212 Practice of Aerostation,

If it be asked, Why does not the hot airescape out of its apexture ? the answer is,Because the hot air is impelled by the colderair towards the apex of the cone, and conse-quently cannot go over the, edge of theaperture of the bag, except the cone wasto be inverted •, for then it would escape, inthe same manner as a bubble of air escapesout of a glass inverted in water, and inclin-ed a little, as was mentioned towards thebeginning of this chapter.

Besides common air, there are many otherelastic fluids, which are invisible and com-pressible like common air, but are distin-guishable from that element on account ofsome other properties that are particular toeach or divers of them; for instance, somewill not assist respiration, and which arecalled gaffes \ others are soon absorbed bywater, &cc. Amongst these gaffes there isone called inflammable air, from its pecu-liar property of burning when fire is com-municated to it by means of a candle, anelectric spark, &c. This elastic fluid, be-sides its inflammability, has another re-markable