XII. Such bodies as had hitherto been put into the air pump were gradually subjected to the action of a vacuum. Hence it appeared that the apparatus above described might be attended with a considerable degree of utility; as in future, any body whatever, whether solid or fluid, might be subjected to this action at once. For, the lower part of the receiver being deprived of its internal air, if the bladder be burst by means of the glass rod, the fluid will run down and occupy the space emptied of air, leaving a vacuum in the upper part. When the fluid has thus fallen to the lower part of the receiver, solid bodies even, if any were immersed in it while in the upper part of the receiver, will also experience the action of the vacuum. But liquid bodies are of such a nature, that they have united with them certain aëriform fluids, which, when the pressure of the air is removed, readily expand. A fluid, therefore, when it has fallen to the lower part of the receiver, being agitated and thrown into a state of perturbation by the motion, its most subtle principles will be extricated, and fill the capacity of the bell. The vacuum will then be disturbed by the evaporation, which, acting on the mercury in the barometer connected with the air pump, will cause it to fall. But every one acquainted with the principles of philosophy must know, that the depression of the mercury in the barometer will be greater, according as a greater quantity of aëriform fluids has been disengaged in the receiver; and if the degree of pressure in the barometer varies according to the variety of aëriform principles, it may be readily seen, that this method may be employed to determine the quantity of them, or their elasticity, since they are cut off from all communication with the surrounding atmosphere, though it still exercises its pressure upon them.