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(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 (ssss1.), 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.