There is a law in Physics which expresses the relation between expansion and elasticity of gases, which may be said to be as follows:— The elasticity (of a gas) is in inverse ratio to the space it occupies, and therefore by compressing air into a small space we can obtain a great force, as in the air-gun and the pop-gun of our youthful days. [Illustration: Fig. 43.—Hard boiled egg, divested of its shell, passing through the neck of a glass bottle, under the influence of atmospheric pressure.] In the cut below we can illustrate the principle of the pop-gun. The chamber full of air is closed by a cork and by an air-tight piston (S) at _p_ and _p_. When the piston is pushed into the chamber the air is compressed between it and the stopper, which at length flies out forcibly with a loud report [Illustration: Fig. 44.—The principle of the pop-gun.] We have said that the tendency of air particles is to fly away from each other, and were it not for the earth’s attraction the air might be dispersed. The height of the atmosphere has been variously estimated from a height of 45 miles to 212 miles in an attenuated form; but perhaps 100 miles high would be a fair estimate of the height to which our atmosphere extends. [Illustration: Fig. 45.—Weighing the air.] The pressure of such an enormous body of gas is very great. It has been estimated that this pressure on the average human body amounts to fourteen tons, but being balanced by elastic fluids in the body, the inconvenience is not felt. The _Weight_ of Air can easily be ascertained, though till the middle of the seventeenth century the air was believed to be without weight. The accompanying illustration will prove the weight of air. Take an ordinary balance; and suspend to one side a glass globe fitted with a stop-cock. From this globe extract the air by means of the air-pump, and weigh it. When the exact weight is ascertained turn the stop-cock, the air will rush in, and the globe will then pull down the balance, thus proving that air possesses weight. The experiments of Torricelli and Otto von Guerike, however, demonstrated that the air has weight and great pressure. Torricelli practically invented the barometer, but Otto von Guerike, by the cups known as _Magdeburg Hemispheres_, proved the pressure of the outward air. This apparatus is well known, and consists of two hollow copper hemispheres which fit very closely. By means of the air-pump which he invented in 1650, Otto von Guerike exhausted the air from the closed hemispheres. So long as air remained in them, there was no great difficulty in separating them; but when it had been finally exhausted, the pressure of the surrounding atmosphere was so great that the hollow spheres could not be dragged asunder even by horses harnessed to rings which had been inserted in the globes. [Illustration: Fig. 46.—Magdeburg Hemispheres.]