(36.) It has since been proved, that water, and other liquids, are compressible. In the year 1761, Canton communicated to the Royal Society the results of some experiments which proved this fact. He provided a glass tube with a bulb, such as that described in (28), and filled the bulb and a part of the tube with water well purified from air. He then placed this in an apparatus called a condenser, by which he was enabled to submit the surface of the liquid in the tube to very intense pressure of condensed air. He found that the level of the liquid in the tube fell in a perceptible degree upon the application of the pressure. The same experiment established the fact, that liquids are _elastic_; for upon removing the pressure, the liquid rose to its original level, and therefore resumed its former dimensions. (37.) Elasticity does not always accompany compressibility. If lead or iron be submitted to the hammer, it may be hardened and diminished in its volume; but it will not resume its former volume after each stroke of the hammer. (38.) There are some bodies which maintain the state of density in which they are commonly found by the continual agency of mechanical pressure; and such bodies are endued with a quality, in virtue of which they would enlarge their dimensions without limit, if the pressure which confines them were removed. Such bodies are called _elastic fluids_ or _gases_, and always exist in the form of common air, in whose mechanical properties they participate. They are hence often called _aeriform fluids_. Those who are provided with an air-pump can easily establish this property experimentally. Take a flaccid bladder, such as that already described in (27.), and place it under the glass receiver of an air-pump: by this instrument we shall be able to remove the air which surrounds the bladder under the receiver, so as to relieve the small quantity of air which is inclosed in the bladder from the pressure of the external air: when this is accomplished, the bladder will be observed to swell, as if it were inflated, and will be perfectly distended. The air contained in it, therefore, has a tendency to dilate, which takes effect when it ceases to be resisted by the pressure of surrounding air.