The Refractory Cork
A very interesting and amusing experiment may be performed with a bottle and a cork.
Take a cork of a diameter less than the internal diameter of the neck of the bottle you propose using, and ask a friend to make it enter the bottle by blowing upon it.
At first sight this seems a very easy task, and your friend at once proceeds to blow strongly upon the cork. This, however, instead of making the cork enter the bottle, causes it to fly out.
Again your friend tries to overcome the troublesome cork, on the next occasion by blowing very gently, but again it flies out ([Fig. 1]).
The explanation of this is as follows:—
In blowing upon the cork, a certain amount of air at the same time enters the bottle, the air in which becomes so compressed that it rapidly ejects the cork. There are, however, three ways in which the refractory cork may be overcome.
Fig. 1.—The refractory cork.
Since you know that by blowing on the cork it is at once ejected, try to achieve success by performing the contrary action—that is, by withdrawing some of the air from the bottle.
Indeed, the experiment will prove to you that, by so doing, you create in the bottle a partial vacuum, and as soon as your mouth leaves the neck of the bottle air enters it owing to atmospheric pressure. This incoming current of air pulls with it the cork, which at once slides into the aperture.
The same result may also be achieved by first warming the bottle, when, owing to the expansion of the air, a part of it is expelled. Directly the air inside the bottle cools, a vacuum is created, and a current of air from without enters. If you add to this current of air by blowing air from your mouth, you will find this quite sufficient to cause the cork to enter the bottle. Then, again, having a straw or a pipe-stem handy, all you have to do is to blow through the tube, directing the air exactly on the base of the cork, which will once again enter the aperture.
Whichever of these means is adopted, you must always take the precaution of seeing that the bottle is perfectly dry. It should be wiped every time. The moisture formed in the neck is sufficient to prevent the cork from gliding along the glass.