contagious Animals or Plants, all the disagreeable Effluvia from the Bodies of Men and Beasts, and whatever elfe of other Infections in the Air might be added hereto : Ifay, if all things were fet before his Eyes in the fame manner, would he not loath and nauseate the very fight of them? The fame would certainly befal him, if he were capable of feeing with his Eyes, the Air that he must constantly breath, fill'd with so many impure and unwholsome Particles; would he not live in a continual fear of being poifon'd by them? Would he not employ all the Powers of his Mind, even till he was tired, to find, if it were poffible, among fuch a loathfome heap of disagreeable things, fomething that was clean, and could be fuckt in without naufeating? Should we not fee Rich Men offering more Money for Places where the Air was pure and wholfome, than they now bestow for stately Houses and Country Seats? Now it has pleafed the gracious Director of all things fo carefully to provide against these Inconveniences (that what befals us every Moment of our whole Lives, namely, the Infpiration and Expiration of Air, might be perform'd with pleasure, or, at leaft, without producing in us any difagreeable Senfations) as to render invifible to us, that Air which would otherwise set before our Eyes a perpetual Swarm of deteftable Objects; and by this means only (tho' they fhould not be dangerous to our Health or Life) release us from inceffant Cares and Fears, of drawing into the Lungs by the Mouth and Wind-Pipe fuch a quantity of odious things.

The like Averfion and Dread of fo many Particles floating in the Air would befal us, but in a much higher degree, if they fhould become fenfible to our Taft. Ought not then every Man to acknowledge his Obligations to the Wisdom and Mercy of the great Ruler of this World? Who, tho' he caufes

causes us to Hear this compounded Air in Flutes and Organs, to Feel it in Winds and Storms, and to Smell it too in many Cafes; yet, that he might not make us miferable, has form'd it after fuch a manner, that notwithstanding its being impregnated and laden with such a diversity of Parts, it can be neither feen nor tafted, except in fome particular and very rare Cafes; by which an Atheist may be convinced, that He who brings this about, does it of his free Will and Pleafure; but by no means can it be faid to be thus order'd by neceffary Confequences, and much lefs by Chance. Accordingly we find, for instance, that when an Apothecary has pounded a good quantity of Aloes, and that the finest Parts thereof fly up, and mingle themfelves with the Air, their Bitterness discovers itfelf to the Taft of thofe that fuck in that Air: And to fhew, that the Air is likewife in its own Nature visible, we need only comprefs a good quantity thereof together in an Air-Pump, and then let it out again as quick as we can, and it will presently fhew it felf to our Eyes like a Fog or Mift.

SECT. XV. The Obfervation of Meteors refumed.

BUT to return to the Meteors; If we fhould attempt to fhew the Causes thereof fully and clearly, we must do it by a number of Natural and Chymical Experiments, which might be render'd Analagous and Uniform to the fame in Little : But this would engage us in too large a Field; we fhall however produce fome few, to fhew how the fame are generated in the Air, without pretending that they may not come to pass many other ways; for as fome of these that are now known to us were hid from the Ancients, fo perhaps fome may be discover'd by our Pofterity, of which we are hitherto ignorant. SECT.

SECT. XVI, and XVII. Mifts and Fogs produced by many Exhalations, and by the Rarefaction of the Air, fhewn experimentally.

To fay fomething first of Mifts and Fogs: It is plain from what has been faid, that unspeakable numbers of watry Vapours and other Exhalations do mingle themselves with Air, by which they render it Thick and Untransparent or Dark: As first, when they arife in too great a Quantity, and are fo closely compreffed together, as to fill the Air, and to obftruct a free Paffage of Light. In the fame manner we see in Chambers, where the Smoak does not go directly up the Chimney, as alfo by the thick Steams of boiling Water in Kettles, the Air render'd in fome manner untranfparent and foggy: The fame happens by the numerous Vapours that arife in cold Weather in Winter, and here in Holland, upon the Breaking and Opening the Ice.

The fecond way of producing Fogs and Vapours is, when the Air is more rarified than ufual, and thereupon becoming lighter, is no longer able to balance the more heavy watry Vapours, and to keep them floating in its own Region. A plain Inftance thereof we may fee in Tab. XIV. Fig. 5. by taking fome of the Water out of the Glass Globe A B (from whence the Air was first exhausted, in order to fill it by the fpouting in of Water, as has been fhewn before on another Account in Contemplation XVII.) and then fastening or fcrewing it on to the Air-Pump at D, fo that the very small quantity of Air that remain'd in it at S, will appear above the Water N PR; after which, a Vacuum being made in the Pump, the Cocks E and K must be open'd; by which means the Air, which at S gravitated upon the Water NP, meeting with no refiftance, will drive it down towards the Pump,

and

and fo the Space A NP, becoming larger, the Air that is in it will be likewife more expanded or rarified. Now, as it does alfo become lighter thereby, the Watry Vapours in it will fink down, and produce a visible and whitifh Fog in the Globe, and many times little Clouds, exactly mimicking those that we see in the open Air. But thefe Mifts and Clouds, upon the Re-admiffion of the Air QWR thro' the Water, and by the Encrease and Compreffion of the Air at S, do immediately difappear again, and the faid Air at S, as foon recovers its former Tranfparency; and fo, toties quoties, becomes foggy and cloudy when it has an Opportunity of dilating itself, and of forcing the Water out of the Globe, upon exhaufting the Air ; and again becomes clear and tranfparent, upon the letting in of fresh Air: So that clear and foggy Weather may be as alternately reprefented as often as you pleafe after this manner; and even when there remain watry Vapours enough in the Air, this may be still produced, provided the Bubble be but a little moift within, tho' altogether empty of Water.

SECT. XVIII. Reflections and Obfervations upon the fame.

WE have made these Experiments very frequently, and from thence obferv'd; First, that these Vapours, when the Air appear'd heavy in a Barometer, were not feen at the first Pumping, nor did fhew themselves fooner, till after fome Expansions of the included Air, it became lighter and thinner. Secondly, This Experiment did not fucceed well when the Water and Air were cool; probably, because there were not watry Vapours enough mixed with the Air: Wherefore hot Water, in a little Glass Veffel (Tab. XIV. Fig. 4.) MN, being placed under the Bell, prefently fill'd the Air with

the Steams which exhaled from it, but upon the admiffion of fresh Air, vanished as before.

It was likewife observed at another time, that no Mift appearing in the Glafs Globe in cold Weather, upon making a Fire in the Room, and the Air in a Thermometer fhewing itself warmer, we renewed our Pumping a little while after, and the Fog became immediately visible. Thirdly, We found likewife, that the Mift which had been thus produced in the Glass, fubfided by degrees, and the Glass became clearer, without admitting fresh Air into it; As alío, Fourthly, That thefe Mifts, by letting in fresh Air upon them, and by the Wind which the fame produced, being put into Motion, occafion'd an agreeable Reprefentation of the irregular Course of the Clouds in the Air in the time of Storms and Tempefts.

I have related this Experiment fomething the more particularly, because it did not always fucceed, and forafmuch it feem'd to give us a great deal of Light into the Nature of Mifts and Clouds.

Now that the Natural Mifts, and Fogs, and Clouds are of the fame kind with thefe Artificial ones, feems deducible from hence, that most commonly when the Air lofes its Clearnefs, and becomes more dark and obfcure, the Mercury in the Barometers defcends, and fhews thereby, that the Air is become lighter.

I have likewife often obferved with Astonishment, that when the Air appeared clear all above and round about us, in a very fhort while after, the whole Heavens grew dark and were cover'd over with Clouds. Whether this may be deduced from a fudden thinning of the Air (because we know of no other Reason befides, that in fo little a space of time can operate fo quick over the whole Face of the Heavens) I leave to others. The Barometer may be compared therewith. VOL. II.

Cc

SECT.