J. A. Coolidge

Gases and liquids, because of their similarity in some respects, are called fluids. They are carried in vessels of any shape, have no difficulty in adapting themselves to the form of vessel into which they are put, and may be forced through pipes many miles in length. They have some points in which they differ widely. While liquids are almost incompressible, an immense force exerted on a cubic foot of water hardly diminishes its bulk so that any difference is noticeable ; a cubic foot of air, on the other hand, may be easily made to occupy a space one-half or even one-fourth as large. Then, again, the density of gases is very much smaller than that of liquids. Many solids may be be found lighter than the lightest liquid, but the lightest liquid is probably as many as a thousand times as heavy as any gas known. Our work in this issue deals with both gases and liquids, some of their characteristics and how they are related.

Experiment XXVIII. For all who have a bicycle pump an interesting experiment may be performed illustrating several things. A large. strong bottle, fitted tightly with a rubber stopple, and having a valve from a bicycle tire with the valve stem fitted in, as in Fig. 30, will serve as apparatus. It may be necessary to tie the cork in with a stout thread. First, weigh the bottle with cork and fittings, next, fasten to the pump and pump in several strokes. Weigh again and notice the increased weight. The bottle, filled with condensed air, weighs more than before. Follow out this reasoning and you will see that the bottle, without any air, would weigh less, and that the air in the bottle has a definite weight. Let some of the air out of the bottle and notice the force with which it escapes. If there were more of it, and the supply could be maintained, almost any kind of mechanical motion might be produced. Recall some of the uses of compressed air. Brakes on cars are made to act, packages are sent flying through pneumatic tubes, and cars are made to go by means of compressed air. Plunge the bottle, filled with compressed air, into a tub of water and again open the valve. Instead of seeing the water enter the bottle the air comes bubbling out. Imagine our bottle to be a tunnel under a river, as we have in Boston, under the harbor. If air is forced into the tunnel and kept there compressed, a small leak will not let water through, because the pressure of air against the hole trying to get out is greater than the force of the water trying to get in. Such a condition is today quite frequent. A diving bell filled with compressed air has the power to prevent the entrance of water from below. A simple illustration of this may be seen in the forcing of an ordinary glass tumbler, inverted, down into a pail of water. See Fig. 31. A little block of wood under the glass makes it more apparent that the water does not enter the glass but is prevented by the air already there.

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The air all around us exerts pressure upon all things although, for the most part, this pressure is unnoticed Its pressing force is felt when we try to open a bottle or to take the top off a glass covered fruit jar. Experiment XXIX.

Place a small quantity of water in a bottle and then place the bottle in a pan of moderately warm water. Bring the water to the boiling point and when boiling briskly remove from the fire. As soon as the water ceases to boil cork the bottle tightly and allow it to cool. After it is cool you can see that the cork hasbeen driven deeper into the neck of the bottle, that it requires considerable force to removed it and, that when pulled out, the air rushes in with a slight explosive sound. The air presses upon the cork with a force great enough to push it down into the bottle; the steam, when cooled off, is condensed into water and occupies so much less space than as steam that there is a partial vacuum into which the air rushes when the cork is pulled out.

Experiment XXX.

A circular piece of thick leather, two inches in diameter, with a string passing through the center and knotted, makes what is known as a "sucker". Make the knot as flat as possible by pounding, soak the leather in water and press it firmly upon a very smooth, flat stone or pane of glass, so that no air shall be left between the leather and the glass. Make a loop in the string and fasten to the spring balance (see Fig. 32) and see how many ounces you can pull before the leather is separated from the glass. Make at least three trials. The air pressure upon the leather is so great that an opposing force still larger must be used to overcome this.

Experiment XXXI.

The effect of heat upon a body of air may be seen in this experiment. Take the bottle used in Experiment XXVIII., remove the valve from the stem, connect to the stem the rubber tube of the pump after unserewing it from the pump, and plunge the bottle in a pail of warm water, holding the open end of the tube under water, as in Fig. 33. The heat expands the air and causes it to force its way out through the opening A. This expanded air, being less dense, will rise. The air around a stove being heated rises, and cooler air rushes in to take its place. The products of combustion in our stoves, consisting of heated air and gases rise, pass out one chimney, and are replaced by cool air coming in at the bottom of the stove. The hot air in the top of a furnace rises because of the pressure it exerts, the cold air comes in to take its place, forcing it up and thus the air is carried through the pipes to the rooms of the house. The expansion of gases due to heat, the pressure of colder, heavier ones crowding down to take their place, explains the principle of chimneys and ventilators.

Experiment XXXII.

Take a common clay pipe and a thin piece of sheet rubber about two inches square. The rubber can be obtained from any dentist. Tie the rubber with a thread so as to make a diaphragm covering over the open end of the pipe. By drawing in the breath the air may be removed frem the inside of the robber diaphragm, leaving the air on the outside free to press as it will. No matter what the direction of the bowl of the pipe, the diaphragm is pressed in whenever the air on the inside is drawn out, and returns when the air is allowed to flow in. This experiment. perhaps, better than any other, illustrates the fact that air presses in all directions.

Elementary Mechanics X Compressed Air 43

Experiment XXXIII. Take a tumbler and a piece of cardboard just covering it. Fill with water, cover the card and invert, holding the hand on the card until level. See Fig. 34. The card does not fall off; the water does not run out. If done correctly considerable force may be used to jerk the card off without causing it to fall. Of course the water and card would fall of their own weight were they not held in place by a larger force pressing up. This upward force must be the air, as there is nothing else there that can do it. We have seen some illustration of air pressure; in our next paper we will try some experiment with siphons, pumps, etc.