La copie se trouve à Leiden, coll. Huygens.
The objections of Monsieur Hugens (which you were pleasd to acquaint me with) as they may be very well made against my hypothesis, before it be more fully explicated, then it is in that short attempt which J presum'd to trouble you with, concerning the rarefaction of the air. Soe J doubt not but vpon a more copious explication they will very easily be remou'd, by the ingenious Objector himself.
For my first hypothesis (in which it is the difficulty lyes) being Epicurean, supposes first an internall motion in the particles of bodyes especially of such as are fluid (a principle generally granted by that sect) which therefore though it may be retarded by the occursion of other bodys, either contrarily moued or at rest, yet those impediments are noe sooner remou'd, then the freed particles begin again their naturall and congenite motion, which in the particles of the air is here suppos'd circular. Next the parts them selues being supposd much of the shape of a watch-spring, or coyle of wire, And to haue a circular motion, like that of ye meridian of a Globe vpon it's poles doe thereby become potentiall sphaeres or globules (if I may soe speake) that is, they defend a sphaericall space from being entred into by any other of the like globules, vnless they be thrust on with a sufficient strengh. By which meanes the coyled particles when by externall pressure they are driuen into lesse room doe not loose their naturall circular motion, and consequently not their power of maintaining a globular space, though now indeed made less & much contracted. That is, both the one and the other haue almost the same propension or aptitude to slide by each other without sticking, as
by reason of their internall motion they are noe ways sitt, whilst that remaines, to compose a solid body.
Soe that whether we consider them as crowded together by some externall pressure (in which state each particle hauing less room to perform its circular motion in, will consequently maintain a less sphaere) or expanded by being left more at their liberty, they may still be supposd potentiall Sphaeres or Globuls, now smaller & more massy, and consequently more difficult to be remou'd, and soe composing a more sluggish or retarding medium, such as we find comprest air to be by its hindring not only the motion of light descending bodys, but euen that of light as appeares by its greater refraction, such are water and other liquors which in this hypothesis are suppos'd to consist of the same kind of Particles though specificated by some other proprietys, not necessary here to be mentiond; whereas at other times these potentiall globules when bigger are consequently more spongy (if J may soe speak) and soe compose a fluid body that does more readily yeald to the transcursion of the rayes of light, or to the vibrations of a Pendulum made with wooll or feathers. But though the composd body be sometimes more dens at other times more rare yet may it as properly be call'd a fluid body then as now. As quicksiluer is as properly termed a fluid body as air it self.
This is in short Sir what J haue to reply to the jngenious scruples of Monsieur Zulichem jn defense J say not of my opinion, but hypothesis, whose Principles I doe not here vndertake to vindicate, but only that those being granted, J suppose not only all those which Franciscus Linus has instanced in, But euen all the phaenomena of rarefaction may be at least as well, if not more intelligibly explicated then by that of Aristotle. Whose hypothesis that newly mention'd authour soe far maintaining, as not only to assert it the most probable, but to brand all other with impossibility, J was5) a litle transported with zeale for the honour of some moderne Philosophers as well as of some that praeceded Aristotle (And Particularly because he had oppos'd it to your Doctrine of ye Spring of the air, soe firmly founded on those numerous experiments & reasons which you had alledg'd for it. And that without bringing any considerable either reason or Experiment against it). And at some leasure howres drew vp & presented to you the sum of my thoughts on yt particular. Which since they haue passt your seuere scrutiny, and that you were pleas'd to honor soe far as to publish with some of your owne, I think my self now obleig'd to vindicate, at least by a further explication of it as to the resoluing the Doubts of that Noble Virtuoso. Which is the occasion of the trouble that is at present giuen you by
Your Honors most obleig'd & most humble seruant