ELASTICITY OE ATMOSPHERIC AIR.

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dense and more ponderous fluid. We are not sensible of theweight of the air, on account of its being equally applied onevery side, besides, that we have been accustomed to support itfrom our earliest infancy. Sensations to which we have at alltimes been habituated, are not felt; we can perceive no differ-ence where we have no standard to judge of variations.

ELASTICITY OF ATMOSPHERIC AIR.

That the air is an elastic fluid, or capable of compression andexpansion may be proved by a great number of experiments.In fact the elasticity or spring of the air produces the same effectas the pressure, because the pressure is equal to the compressingforce ; for if it were less, it is clear it would yield and be morecompressed; were it greater, it would not be so much reduced;for action and re-action are always equal.

The air-gun, the effects of which are generally known, illus-trates the elasticity of the air and shows the compression it issusceptible of.

A foot-ball or bladder filled with air, rebounds from any hardbody.

The elasticity of the air is also proved by fish and other ani-mals. The former have received from nature various means ofaction; among these are an air-bladder, which they contract ordilate at pleasure, and by this double property they ascend ordescend in the water.

INVISIBILITY OF AIR.

Atmospheric air when confined in vessels is absolutely invisi-ble, and cannot be distinguished from the glass which contains it.It owes this property to the ready passage it affords to the raysof light, which are refracted without being reflected; and it istherefore destitute of colour; though some philosophers havepersuaded themselves that they discovered large masses of it to be