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The energy produced by the phenomena of diffusion is exhibited in lecture courses by placing a bell glass filled with hydrogen over a porous vessel at whose base is fixed a glass tube that dips into water. The hydrogen, in diffusing, enters the porous vessel, increases the internal pressure, and a number of bubbles escapes from the tube. On withdrawing the bell glass of hydrogen, the latter becomes diffused externally, a lower pressure occurs in the porous vessel, and the level of the water rises.
The arrangement devised by Mr. C.J. Woodward, and recently presented to the Physical Society of London, is an adaptation of this experiment to the production of an oscillating motion by alternations in the internal and external diffusion of the hydrogen.
The apparatus, represented herewith, consists of a scale beam about three feet in length that supports at one end a scale pan and weights, and, at the other, a corked porous vessel that carries a glass tube, c, which dips into a vessel containing either water or methylic alcohol. Three or four gas jets, one of which is shown at E, are arranged around the porous vessel, as close as possible, but in such a way as not to touch it during the oscillation of the beam. These gas jets communicate with a gasometer tilled with hydrogen, the bell of which is so charged as to furnish a jet of sufficient strength. Experience will indicate the best place to give the gas jets, but, in general, it is well to locate them at near the center of the porous vessel when the beam is horizontal.
It is now easy to see how the device operates. When the hydrogen comes in presence of the porous vessel it becomes diffused therein, and the pressure exerted in the interior then produces an ascent. When the bottom of the porous vessel gets above the jets, the internal diffusion ceases and the hydrogen becomes diffused externally, the internal pressure diminishes, and the vessel descends. The vessel then comes opposite the jets of hydrogen and the same motion occurs again, and soon indefinitely. The work produced by this motor, which has purely a scientific interest, is very feeble, and much below that assigned to it by theory. In order to obtain a maximum, it would be necessary to completely surround the porous vessel each time with hydrogen, and afterward remove the jets to facilitate the access of air. All the mechanical arrangements employed for obtaining such a result have failed, because the friction introduced by the maneuvering parts also introduces a resistance greater than the motor can overcome. There is therefore a waste of energy due to the continuous flow of hydrogen; but the apparatus, for all that, constitutes none the less an original and interesting device.—La Nature.
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