Pneumatics treat of the mechanical properties of air, gases, and vapours. Ail air, gases, and vapours not in contact with the liquids from which they rise, partake of the same general properties; that is, they all possess weight and inertia, impenetrability, compressibility, and elasticity. The impenetrability of air may be made manifest by the impossibility of bringing together the opposite sides of a blown bladder. It may be also shown, by taking a cylinder with a smooth bore, and fitting a piston or plug into it so closely that the air may not pass between its sides and the tube; it will then be found that no power we can command will force the plug to the bottom of the cylinder. In making this experiment, however, we observe two of the most important properties of air, viz. its compressibility and elasticity; for although the plug cannot be forced to the bottom of the cylinder, yet it may be considerably depressed, so that the air is reduced to a much smaller volume, and, consequently, is compressible. On the withdrawal of the pressure another remarkable phenomenon presents itself, that is, the plug is forced upwards to its original position.
The especial properties of air are as follows:-
It possesses weight and inertia;
It exerts an equal pressure in every direction;
It is compressible and elastic. We shall speak of each of these properties in succession; but first it may be necessary to describe the air pump, an instrument which is in the highest degree useful in pneumatic experiments. In the annexed sectional representation, A and B are the barrels of the pump, which must be perfectly cylindrical and smooth within. C is a glass receiver placed upon the pump plate E; and D is a pipe communicating with the receiver and the two barrels: b and d are valves at the bottom of the barrels, opening upwards; a and c are valves in the pistons, which must fit into the barrels with the greatest accuracy; e and / are racks attached to the pistons, and which are moved upwards and downwards by means of the toothed wheel g, which is turned by a winch fixed on its axis. By comparing this instrument with a common water pump, its principle will be found identical; but as air is a much lighter and more elastic fluid, it will require the workmanship in the air pump to be of the most accurate description.
In working this pump it will be seen that as the piston in B is raised, the air which previously filled only the receiver and the pipe D will be expanded by its elasticity so as to fill the barrel also; by the next motion of the handle the piston is depressed, and the air within the barrel becoming compressed will close the valve b, and open that at a, through which it will escape into the atmosphere. When the piston is again raised, the air left in the receiver will be again expanded so as to fill the barrel, and on being depressed, the air will escape as before. We have only in this process noticed one barrel, but the action of both is similar; while one is filling by the expansion of the air in the receiver, the other is emptied into the surrounding atmosphere. By this alternate action of the pistons, the air within becomes considerably rarefied, but as the portion withdrawn is always a definite part of what was previously in the receiver, it is manifest that a perfect vacuum cannot be obtained. We have stated that air has weight: this may easily be shown by means of the air pump.
If we take a glass or other vessel holding exactly a quart, and furnished with a stop-cock that will fit into the hole in the middle of the pump plate, we may exhaust or withdraw nearly the whole of the air from the vessel. Now, if we weigh the empty vessel, and afterwards, by turning the stop-cock, let in the air, the difference of weight in the bottle will show the weight of the quantity of air admitted: this will be about seventeen grains, varying at different times, both on account of changes of density which take place in the atmosphere, and the varying quantity of aqueous vapour that it may contain. The inertia of air may be seen in the resistance it offers to the motions of bodies immersed in it. Two sets of small brass vanes are sometimes put into motion by the same force under the receiver of an air pump. While the vanes in both are turned one way, they revolve for the same length of time whether in the air or in a vacuum; but if in one of them the broad surfaces of the vanes are turned in the direction of the motion, and in the other the narrow edges, a marked difference is observed.
In an exhausted receiver they continue in motion during equal times, but in the air, that which cuts the atmosphere with its edges continues moving for some time after the other is at rest.
Another experiment illustrative of the same fact is termed the guinea and feather experiment. A long receiver is placed upon the pump plate, and a guinea and a feather are attached at the top to a little piece of apparatus by which they may be disengaged at the same instant. While the receiver is. full of air, the guinea reaches the pump plate before the feather, but when the air is taken from the receiver, the guinea and feather fall with exactly the same velocity.
Since air is fluid, it will manifest the common properties of fluids; as, for example, pressure. If a small vessel, similar to the one here represented, be placed over the hole in the pump plate, and the hand placed closely over the top, when the pump is worked, the hand will be held firmly on the glass by means of the downward pressure. In the same way, the glass receivers are held firmly on the pump plate. If a bladder be made wet, and tightly stretched over the top of the glass, then dried and placed over the hole of the air pump, as soon as the pump is worked, the bladder will appear concave at the top, and will eventually be burst by the great pressure of the superincumbent air. Another apparatus, admirably adapted to evince the great pressure of the air in all directions, is what are termed the Magdeburg hemispheres. It consists of two hemispheres of brass, having their edges accurately ground, so that they may fit together, as in the annexed representation. The part a of the lower hemisphere is screwed into the hole of the pump plate, and the air may then be exhausted. If then the handle b be screwed on, two persons may endeavour to separate them by pulling in opposite directions, or they may be suspended, and a weight attached to the lower one.