Of the Air, its various Properties. 147 A. What is the Reafon the Air is fo transparent, as to be invifible?

B. Because of the great Porofity thereof; the Pores and Interftices of Air being fo very great and large, it admits the Light not only in right Lines, but in fuch great and plentiful Rays, that the Brightness and univerfal Luftre thereof, not only renders the Air Diaphanous, but entirely hinders the Opacity of the very fmall Particles of Air from being at all feen; and therefore the whole Body of Air must confequently be invisible.

A. You obferved next, the Air hath the Property of being rarified and condensed; pray how is this demonftrated ?

B. That Air may be rarified is proved feveral Ways; as thus, if you take a Bladder, entirely empty as you think, and tie its Neck with a Thread, and lay it before the Fire, the Heat will fo rarify the little inclofed Air, as to make it extend the Bladder to its utmoft Stretch, and, if continued, will break through it with the Report of a Gun: Also, that Air may be so condenfed by Art as to take up but Part of the Space it poffeffed before, is proved by various Experiments.

A. Pray how do you prove the Air's Elafticity?

B. By various Experiments of the Airpump, and otherwife: One very plain, is L 2 thus;

thus; an empty Bladder, whofe Neck is faft tied, being put into the Receiver, and the external Air therein exhausted, the fmall Matter of the inclosed Air, will by its own proper Spring, or elaftick Force, gradually expand it felf, and at laft will fo extend the Bladder as to break it: Thus alfo, the Air compreffed in the Wind-gun, will by its elaftic Force (being difcharged) drive a Bullet through a Board at the Diftance of feveral Yards, in the fame Manner as with Gun-powder: Yea, Mr. Boyle hath found that Air by its Spring, or Elafticity, will fo far dilate or expand it felf as to take up 13769 Times a greater Space than before: This Power of Elafticity is as the Denfity of the Air.

A. Pray how do you become acquainted with the Air's Gravity?

B. By Experiments of the Air-Pump, Barometer, &c. The Weight of the Air is greater, the nearer it is to the Earth's Surface: The Mercury, by the Preffure of the Air, is raised in the Barometer to the Height of 28, 29, 30, or 31 Inches; therefore fuch a Column of Mercury is equal in Weight to a Column of Air (of an equal Bafis) which proceeds from the Mercury in the Tub to the uppermoft Part of the Atmosphere: Alfo becaufe Mercury is about 14 Times heavier than Water, therefore Water will rife in a Tube to 32 or 33 Feet

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The Weight of the Atmosphere. ia Height; and therefore every Square Foot in any Superficies fuftains the Weight of a Column of Water of 32 or 33 folid Feet; now a cubic Foot of Water weighs about 63 Pounds, hence the Weight of Air on every fuperficial fquare Foot, is above 2000 Pound Weight.

A. And, pray, what do you infer from thence ?

B. Why, thence it appears, that if we allow the whole Surface of a Man's Body of 6 Feet Stature) to be about 14 fquare Feet, then the Weight of Air preffing on the Body of fuch a Man is equal to 28000 Pounds, or 250 hundred Weight; that is 12 Tun: Alfo, fince the Number of 1/ fquare Miles on the Earth's Superficies is computed 199250205, and in one fquareMile are 27878400 fquare Feet, the square Feet on the Earth's Superficies will be fome-. what above 5547800000000000; whence the Weight of the whole Atmosphere, or its Preffure on the Superficies of the whole Earth, is more than 11095600000000000000 Pounds, or much about 500000oooooooooo Tuns; that is, the Atmosphere compreffeth the Earth with a Force, or Power, nearly equal to that of five thousand Millions of Millions of Tuns.

A. This is extreamly wonderful! Buty pray, how happens it that Men, Beafts, Houfes, &c. are not crushed to Pieces, if

they are preffed with fuch an intolerable Weight of Air?

B. By the Equilibrium of the internal Air, or the Air within all Bodies; which though it be small, and not worth naming, yet, can ballance, refift, and equiponderate the Force of the external Air (as is proved by various Experiments) how great a Quantity foever it be: The Experiments of the Air-pump which confirm this are very surprizing.

4. I think, Sir, all you have faid of the Weight and Gravity of the atmosphere, or Air, is full of Aftonishment; and, pray, can you tell any Thing of the Height of it?

B. Nothing certainly can be determined about that, because the higher you go, the rarer the Air is ; and there being no certain Means to determine in what Proportion the Air becomes rarer and rarer through the whole Extent thereof, there can of Courfe be no precife Account of its Altitude; However, they compute the Air at the Height of 42 Miles, to be 4096 Times more rare, or thin, than with us; and this being next to nothing, the Height of the Air may be reckoned about 40, or 45 Miles agreeably hereto; Dr. Keil hath calculated its Height to be 44 Miles by an Obfer vation of the Twilight.

4. Pray

The wonderful Advantages of the Air. 151 A. Pray what other very remarkable Properties hath the Air?

B. It is not only the Means, but as it were the Matter of Life itself, and therefore abfolutely neceflary.

A. How do you fhew it to be the Means of Life?

B. By putting certain Animals into the exhaufted Receiver; where it is surprising to see the Effects of withdrawing the Air by the Pump on the Bodies of thofe Creatures: You will thus fee Dogs, Cats, Rats, Mice, &c. turn up and expire in half a Minute, and look as thin as a Rag: A Mole dieth in one Minute: Infects, as Wafps, Bees, Hornets, Grafhoppers, &c. in two Minutes feem dead; and will continue a whole Day and Night without Air, and afterwards revive in open Air: Earwigs, Beetles, Snails, &c. endure the Air-pump prodigioufly; and Frogs will longer preserve their Lives in Vacuo than Toads; yea, thofe invisible Animalcules in Pepper-water, will revive in the open Air, after having lain Hours in Vacuo.

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A. It must be curious, indeed, to see thofe artificial Deaths and Refurrections in dumb Creatures! But, pray, how do you understand Air to be the Matter of Life?

B. 'Tis certain Air is impregnated with a vivifying Spirit, or Matter, which is imL 4 medi