Part 78 (1/2)
To ascertain the strength of brine for salting meat, it is usual to put an egg in the boiling water, and gradually put in salt until the egg be made to swim.
_The following Experiments shew the Pressure and Elasticity of Air._
Put an empty bottle with a cork in it near the fire; the cork will be driven out.
Get a vessel of hot water, and put a phial into it, with the mouth downwards; the expanded air will bubble out. Let the water cool, or pour cold water on the phial, of which the mouth has not been drawn above the surface of the water, and as the air is now cooled, and occupies less s.p.a.ce, a considerable part of the bottle will be filled with water.
Boil a little water in a gla.s.s phial over a candle for a few minutes; then invert the mouth of the phial in water, and, as it cools, the air will contract, and water will be forced up the bottle, by the external air, to occupy the vacant s.p.a.ce.
Lay a weighty book on a bladder, and blow into it with a pipe, and the book will be raised. Increase the weight on the bladder very much indeed, and you may still raise it as before.
A bladder filled with air may be compressed, and the moment the force is removed, it will recover its size. If thrown on the ground it will rise like a ball.
Take a cup, and burn a few pieces of paper in it, the heat will expand the air in it. Invert the cup now in a saucer of water, and, as the enclosed air cools, it will return to its former density, and leave a vacuum, and the pressure of the external air will force a great deal of water up into the cup. If this experiment be performed with a large drinking-gla.s.s, the water may be seen to rise in the gla.s.s.
The pressure of the air may be very sensibly felt, by putting the hole of a common bellows over the knee, and then attempting to raise the upper part of it.
Boil water in a gla.s.s phial over a candle for a few minutes, then suddenly removing it, tie a piece of wetted bladder over the mouth, making it fast with a string; the pressure of the air will stretch the bladder, if it do not burst it.
Get a gla.s.s vessel, as a common tumbler, if no better be at hand, and put a piece of wetted bladder over the mouth, pressing it down in the middle, and then tie it firm with a string; then lay hold of the bladder in the middle, and try to pull it straight, or level with the rest, and the pressure of the external air will not permit it.
Do exactly the same as before, except that the vessel must be nearly full of water. Turn the vessel upside-down, and the bladder will still continue as it was placed, the pressure of the air overcoming the weight of the water.
Though air be capable of compression, it makes a resistance, and that very considerable. The ball of an air-gun has been burst asunder by overcharging it. If bottles are filled too much, they may be burst in attempting to cork them, from the air between the cork and the liquor being too much condensed.
Put a common wine-gla.s.s, with the mouth downwards, into water; and to whatever depth it may be plunged, the air will not allow much water to rise into it, as may be seen by the inside of the gla.s.s not being wet. If a bit of cork float inside of the gla.s.s, it will point out to the eye still more clearly how high the water rises. This experiment, though so very simple will ill.u.s.trate the nature of the diving-bell.
_Experiments respecting Sound._
Hold a tumbler sideways, and sprinkle a little dust, or powder of any sort, on it; then strike the gla.s.s, and make it sound:--the dust keeps dancing about whilst the sound continues; stop the sound, and the dust is at rest.
The sound of a watch laid upon a long table, or upon a plank of wood, will be heard much farther than it otherwise would.
When a vessel on the fire begins to boil, let a communication be made between it and the ear, by means of the poker, and the sound is more distinctly heard.
Tie a string round the end of a poker, and then, winding one end of the string round the fore-finger of the one hand, and the other end of the string round the fore-finger of the other; put the fingers into the ears, and make the poker strike against a table, or any other object, and it will sound like the bell of a church.
Tie a string round the end of a poker, as before, and hold the string with your teeth; when the poker is made to strike against any object, as in the last experiment, the same kind of sound will be transmitted through the teeth.
Make a watch touch your teeth, and you will hear its beating more distinctly.
When a pitchfork is struck, in order to pitch a tune, its end is put on the table, and a greater sound is produced. If the pitchfork, after being struck, be held to the teeth, its sound is still more distinct.
Having shut up both ears with cotton very closely, put your fingers on the teeth of a person who speaks to you, and you will hear his voice.
_Electrical Experiments._
If a piece of sealing-wax be rubbed briskly against the sleeve of your coat, or any other woollen substance, for some time, and then held within an inch or less of hair, feathers, bits of paper, or other light bodies; they will be attracted, that is, they will jump up, and adhere to the wax.