220

DIL AT ABILITY OF AIR.

of a blue colour, as is the case in the sky, or in viewing exten-sive landscapes. But this is certainly a mistake; the blue colouris occasioned by the vapours which are always mixed with theair, and which have the property of reflecting the blue rays morecopiously than any other.

It appears from the very accurate observations of Saussure,that the colour of the sky acquires a deeper shade in proportionas it is viewed from a more elevated situation.* Consequentlyat a certain height the blue will disappear altogether, and thesky appear Hack, that is to say it will reflect no light at all.

DILATABILITY OF AIR.

/

We have noticed already (page /O,) that if a flaccid or half-blown bladder be exposed to the action of caloric, the air willbe dilated so as to burst the bladder.

Experiment I.

To the bottom of a hollow glsss-ball let an open bended tubebe affixed. Let the lower part of the bended tube be filled withmercury; the external surface will be pressed by the weight ofthe atmosphere, and the air inclosed in the ball of the tube bymeans of the mercury will be equally pressed by the spring ofthe air within the vessel. If the ball be immersed in boilingwater, the increased elasticity of the included air will raise themercury in the tube.

Remark. ..The theory of the Mongolfiers depends on the rare-faction of die air. In this case a given volume of atmosphericair is rarefied by heat, and kept separated from the commonmass, by a hollow vessel of cloth. This rarefied space may diere-fore be considered as consisting of a mass of air of greater levitythan atmospheric air. It must therefore make an effort to risein it.

« Journal de Physique, March 1777, and Saussure's Voyages dans lcs Alpes.