These various objects being prepared, will serve to demonstrate once more the atmospheric pressure. Lift the valve up, force the cork into the tube towards the middle, and when there, put the valve in its place again, and pull quickly on the string as if trying to pull the cork out. The latter will not come out, for the reason that producing a vacuum before itself, the atmospheric prepare will prevent it from coming out. But if instead you draw it gently towards you, it will offer much less resistance, because the outer air will enter through the smallest interval between the cork and the glass, and partly destroy the exterior pressure. Petroleum Pulverizer. With the aid of the compressed air reservoir you are able to conduct various soldering operations, requiring often great heat. Let us construct the following pulverizer: Into a bottle of the shape shown in the figure, put some petroleum, and introduce a glass tube that does not quite reach the bottom. Close with wax that no air can enter, and at the upper extremity of the tube let a fitting be embedded, a section of which is shown in the engraving. This fitting has three openings, two horizontals and one corresponding with the tube that is plugged into the petroleum. [Illustration] Adjust the india-rubber tube to the reservoir, and when the pressure is exercised on the surface of the liquid it will force the petroleum upwards through the tube, and thence it will be blown in a fine spray, which burns as if coming out of an ordinary burner. The particles of oil are mixed with air, and consequently the atoms of air are heated to a high degree. This jet develops a heat of great intensity. This pulverizer may serve also to disinfect rooms. You have only to replace the petroleum with an antiseptic liquid. Electric Attraction and Repulsion. The poles of the same sign repel, and the contrary poles attract each other, or, in other words, the negative, or the positive electricity attracts the electricity of a contrary sign, whereas the electricities of the same signs repel each other. In order to demonstrate this principle we will contrive a little plaything which will be as interesting as amusing to see in operation. [Illustration] For a pivot take a needle stuck in a cork, and, as magnetic needles, two old corset steels will do very well. If these cannot be had a clock spring may do instead. Magnetize these two steel rods by rubbing them with a magnet. In the middle of one of the rods punch a small hollow so that it may freely move on the needle in the cork without fear of falling off. You have thus manufactured a rough compass. Then cut out four dolls in paper, two gentlemen and two ladies, and stick them in the extremities of the two magnetized needles. Remember to put at each end a figure of the opposite sex. Now, each time you present a man to the other man, which is placed on the magnetized needle, they will repel each other; if to a lady, the dolls are attracted.