tural, or effential properties, and makes fuch a notable difcovery, which was scarce ever attempted before, at least with any fuccefs, namely, that the fame æther which was fo much like air, was 700000 times more fine or rare, and yet 700000 times more elastic than that. That Ι may not marr the fenfe of his reafoning, I fhall cite the whole paffage, in the following chapter, and conclude the prefent with a few remarks on the most predominant accidental quality of æther, viz. that of fubtilty.

140. Now fubtilty is that property of æther which by means of the rarity, or exceeding fineness of its first elements or corpufcles, in conjunction with its elafticity, feems necef-farily as it were, to pervade or infinuate into the intersticial vacuities of grofs bodies, to efcape from the incumbent preffure, and repelling fpring of the furrounding atmosphere*; fubtilty therefore, in ftrictness, is by no means fo properly a natural quality, or effential property,. as either rarity or elafticity.- As a confe

quence then of this property of fubtilty, &c. we find it in the pores of all grofs bodies, particularly thofe term'd non-electrics, whether of metals, water, or animals; or vegetables pro

* To all minute vacuums (if the term be allowedy i. e. the pores contained in the most dense and compacted bodies, provided they are non-electrics, and those whofe pores are fo fmall that the grofs air is excluded, and where nothing but the fame æther, or primary air can infinuate.

vided they are green and replete with Juices Those being the only bodies to which it escapes' the most freely, fince it is not repell'd by them: Any of those kinds of bodies, when perfectly infulated or cut off from all communication. with the earth by means of filken ftrings, or other original electrics, are thofe to which it escapes and accumulates all the time the machine is in motion, if any of those infulated bodies are within the reach of it; accumulates I fay, the pores feeming always fo replete withthe fame electrical principle, as if incapable of receiving any addition, as will more fully ap pear farther on.

N. B. The pores of all other bodies even those that are the moft perfect electrics per fe, appear as replete with this principle, as thote term'd non-electrics; but in thofe bodies it is more firmly fix'd,

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CHA P. V.

PART I.

141.

SECTION

Farther Accounts of the Wonderful ESSENTIAL Properties of Æther.

Come now to confider the Rarity, or Thinnefs, and the Elafticity, or Springi nefs of Ether, with the Proportion obferved by Sir Ifaac Newton between K 2

thefe

thefe Properties in that, and the fame Properties in common Air, and the means by which he investigated that Proportion.

142. Firft. That the medium which propagates Sound is the common air; and confequently, that the finer and thinner elastic air or æther beyond the atmosphere is the medium which conveys or propagates light from the heavenly bodies.

143. Secondly. That found is obferved to move the space of an hundred miles, in much about the fame space of time, that light moves from the Sun to the Earth, which diftance Sir Ifaac computed at 70 millions of miles; therefore as 100 is to 1, fo is 70000000 of miles, to 700000 miles: From thence he infers, that not only the different velocities of light and found, are in the proportion of 700000 to 1, but the denfity, rarity, and elasticity, are in the fame proportion alfo. See his own words.

144. Sounds move about 1140 english feet in a second minute of time, and in seven or eight minutes of time they move about one hundred english miles. Light moves from the Sun to us in about seven or eight minutes of time, which distance is about 70000000 english miles, fuppofing the horizontal parallax of the fun to be about 12". And the vibrations or pulfes of this medium, that they may cause the alternate fits of eafy ⚫ tranfmiffion and eafy reflexion, must be swifter than light, and by confequence, above

700000 times fwifter than founds; and therefore the elaftic force of this medium, in proportion to its denfity, must be above 700000 multiplied by 700000, that is, above 490000000000 times greater than the elastie • force of the Air is in proportion to its den, fity. For the velocities of the pulfes of elastic 'mediums are in a fubduplicate ratio of the Elafticities and the Rarities of the Mediums • taken together.' Optics. Quære 21.

145. The latter part of the above Quære, and the next following it, containing what may ferve to illuftrate the foremention'd reasoning concerning the rarity of the æther or pure air, as alfo its expanfive force, are as follow:

146. As Attraction is ftronger in fmall Magnets than in great ones, in proportion to their bulk; and Gravity is greater in the fur• faces of fmall Planets than in those of great ones, in proportion to their bulk; and fmall bodies are agitated much more by electric ⚫ attraction than great ones: So the smallness ⚫ of the rays of light may contribute very much ⚫ to the power of the agent by which they are • refracted. And so if any one should fuppofe, that æther (like our air) may contain particles which endeavour to recede from one ⚫ another (for I do not know what this Æther is and that its particles are exceedingly • fmaller than those of air, or even than those of light: The exceeding smallness of its par⚫ticles. may contribute to the greatness of the ⚫ force

force by which those particles may recede • from one another, and thereby make that • medium exceedingly more rare and elastic * than air, and by confequence exceedingly lefs • able to refift the motions of projectiles, and • exceedingly more able to prefs upon grofs bodies, by endeavouring to expand itself.

147. Qu. 22. May not Planets and Comets, and all grofs bodies, perform their mo⚫tions more freely and with lefs refiftance in ⚫ this ætherial medium than in any fluid, which fills all Space adequately, without leaving any pores, and by confequence is much denfer than Quickfilver or Gold? And may not its re• fiftance be so small as to be inconfiderable? For instance; If this æther (for fo I will call it) fhould be fuppos'd 700000 times more • elastic than our air, and above 700000 times more rare, its refiftance would be above • 600000000 times lefs than that of water; and so small a refiftance would scarce make any fenfible alteration in the motions of the Planets in ten thousand years. If any one

• would ask how a medium can be fo rare, let him tell me how the air in the upper parts • of the atmosphere can be above an hundred • thousand thousand times' (i. e. an hundred millions of times) rarer than gold? Let him

alfo tell me how an electric body can by • friction emit an exhalation fo rare and fubtile,

† An Allufion to the Fluid of Des Cartes.

and