continues, until the air in the vessel becomes too much rarefied to lift the delicate valve and make its escape. The vacuum thus produced is by no means so perfect as the “Torricellian vacuum,”[12] the name given to the unoccupied space above the column of mercury in a barometer. Various substances may be placed in a receiver to show the expansive tendency of air. A piece of wood immersed in a jar of water will throw off thousands of little bubbles. A shriveled apple will become round and plump. The air in an empty rubber bag will often expand so as to fill the receiver. Air in a thin glass vessel, tightly corked, will expand so as to burst the vessel into fragments. The following simple but useful piece of apparatus can easily be made: Take a pint bottle with a nicely fitted cork, through the cork insert a small glass tube so as to be perfectly air-tight (melted sealing wax is convenient for making tubes or glass tight;) a perforated _rubber_ stopper is better. Let the end of the tube inserted be drawn out in the flame of an alcohol lamp (this is not essential, but will make the experiments more interesting), suck the air from the bottle, close the end of the tube at once with the finger and place it in a glass of water, and a miniature fountain, in vacuo, will be revealed. After performing this pretty experiment remove the tube and reinsert it with the larger end down, having filled the bottle two-thirds full of water. With the lips force a quantity of air into the bottle, upon removing the mouth the water will rise in the tube and fall in a fine spray from the small aperture at the top. This last experiment is particularly interesting, as it is a perfect illustration of a flowing oil well. Closely allied to the expansibility of air is its ELASTICITY, Or tendency to regain its former volume after being compressed. Many a school-boy has observed this property, while manipulating his fascinating popgun. When he places his finger over the open end of the piece of elder, utilized as a gun, and suddenly pushes down the piston upon the wad, he notices that it quickly flies back. An inflated bladder thrown upon the floor bounds like a rubber ball; force pumps in our houses act upon this principle. The air in the chamber of the pump is first compressed by the entrance of the water; it reacts like a spring, and forces the water through the pipes to the rooms above. [Illustration: EXPANDING RUBBER BAG IN AN EXHAUSTED RECEIVER.]