AIR IN THE EGG.

Take a fresh egg, and cut off a little of the sh.e.l.l and film from its smaller end; then put the egg under a receiver, and pump out the air; upon which all the contents of the egg will be forced out by the expansion of the small bubble of air contained in the great end between the sh.e.l.l and the film.

THE DESCENDING SMOKE.

Set a lighted candle on the plate, and cover it with a tall receiver.

The candle will continue to burn while the air remains, but when exhausted will go out, and the smoke from the wick, instead of rising, will descend in dense clouds towards the bottom of the gla.s.s, because the air which would have supported it has been withdrawn.

[Ill.u.s.tration: AIR IN THE EGG.]

[Ill.u.s.tration: DESCENDING SMOKE.]

THE SOUNDLESS BELL.

Set a bell on the pump-plate, having a contrivance so as to ring it at pleasure, and cover it with a receiver; then make the clapper sound against the bell, and it will be heard to sound very well; now exhaust the receiver of air, and then when the clapper strikes against the sides of the bell the sound can be scarcely heard.

[Ill.u.s.tration: SOUNDLESS BELL.]

[Ill.u.s.tration: FLOATING FISH.]

THE FLOATING FISH.

If a gla.s.s vessel containing water, in which a couple of fish are put, be placed under the receiver, upon exhausting the air the fish will be unable to keep at the bottom of the gla.s.s, owing to the expansion of the air within their bodies, contained in the air bladder. They will consequently rise and float, belly upwards, upon the surface of the water.

THE DIVING BELL.

The diving bell is a pneumatic engine, by means of which persons can descend to great depths in the sea, and recover from it valuable portions of wrecks and other things. Its principle may be well ill.u.s.trated by the following experiment. Take a gla.s.s tumbler, and plunge it into the water with the mouth downwards, and it will be found that the water will not rise much more than half way in the tumbler.

This may be made very evident if a piece of cork be suffered to float inside the gla.s.s on the surface of the water. The air within the tumbler does not entirely exclude the water, because air is elastic, and consequently compressible, and hence the air in the tumbler is what is called condensed. The diving bell is formed upon the above principle; but instead of being of gla.s.s, it is a wooden or metal vessel, of very large dimensions, so as to hold three or four persons, who are supplied with air from above by means of powerful pumps, whilst the excess of air escapes at the bottom of the bell, as may be seen any day at the Royal Polytechnic Inst.i.tution.

[Ill.u.s.tration]

[Ill.u.s.tration]

EXPERIMENTS.

1. Place a cylinder of strong gla.s.s, open at both ends, on the plate of the air-pump, and put your hand on the other end, and you will of course be able to remove it at pleasure. Now exhaust the air from the interior of the cylinder, and at each stroke of the pump you will feel your hand pressed tighter and tighter on the cylinder, until you will not be able to remove it: as soon as the air is again admitted to the interior of the cylinder, the pressure within will be restored, and the hand again be at liberty.

2. Tie a piece of moistened bladder very firmly over one end of a similar gla.s.s cylinder, and place the open end on the plate of the pump.

As soon as you begin to exhaust the air from the interior, the bladder, which was previously quite horizontal, will begin to bulge inwards, the concavity increasing as the exhaustion proceeds, until the bladder, no longer able to bear the weight of the superinc.u.mbent air, breaks with a loud report.

3. The elasticity of air, or indeed of any gaseous body, may be shown by introducing under the air-pump receiver a bladder containing a very small quant.i.ty of air, its mouth being closely tied. As you exhaust the air from the receiver, that portion contained in the bladder being no longer pressed upon by the atmosphere, will gradually expand, distending the bladder until it appears nearly full: on readmitting the air into the receiver, the bladder will at once shrink to its former dimensions.

A shrivelled apple placed under the same conditions will appear plump when the air is removed from the receiver, and resume its former appearance on the readmission of the air.

4. There is a very pretty apparatus made for the purpose of showing the pressure of the atmosphere, consisting of a hollow globe of bra.s.s, about three inches in diameter, divided into two equal parts, which fit very accurately together. It is furnished with two handles; one of them screwed into a hollow stem, communicating with the interior of the globe, and fitting on to the air-pump: the other is attached to a short stem on the opposite side of the globe. In the natural state the globe may easily be separated into its two hemispheres by one person pulling the handles, but after the air has been exhausted from the interior it requires two very strong men to separate the parts, and they will often fail to do so. By turning the stopc.o.c.k, and readmitting the air into the interior of the globe, it will come asunder as easily as at first.

We are indebted to the weight of the atmosphere for the power we possess of raising water by the common pump; for the piston of the pump withdrawing the air from the interior of the pipe, which terminates in water, the pressure of the atmosphere forces the water up the pipe to supply the place of the air withdrawn. It was soon found, however, that when the column of water in the pipe was more than thirty feet high, the pump became useless, for the water refused to rise higher. Why? It was found that a column of water about thirty feet high exerted a pressure equal to the weight of the atmosphere, thus establishing an equilibrium between the water in the pipe and the atmospheric pressure.

[Ill.u.s.tration]

This is the principle on which the barometer, or _measurer of weight_, as its name imports, is constructed. The metal Mercury is about thirteen and a half times heavier than water; consequently, if a column of water thirty _feet_ high balances the pressure of the atmosphere, a column of mercury thirty _inches_ high ought to do so also--and this is in fact the case. If you take a gla.s.s tube nearly three feet long, and closed at one end, and fill it with mercury; then, placing your finger on the open end, invert the tube into a basin or saucer containing some of the same metal; upon removing your finger (which must be done carefully, while the mouth of the tube is completely covered by the mercury,) it will be seen that the fluid will fall a few inches, leaving the upper part of the tube empty. Such a tube with a graduated scale attached is in truth a barometer, and as the weight of the atmosphere increases or decreases, so the mercury rises or falls in the tube. This instrument is of the greatest value to the seaman, for a sudden fall of the barometer will often give notice of an impending storm when all is fine and calm, and thus enable the mariner to make the preparations necessary to meet the danger.

It was discovered by an Italian philosopher named Torricelli, and from him the vacuum formed in the upper end of the tube above the surface of the mercury has been called the Torricellian vacuum. It is by far the most perfect vacuum that can be obtained, containing necessarily nothing but a minute quant.i.ty of the vapour of mercury.

EXPERIMENT.

Pa.s.s a little ether through the mercury in the tube, and as soon as it reaches the empty s.p.a.ce it will boil violently, depressing the mercury, until the pressure of its own vapour is sufficient to prevent its ebullition. If you now cool the upper part of the tube, so as to condense the vapour, the pressure being thus removed, the ether will again begin to boil, and so alternately, as often as you please, in order to show this fact with effect, the bore of the tube should not be less than half an inch in diameter.

[Ill.u.s.tration]

EXPERIMENT.

To show that the heat abstracted by the boiling of one liquid will freeze another, fill a tall narrow gla.s.s about half full of cold water (the colder the better), and place in it a thin gla.s.s tube containing some ether. Put them under the receiver of an air-pump. As you exhaust the air, the ether will begin to boil, until at length, by continuing the exhaustion, the water immediately surrounding the tube of ether will freeze, and a tolerably large piece of ice may thus be obtained.

Ether evaporates so rapidly even under the pressure of the atmosphere, that a small animal, such as a mouse, may be actually frozen to death by constantly dropping ether upon it. If poured on the hand, it produces a degree of cold that soon becomes, to say the least, unpleasant.

EXPERIMENT.

Place a flat saucer containing about a pound of oil of vitriol under the receiver of the air-pump, and set in it a watch gla.s.s containing a little water, supported on a stand with _gla.s.s_ legs. Exhaust the receiver, when the water will evaporate, but without boiling; and the vapour being absorbed as it forms by the oil of vitriol, the vacuum is preserved, and the evaporation continues, until the vapour has abstracted so much caloric from the remainder of the water that it is all at once converted into ice.

In most elementary works on chemistry may be found a long table of freezing mixtures, as they are called, some with and others without ice or snow. We have selected a few from each division.

WITH ICE OR SNOW.

{ Snow or powdered ice 2 parts.

{ Powdered common salt 1

{ Snow 5 { Powdered common salt 2 { Powdered sal ammoniac 1

{ Snow 3 { Dilute sulphuric acid 2

{ Snow 2 { Crystallized muriate of lime 3

WITHOUT SNOW OR ICE.

{ Sulphate of soda 3 parts.

{ Dilute nitric acid 2

{ Nitrate of ammonia 1 { Water 1