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.

If a tube of gla.s.s, or small phial, be rubbed in a similar manner, it will answer much better. The bottle thus rubbed becomes electric; and when the operation is performed in a dark room, small flashes of divergent flame, ramified somewhat like trees bare of leaves, will dart into the air, from many parts of the surface of the tube, to the distance of six or eight inches, attended with a crackling noise; and sometimes sparks will fly along the tube to the rubber at more than a foot distant.

Cut two bits of cork into the shape and size of a common pea. With a needle, draw a thread through each of the corks, so that they may be made to hang at the ends of the threads with a knot below them. Let the other ends of the threads be inserted in the notch of a small piece of wood, about a foot long, and an inch broad, and the thickness of a common match.

Lay the piece of wood over two wine-gla.s.ses, a few inches asunder, so that the end of it, in which the threads are, may project over the edge of the gla.s.s nearest it, and the corks may be in contact one with another. Take another wine-gla.s.s, and, having rubbed it briskly with a piece of flannel, or upon the skirt or sleeve of a woollen coat, hold its mouth to within about an inch of the corks, and they will suddenly start asunder, and continue so for some time.

Lay a pocket-watch upon a table, and take a common tobacco-pipe, and place it on the face of the watch so that it may balance thereon; then, after rubbing a wine-gla.s.s, as described in the former experiment, bring it to within an inch of the smaller end of the tobacco-pipe, and by moving the gla.s.s gently round in an horizontal circular track, you will cause the pipe to turn round on the watch-gla.s.s, as the needle turns on its centre in a mariner's compa.s.s.

_A curious Experiment made by Mr. Symmer, on the Electricity of Silk Stockings._

This gentleman having frequently observed, that on putting off his stockings in the evening, they made a crackling or snapping noise, and that in the dark they emitted sparks of fire, was induced to examine on what circ.u.mstances these electrical appearances depended. After a considerable number of observations, directed to this point, he found that it was the combination of white and black which produced the electricity, and that the appearances were the strongest when he wore a white and a black stocking upon the same leg. These, however, discovered no signs of electricity while they were upon the leg, though they were drawn backwards and forwards upon it several times; but the moment they were separated, they were both of them found to be highly electrified, the white positively, and the black negatively; and when they were held at a distance from each other, they appeared inflated to such a degree, that they exhibited the entire shape of the leg.

When two black or two white stockings were held together, they would repel one another to a considerable distance; and when a white and black stocking were presented to each other, they would be mutually attracted, and rush together with great violence, joining as close as if they had been so many folds of silk; and in this case their electricity did not seem to have been in the least impaired by the shock of meeting, for they would be again inflated, attract, repel, and rush together, as before.

When this experiment was performed with two black stockings in one hand, and two white ones in the other, it exhibited a still more curious spectacle. The repulsion of those of the same colour, and the attraction of those of different colours, threw them into an agitation, and made each of them catch at the opposite colour in a way that was very amusing.

What was also very remarkable in these experiments with a white and black stocking, was, the power of electrical cohesion which they exhibited; Mr.

Symmer having found, that when they were electrified, and allowed to come together, they frequently stuck so close to each other, that it required a weight of sixteen or seventeen ounces to separate them, and this in a direction parallel to their surfaces.

When one of the stockings was turned inside-out, it required twenty ounces to separate them; and by having the black stockings new dyed, and the white ones washed, and whitened in the fumes of sulphur, and then putting them one within the other, it required three pounds three ounces to separate them.

Trying this experiment with stockings of a more substantial make, he found that, when the white stocking was put within the black one, so that its outside was contiguous to the inside of the other, they raised near nine pounds; and when the white stocking was turned inside-out, and put within the black one, so that their rough surfaces were contiguous, they raised fifteen pounds, which was ninety-two times the weight of the stockings.

And, in all these cases, he found that pressing them together with his hands contributed much to strengthen the cohesion.

When the white and black stockings were in cohesion, and another pair, more highly electrified, were separated from each other, and presented to the former, their cohesion would be dissolved, and each stocking of the second pair would catch hold of, and carry away with it, that of its opposite co-lour; but if the degree of electricity of both pairs were equal, the cohesion of the former would be weakened, but not dissolved, and all the four would cohere together in one ma.s.s.

Mr. Symmer also observed, that white and black silk, when electrified, not only cohered with each other, but they would also adhere to bodies with broad, and even polished, surfaces, though those bodies were not electrified. This he discovered, by throwing accidentally a stocking out of his hand, which stuck to the paper-hangings of the room, and which, in another experiment of this kind, continued hanging there nearly an hour.

Having stuck up the black and white stockings in this manner, he came with another pair of stockings, highly electrified, and applying the white to the black, and the black to the white, he carried them off from the wall, each of them hanging to that which had been brought to it. The same experiment also held with the painted boards of the room, and likewise with the looking-gla.s.s, to the smooth surface of which, the white and black stockings appeared to adhere more tenaciously than to either of the former.

_To suspend a Ring by a Thread that has been burnt._

The thread having been previously soaked in chamber lye, or common salt and water, tie it to a ring, not larger than wedding-ring. When you apply the flame of a candle to it, though the thread burn to ashes, it will yet sustain the ring.

_Chemical Illuminations._

Put into a middling-sized bottle, with a short wide neck, three ounces of oil or spirit of vitriol, with twelve ounces of common water, and throw into it, at different times, an ounce or two of iron filings. A violent commotion will then take place, and white vapours will arise from the mixture. If a taper be held to the mouth of the bottle, these vapours will inflame, and produce a violent explosion; which may be repeated as long as the vapours continue.

_To make the Appearance of a Flash of Lightning when any one enters a Room with a lighted Candle._

Dissolve camphor in spirit of wine, and deposit the vessel containing the solution in a very close room, where the spirit of wine must be made to evaporate by strong and speedy boiling. If any one then enters the room with a lighted candle, the air will inflame; but the combustion will be so sudden, and of so short duration, as to occasion no danger.

_The Fiery Fountain._