PNEUMATICS.
“There is a tricksy spirit in the air
That plays sad gambols.”—Ben Jonson.
The branch of the physical sciences which relates to the air and its various phenomena is called Pneumatics. By it we learn many curious particulars. By it we find that the air has weight and pressure, colour, density, elasticity, compressibility, and some other properties with which we shall endeavour to make the young reader acquainted by many pleasing experiments, earnestly impressing upon him to lose no opportunity of making physical science his study.
The common leather sucker by which boys raise stones will show the pressure of the atmosphere. It consists of a piece of soft but firm leather, having a piece of string drawn through its centre. The leather is made quite wet and pliable, and then its under part is placed on the stone and stamped down by the foot. This pressing excludes the air from between the leather and the stone, and by pulling the string a vacuum is left underneath its centre; consequently the leather is firmly attached to the stone, which enables you to lift it.
WEIGHT OF THE AIR PROVED BY A PAIR OF BELLOWS.
Shut the nozzle and valve-hole of a pair of bellows, and after having squeezed the air out of them, if they are perfectly air-tight, we shall find that a very great force, even some hundreds of pounds, is necessary for separating the boards. They are kept together by the weight of the air which surrounds them in the same manner as if they were surrounded by water.
THE PRESSURE OF THE AIR SHOWN BY A WINE-GLASS.
Place a card on a wine-glass filled with water, then invert the glass: the water will not escape, the pressure of the atmosphere on the outside of the card being sufficient to support the water.
ANOTHER.
Invert a tall glass jar in a dish of water, and place a lighted taper under it; as the taper consumes the air in the jar, the water, from the pressure without, rises up to supply the place of the oxygen removed by the combustion. In the operation of cupping the operator holds the flame of a lamp under a bell-shaped glass. The air within this being rarefied and expanded, a considerable portion is given off. In this state the glass is placed upon the flesh, and as the air within it cools it contracts, and the glass adheres to the flesh by the difference of the pressure of the internal and external air.
ELASTICITY OF THE AIR.
This can be shown by a beautiful philosophical toy, which may easily be constructed. Procure a glass jar such as is here represented, and put water into it. Then mould three or four little figures in wax, and make them hollow within, and having each a minute opening at the heel, by which water may pass in and out. Place them in the jar, as seen in the figure, and adjust them by the quantity of water admitted to them, so that in specific gravity they differ a little from each other. The mouth of the jar should now be covered with a piece of skin or india-rubber, and then, if the hand be pressed upon the top or mouth of the jar, the figures will be seen to rise or descend as the pressure is gentle or heavy; rising and falling, or standing still, according to the pressure made.
REASON FOR THIS.
The reason of this is, that the pressure on the top of the jar condenses the air between the cover and the water surface; this condensation then presses on the water below, and influences it through its whole extent, compressing also the air in the figures, forcing as much more water into them as to render them heavier than water, and therefore heavy enough to sink.
THE AIR-PUMP.
The time was, and that not very long ago, when the air-pump was only obtainable by the philosophical professor or by persons of enlarged means. But now, owing to our “cheap way of doing things,” a small air-pump may be obtained for about a guinea, and we would strongly advise our young friends to procure one, as it will be a source of endless amusement to them; and, supposing that they take our advice, we suggest the following experiments.
The air-pump consists of a bell glass, called the receiver, A, and a stand, upon which is a perforated plate, B. The hole in this plate is connected with two pistons, the rods of which are moved by a wheel handle backwards and forwards, and thus pump the air out of the receiver. When the air is thus taken out, a stop-cock is turned, and then the experiments may be performed.
Under the receiver of an air-pump, when the air has been thoroughly exhausted, light and heavy bodies fall with the same swiftness. Animals quickly die for want of air, combustion ceases, a bell sounds faint, and water and other fluids change to vapour.
TO PROVE THAT AIR HAS WEIGHT.
Take a florence flask, fitted up with a screw and fine oiled silk valve. Screw the flask on the plate of the air-pump, exhaust the air, take it off the plate, and weigh it. Then let in the air, and again weigh the whole, and it will be found to have increased by several grains.
TO PROVE AIR ELASTIC.
Place a bladder out of which all the air has apparently been squeezed under the receiver, upon it lay a weight, exhaust the air, and it will be seen that the small quantity of air left within the bladder will so expand itself as to lift the weight. Put a corked bottle into the receiver, exhaust the air, and the cork will fly out.
SOVEREIGN AND FEATHER.
Place a nicely-adjusted pair of forceps at the top of the receiver, communicating with the top of the outside through a hole, so that they may be opened by the fingers. Then place on each of the little plates a sovereign and a feather. Exhaust the air from the receiver; and having done so, detach the objects, so that they may fall. In the open air the sovereign will fall long before the feather, but in vacuo, as in the receiver now exhausted of its air, they will fall both together, and reach the bottom of the glass at the same instant.
AIR IN THE EGG.
Take a fresh egg, and cut off a little of the shell 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 shell 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 glass, because the air which would have supported it has been withdrawn.
AIR IN THE EGG.
DESCENDING SMOKE.
AIR IN THE EGG.
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.
SOUNDLESS BELL.
FLOATING FISH.
SOUNDLESS BELL.
FLOATING FISH.
THE FLOATING FISH.
If a glass 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 glass, 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 illustrated by the following experiment. Take a glass 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 glass 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 glass, 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 Institution.
EXPERIMENTS.
1. Place a cylinder of strong glass, 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 glass 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 superincumbent 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 quantity 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 brass, 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 stopcock, 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.
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 glass 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 quantity of the vapour of mercury.
EXPERIMENT.
Pass a little ether through the mercury in the tube, and as soon as it reaches the empty space 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.
EXPERIMENT.
To show that the heat abstracted by the boiling of one liquid will freeze another, fill a tall narrow glass about half full of cold water (the colder the better), and place in it a thin glass 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 glass containing a little water, supported on a stand with glass 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 | „ | ||
| - | Phosphate of soda | 2 | „ | |
| Dilute nitric acid | 1 | „ | ||
| - | Sulphate of soda | 2 | „ | |
| Muriatic acid | 1 | „ | ||
The effects of most of these mixtures may be considerably increased by previously cooling the ingredients separately in other freezing mixtures.
In connexion with this branch of science, and especially with chemistry, the youthful philosopher should practise the art of decanting air from one jar to another standing over water, beginning by passing it from a small to a larger jar, then with two of equal size; and when he can accomplish the transfer without permitting even one bubble to escape, he may essay the much more difficult task of transferring the air from a large to a smaller jar.
He should also practise using the blowpipe until he can keep up a steady and uninterrupted flame for ten minutes or a quarter of an hour, without stopping for breath. It is quite possible to replenish wind in the mouth, which alone ought to be used, without interrupting the breathing for an instant, but it requires some practice.
(1) Box blowpipe, A, the stem; B, the mouthpiece; C, the box, in which is placed a piece of sponge to absorb the moisture of the breath; D, the extremity (this is moveable, and each blowpipe has several, with different apertures); E, the hollow pivot, in which the extremity and its arm turn. The box is opened by the milled ring seen at the bottom.