greatest of all at the Surface of the Earth: Likewife, from thefe Principles it will follow, that the Weight of the Air and its elaftic Force are always equal to each other, and therefore produce an universal Equilibrium among the Particles of Air in every Part of the Atmosphere.

Euphrof. According to this Account of the Denfity and Rarity of the Air, what Idea can I form of the Height of it? For, if the lefs the Air be compreffed the more it, is expanded, and the farther you go from the Surface of the Earth, the lefs that compreffive Force will be, where am I to conceive the Bounds or Limits of the Atmosphere to be?

Cleon. No where at all; for the Parts of Air in the upper Regions will be expanded to Infinity, 'till at Length we may very properly underftand, that the Atmospheres of every Body in each System, and of all the Systems in the Univerfe, meet with each other in the diftant Spaces between, and mixing together, conftitute that fine ethereal Medium, which fills the Abyfs of Space.

Euphrof. This is a wonderful Doctrine; but it certainly muit be as you fay from the Nature of Things, as you have now explained it. Therefore, I apprehend it will be in vain to enquire about the Height of the Atmosphere, or Body of the Air.

Cleon. All that can be faid in refpect to the Height of the Air, is this, that the Denfity of it, at the Height of about 45 Miles, is but just fufficient to inflect, or bend a Ray of Light out of its right-line Courfe, which is the Jeaft Effect it can produce, and therefore all beyond that Height must be efteemed an unrefifting Æther.

Euphrof. As the Air is fuch a very extraordinary Subftance, and poffefied of fuch wonderful Qualities, how comes it to pass, that we can fee nothing of it? For if it was not fenfible otherways, I fhould know of no fuch Thing by the Sight.

Cleon. Why even in this Refpect you may not be free from Miftakes. The Air in fome Cafes, tho' very rarely, is fubject to the Sight, or may be perceived by the Eye; but as this feldom happens, we look upon the Air in general as altogether invifible; and it is necefiary it fhould be fo; for, as it is the Medium through which we fce Objects, if the Parts of Air were in the leaft Degree perceptible, it

would

would render the View of those Objects lefs perfect and diftinct; as is well known by the Experiment of viewing Objects thro' a Telescope, which magnifies to a very great Degree; as it fhews the Body of Air, fo it renders the View of other Objects more obfcure and indistinct, which plainly convinces us of the Wisdom of the divine Architect in rightly difpofing this Part of the Constitution of the World.

Euphrof. But you was you was just now mentioning fome particular Cafe in which the Air becomes vifible; pray, what is that? For I fhall have a great Inclination to have a View of fo'rare a Spectacle.

Cleon. That you may do under the following Circumftances: In a very hot Summer's Day, take a Walk to fome open Parts of the Country, and place yourself upon an Eminence or rifing Ground, in a Situation nearly facing the Sun; then, if there be any gentle Wind, or Motion of the Air, it will be fhewn by Reflection of Light from the Body of the Air in the Vale below, and you will as perfectly fee the Undulations or Waves of Air, almoft, as you may thofe of Water, agitated by a gentle Wind.

Euphrof. This will afford me a particular Satisfa&ion, and I fhall take the first Opportunity to gratify myself in this Refpect; and then I fhall know by Experiment, what I otherwife find, that Wind is only Air in Motion.

Cleon. It is very true, Sifter: You have hit off the For fuppofing the whole Philofophy of Wind at once. Air was perfectly quiefcent or ftagnant, there would be no Wind at all; but as the Degrees of Motion, or Agitation in the Body of the Air are greater or leffer; fo we find ftronger or weaker Winds arife; and as the Motion of the Air is this Way or that, fo we ufually fay, the Wind blows from one Quarter or the other, or from one Point of the Compafs or another.

Euphrof. Pray, what are the general Caufes of Wind, or this Motion of the Air that you speak of?

Cleon. The particular Caufes of Wind are many; but one general Caufe is the Inequality of Heat and Cold in different Parts of the Atmosphere; but this will prove the entire Subject of a future Converfation. And as it will be proper, not only to tell you, that the Air has

fuch

fuch and fuch Qualities, but to convince you by Experi ments, that they do really exift, I fhall make fome Preparation for that Purpose against the next Opportunity; and fo for the prefent, Adieu.

DIALOGUE

II.

Of EXPERIMENTS to fhew the natural Production

of AIR.

Euphrofyne.

O! You have spread the Table, I fee, for my Entertainment. I can affure you, to feast my Mind on thefe Things affords me as much greater Pleasure than the common Productions of a Cook's-fhop can afford the Palate, as the Subject of the finest Parts of Philofophy can be supposed to exceed all the Merits of the Art of Paftry.

I make this Sort of Comparison (which I know you will allow to be juft) because I was the other Day in Company, where a certain Perfon, having just before difplayed his Talent in defcanting upon what was fine Eating and Drinking, and how happy it was to have a critical Tafte to diftinguish the Delicacies of a Haut Goût, and the genuine Flavours of Burgundy and Champaign, which took up the beft Part of Dinner-time, and an Hour after; at length, he was pleased to direct his Difcourfe to me, and with an Air of Pleafantry to tell me, that he heard I was intent on the Studies of Philofophy, and that I was already become a very confiderable Adept in it: that I had long been converfing with the Stars; and after much more of fuch Kind of complaifant Raillery, he added (by Virtue of a Bumper of French Port) that it had quite opened a new Scene to his View; that he had always looked on the Fair-Sex till now in a different Light; and " that their highest Pretenfions were to no more Skill than the Oeconomy of a Kitchen; and the pretty Dexterity of making Puddings and Pies-It will not be worth while to infift on what farther paffed between us; but though I was in fome Meafure affected by fuch a fashionable Piece of Ridicule; yet I confidered, that Philofophy was too amiable

Amiable a Science to be bantered out of my Regard, setting afide the natural Propenfity that I always had to the Study of it; therefore let me not interrupt you any longer, but proceed to your Experiments; for I muft ftill long to be acquainted with the Nature of this neceflary Element, and learn how, and by what Means it is produced.

Clean. It is Pity that any, even of our Sex, fhould fo far betray their Want of Prudence as to ridicule a philofophical Difpofition in Others, only for Want of a Tafte for the Science them felves. I must allow this is but too often the Cafe; I only with one Thing for the Honour of our Sex, that many of them had but half the natural Genius and Capacity of Improvement in natural and mathematical Sciences as many of the Ladies are poffeffed of-But to the Purpofe; you remember, I told you, when we laft converfed together, that the Parts of Matter by Attraction and Repulfion were put into Motion, and that whenever by this Means they could be feparated beyond the Sphere of Attraction, they commenced a repelling State, and then, as they were at a greater Distance from each other than before, they muft become very light, and arife into the Body of the Air in Forms of Vapours and Exhalations of various Kinds. Those elaftic Particles that rise visible to the Sight are what we usually call Vapours, or Steam; fuch as we commonly fee rife from the Surface of heated Water, and other moist Substances,

Euphrof. This I am convinced of every Day by my Tea-kettle, in which I have often wondered at that Violence and Impetuofity with which it iffues from the Spout; and I now begin to fee fomewhat of the Reafon and Caufe thereof, viz. that it arifes from the Elafticity of its Parts, which by Experience I know is greatly augmented by Heat; but can you tell the particular Manner in which that is effected?

Cleon. As I fhall fhew hereafter, that all Heat confifts in the great Velocity or fwift Motion of the Parts of Matter, and the Velocity of the Particles of Light is the greatest of any, which we know of in Nature; fo we find, that those Particles by their Action always produce the Senfation of Warmth or Heat, and being plentifully imbibed by all Kinds of Bodies, thefe active Particles are conftantly employed to feparate the Parts of Bodies, and VOL. I. S

by

by this Means to produce the natural Exhalations or Steams from every heated Liquor. These fenfible Vapours rife into the Air, and make a Part of the Atmosphere; for the Air is conftantly filled with Particles of Moisture, which by Heat are fo far attenuated as feldom to be fenfible to the Sight.

Euphrof. Then what you now fay, I prefume, is the Reason why in the Morning or Evening, when the Air is cool, we fee the Vapours arife from the Earth into the Air, and alfo in a very cold Winter's Day, our very Breath becomes vifible to the Eye, as it then wants a fufficient Degree of Heat to rarify the Particles, and make them escape the Sight.

Cleon. You understand this Matter very well; and in the fame Manner you are to imagine, that folid Bodies will have their fuperficial Parts separated from the Action of Heat, and thefe Particles fo feparated will be affected with a repulfive Force; which repulfive Force will caufe them to afcend into the Body of Air, and become a Part of the Atmosphere. Thefe Particles arifing from folid Bodies are always too fine to be vifible to the Eye, and therefore it is, that we rarely know, or even think of any fuch Thing; and we find by Experience, that all Bodies lose their Parts in Proportion as they are more actuated by the Power or Particles of Heat; and when the Degree of Heat is very intenfe, there are but few Bodies, whofe Parts are fo fixed, or cohere fo firmly, as not to be feparated by their Action; and the Experiment which I fhall now fhew you, will be a fufficient Confirmation of this Truth..

Euphrof. Pray, to what Purpose is this fine Pair of Scales and thefe feveral Slips of Paper, which I fee, and the Wax-candle? I fuppofe they are all to be concerned in the Experiment you mention.

Cleon. 'Tis true they are: The Candle is to fhew you the great Power of Heat, or Fire, upon the Parts of thofe Pieces of Paper while they are burning, and they are all firft weighed very nicely by thofe Scales. The Quantity which I have here provided weighs juft 100 Grains, as you fee by placing the feveral Slips in one Scale, and the Grain-weights in the other.

Euphrof. I fee they juft equipoize each other; but how am I to know what Parts are converted into Air in burning?