in the air, it will immediately inflame, and brilliant globules of lead cover the ignited surface. Or, mix three parts of lamp-black, four of burnt alum, in powder, and eight of pearl-ash, and heat them for an hour, to a bright cherry red, in an iron tube. When well made, and poured out upon a glass plate or tile, this pyrophorus will kindle, with a series of small explosions, somewhat like those produced by throwing potassium upon water; but this effect should be witnessed from a distance.