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That it is the air alone which excludes the water from the goblet, in the preceding experiments, can easily be proved. When the goblet is sunk deep in the vessel of water, let it be inclined a little to one side until its mouth is presented towards the side of the vessel; let this inclination be so regulated, that the surface of the water in the goblet shall just reach its edge. Upon a slight increase of inclination, air will be observed to escape from the goblet, and to rise in bubbles to the surface of the water. If the goblet be then restored to its position, it will be found that the cork will rise higher in it than before the escape of the air. The water in this case rises and fills the space which the air allowed to escape has deserted. The same process may be repeated until all the air has escaped, and then the goblet will be completely filled by the water.

(35.) Liquids are compressible by mechanical force in so slight a degree, that they are considered in all hydrostatical treatises as incompressible fluids. They are, however, not absolutely incompressible, but yield slightly to very intense pressure. The question of the compressibility of liquids was raised at a remote period in the history of science. Nearly two centuries ago, an experiment was instituted at the Academy del Cimento in Florence, to ascertain whether water be compressible. With this view, a hollow ball of gold was filled with the liquid, and the aperture exactly and firmly closed. The globe was then submitted to a very severe pressure, by which its figure was slightly changed. Now it is proved in geometry, that a globe has this peculiar property, that any change whatever in its figure must necessarily diminish its volume or contents. Hence it was inferred, that if the water did not issue through the pores of the gold, or burst the globe, its compressibility would be established. The result of the experiment was, that the water did ooze through the pores, and covered the surface of the globe, presenting the appearance of dew, or of steam cooled by the metal. But this experiment was inconclusive. It is quite true, that if the water had not escaped upon the change of figure of the globe, the compressibility of the liquid would have been established. The escape of the water does not, however, prove its incompressibility. To accomplish this, it would be necessary first to measure accurately the volume of water which transuded by compression, and next to measure the diminution of volume which the vessel suffered by its change of figure. If this diminution were greater than the volume of water which escaped, it would follow that the water remaining in the globe had been compressed, notwithstanding the escape of the remainder. But this could never be accomplished with the delicacy and exactitude necessary in such an experiment; and, consequently, as far as the question of the compressibility of water was concerned, nothing was proved. It forms, however, a very striking illustration of the porosity of so dense a substance as gold, and proves that its pores are larger than the elementary particles of water, since these are capable of passing through them.