It is visible in the Form of a Vapour, when the Vapour to which it adheres, becomes overcharged with electric Matter, by Descent into a _cool_ Eknèfiai Stratum below: there forming a luminous and transparent Atmosphere: the Particles of Light and Vapour being repelled to great Distances from each other at so _rare_ a Height. It culminates above the Vapour, because less heavy than the circumambient Air: and may be subject to the Attraction of other Planets. The Aurora Borealis is also seen to issue in Streams and Waves of Light, with inexpressible Velocity, on its Return to the South, in a lower Stratum, as it _passes throu’_ Interstices, between the Vesicles of warm Vapour, raised and dispersed by the turbulent Apogay Winds, in the middle Region. During Summer, the middle Region becomes blended with the lower, throu’ Defect of Cold: and the electric Matter is supposed to be communicated to the Earth, silently, and continually; but by Lightning, when a lower and colder Atmosphere condenses and overcharges the Vapour, and cuts off the Communication. It cannot be seen but in escaping from Vesicle to Vesicle: nor, during Summer, after Sunset, on Account of the Twilight. [83] Air is not unfit for Respiration, by having lost its _vital_ Principle, but because it has imbibed _Floguiston_, which cannot _easily_ be separated from it, but by Agitation in Water. Cavallo, on Air, Pages 479, 670. [84] For if Moisture be one Cause, which keeps the Particles of Air at greater Distances from each other; this Cause decreases at _great_ Altitudes. If also the _Elasticity_ decreases in Proportion, not only to the Height, but the Driness; its Particles must, on both Accounts, approach each other, at great Altitudes: tho’, from the Altitude only; they woud separate according to the Rule, viz. that the Rarity of the Air is proportionable to the Relaxation of the Force compressing it. So that at the Height of 8 or 10 Miles, a Quantity of Air taken from the Surface of the Earth, woud occupy 6 Times its former Space: supposing the Air both below and above to be of the _same Kind_, as well as of the _same mean_ Temperature of 55, on the Thermometer. See “Martin’s Philosophical Grammar, Page 178.” [85] Chalmer describing a Whirlwind, which is a _Storm_ of +collection+ and _Ascent_ of +hot+ Air, &c. by Rarefaction, says, “as the Wind ceased, presently after the Whirlwind passed, the +branches+ and Leaves of various Sorts of Trees, which had been carried into the Air, continued to +fall+ for +half an hour+; and, in their Descent, appeared like Flocks of Birds of different Sizes.” This Circumstance proves that Columns of +hot+ Air must have been raised in a Body, in Succession, to so considerable a Height, that _Branches_ of Trees carried up by them, took _half an Hour_ in falling.