When the air of a room is very calm, have you ever noticed that tobacco smoke rises slowly and in a nearly vertical direction? Have you never watched with interest the grayish or bluish streaks of smoke issuing from the smoke of a cigar or pipe? And in seeing the smoke rise in such a capricious fashion, have you ever ascertained that it is due to the calm of the surrounding air?

One may get some amusement out of the agitation of the atmosphere. For the materials you only require a square or round cardboard box, in the lid of which cut a round hole of about two inches in diameter. In the interior of the box place two sheets of blotting paper, the one impregnated with muriatic acid, the other with ammoniac, in equal quantities. Immediately a whitish smoke will escape through the hole and rise straight to the ceiling. If with both hands you give a series of simultaneous taps on the sides of the box, you will see the smoke issue in well defined rings, which will disperse rapidly in the air, and succeed each other as long as you continue the pressure. These rings are the result of the concussion of the air which you occasion in the box. The experiments can be made also with ordinary tobacco smoke, but it will last longer in the way we have indicated.