Letter from Newton to Robert Boyle, dated 28 February 1678/9
Hond Sr
I have so long deferred to send you my thoughts about ye Physicall qualities we spake of, that did I not esteem my self obliged by promise I think I should be ashamed to send them at all. The truth is my notions about things of this kind are so indigested yt I am not well satisfied my self in them, & what I am not satisfied in I can scarce esteem fit to be communicated to others, especially in natural Philosophy where there is no end of fansying. But because I am indebted to you & yesterday met wth a friend Mr Maulyverer, who told me he was going to London & intended to give you ye trouble of a visit, I could not forbear to take ye opportunity of conveying this to you by him.
It being only an explication of qualities wch you desire of me, I shall set down my apprehensions in ye form of suppositions as follows. And first I suppose that there is diffused through all places an æthereal substance capable of contraction & dilatation, strongly elastick, & in a word much like air in all respects, but far more subtile.
2 I suppose this æther pervades all gross bodies, but yet so as to stand rarer in their pores then in free spaces, & so much ye rarer as their pores are less. And this I suppose (wth others) to be ye cause why light incident on those bodies is refracted towards ye perpendicular; why two well polished metalls cohere in a Receiver exhausted of air: why ☿ stands sometimes up to ye top of a glass pipe though much higher yn 30 inches: & one of ye main causes why ye parts of all bodies cohere. Also ye cause of philtration & of ye rising of water in small glass pipes above ye surface of ye stagnating water they are dipt into: for I suspect ye æther may stand rarer not only in ye insensible pores of bodies, but even in ye very sensible cavities of those pipes. And ye same principle may cause Menstruums to pervade with violence ye pores of ye bodies they dissolve, the surrounding æther as well as ye Atmosphere pressing ym together.
3 I suppose ye rarer æther within bodies & ye denser wthout them, not to be terminated in a mathematical superficies but to grow gradually into one another: the external æther beginning to grow rarer, & ye internal to grow denser at some little distance from ye body superficies of ye body, & running through all intermediate degrees of density in ye intermediate spaces. And this may be ye cause why light in Grimaldo's experiment passing by ye edge of a knife or other opake body is turned aside & as it were refracted, & by yt refraction makes several colours. Let ABCD be a 
dense body whether opake or transparent, EFGH ye outside of ye uniform æther wch is within it, IKLM the inside of ye uniform æther wch is without it; & conceive ye æther wch is between EFGH and IKLM to run through all intermediate degrees of density between yt of ye two uniform æthers on either side. This being supposed, ye rays of ye sun SB, SK, wch pass by ye edge of this body between B & K, ought in their passage through ye unequally dense æther there, to receive a ply from ye denser æther wch is on that side towards K, |&| that ye more by how much they pass nearer to ye body, & thereby to be scattered through ye space PQRST, as by experience they are found to be. Now ye space between ye limits EFGH & IKLM I shall call ye space of ye æther's graduated rarity.
4 When two bodies moving towards one another come neare together I suppose ye æther between them to grow rarer then before, & ye spaces of its graduated rarity to extend further from ye superficies of ye body|ie|s towards one another, & this by reason yt ye æther cannot move & play up & down so freely in ye strait passage between ye bodies as it could before they came so neare together. Thus if 
ye space of ye æther's graduated rarity reach from ye body ABCDFE only to ye distance GHLMRS when no other body is neare it, yet may it reach farther, as to IK, when another body NOPQ approch|ac|hes: & as the other body approaches more & more I suppose ye æther between them will grow rarer & rarer.
These suppositions I have so described as if I thought ye spaces of graduated æther had precise limits, as is exprest at IKLM in ye first figure & GMRS in ye second: for thus I thought I could better express my self. But really I do not think they have such precise limits but rather decay insensibly, & in so decaying extend to a much greater distance then can easily be beleived or need be supposed.
5 Now from ye 4th supposition it follows that when two bodies approaching one another, come so neare together as to make ye æther between them begin to rare{illeg}|f|y, they will begin to have a reluctance from being brought nearer together, & an endeavour to recede from one another: wch reluctance & endeavour will encrease as they come nearer together because thereby they cause the interjacent æther to rarefy more & more. B{illeg}|u|t at length, when they come so neare together that the excess of pressure of ye external æther wch surrounds ye bodies, above yt of ye rarefied æther wch is between them, is so great as to overcome ye reluctance wch ye bodies {illeg} have from being brought together: then will that excess of pressure drive them with violence together & make them adhere strongly to one another, as was said in ye second supposition. For instance in ye second Figure when ye bodies ED & NP are so neare together, yt ye spaces of ye æthers graduated rarity begin to reach to one another & meet in ye line IK; the æther between them will have suffered much rarefaction wch rarefaction requires much force yt is much pressing of ye bodies together: & ye endeavour wch ye æther between them has to return to its former state natural state of condensation will cause ye bodies to have an endeavour of receding from one another. But on ye other hand to counterpoise this endeavour there will not yet be any excess of density of ye æther wch surrounds ye bodies above that of ye æther wch is between them at ye line IK. But if ye bodies come nearer together so as to make ye æther in ye mid-way-line IK grow rarer then ye surrounding æther, there will arise from ye excess of density of ye surrounding æther a compressure of ye bodies towards one another: which when by ye nearer approach of ye bodies it becomes so great as to overcome ye afforesaid endeavour ye bodies have to recede from one another, they will then go towards one another & adhere together. And on ye contrary if any power force them as under to that distance where ye endeavour to recede begins to overcome ye endeavour to accede, they will again leap from one another. Now hence I conceive it is chiefly yt a fly walks on water wthout wetting her feet, & consequently without touching ye water; that two polished pieces of glass are not wthout pressure brought to contact, no not though ye one be plain, ye other a little convex; yt ye particles of dust cannot by pressing be made to cohere, as they would do if they did but fully touch; yt ye particles of tinging substances & salts dissolved in water do not of their own accord concrete & fall to ye bottom, but diffuse themselves all over ye liquor, & expand still more if you ad more liquor to them. Also yt ye particles of vapors exhalations & air do stand at a distanc{illeg}|e| from one another, & endeavour to recede as far from one another as ye pressure of ye incumbent atmosphere will let them: for I conceive ye confused mass of vapors air & exhalations wch we call ye Atmosphere to be nothing els but ye particles of all sorts of bodies of wch ye earth consists, separated from one another & kept at a distance by ye said principle.
From these principles ye actions of Menstruums upon bodies may be thus explained. {illeg}|S|uppose any tinging body as Cochineel or Logwood be put into water, \so soon as/ the water sinks into its pores & wets on all sides any particle, wch adheres to ye body \only/ by ye principle in second supposition: it takes of or at least much diminishes ye efficacy of yt principle to hold ye particle to ye body because it makes ye æther on all sides ye particle to be of a more uniform density then before. And then the particle being shaken of by any little motion, flotes in ye water, & wth many such others makes a tincture; wch tincture will be of some lively colour if ye particles be all of ye same size & density, otherwise of a dirty one. For ye colours of all natural bodies whatever seem to depend on nothing but ye various sizes & densities of their particles: as I think you have seen described by me more at large in another paper. If ye particles be very small (as are those of salts Vitriols & gumms) they are transparent, & as they are supposed bigger & bigger they put on these colours in order black, white, yellow, red; violet, blew, pale green, yellow, orange, red; purple, blew, green, yellow, orange, red &c: as is discerned by ye colours wch appear at ye several thicknesses of very thin plates of transparent bodies. Whence to know ye causes of ye changes of colours \wch are/ often made by ye mixtures of several liquors, it is to be considered how ye particles of any tincture may have their size or density altered by the infusion of another liquor.
When any metal is put into common water, ye water cannot enter into its pores to act on it & dissolve it. Not yt water consists of too gross parts for this purpose, but because it is unsociable to metal. For{illeg} there is a certain secret principle in nature by wch liquors are sociable to some things & unsociable to others. Thus water will not mix with oyle but readily wth spt of wine or wth salts. It sinks also into wood wch Quicksilver will not, but Quicksilvers sinks into metals, wch, as I said, water will not. So Aqua fortis dissolves ☽ not ☉; Aqua regis ☉ & not ☽, &c. But a liquor wch is of it self u{illeg}|n|sociable to a body may by ye mixture of a convenient mediator be made sociable. So molten Lead wch alone will not mix wth copper or wth Regulus of Mars, by ye addition of Tin is made to mix wth either. And water by ye addition of mediation of saline spirits will mix with metal. Now when any metal is put in water {illeg} impregnated wth such spirits, as into Aqua fortis, Aqua Regis, spirit of Vitriol or ye like, the particles of ye spirits as they in floting in ye water, strike on ye metal, will by their sociableness enter into its pores & gather round its outside particles, & by advantage of ye continual tremor the particles of ye metal are in, hitch themselves in by degrees between those particles & ye body & loosen them from it, & ye water entring into ye pores together wth ye saline spirits, ye particles of ye metal will be thereby still more loosed, so as by that motion ye solution puts y{illeg}|m| into, to be easily shaken of & made to Rote in ye water: the saline still particles still encompassing ye metallick ones as a coat or shell does a kernell, after ye manner expressed 
in ye annexed figure. In wch figure I have made ye particles round, though they may be cubical or of any other shape.
If into a solution of metal thus made, be poured a liquor abounding with particles, suppose of salt to wch ye former saline particles are more sociable then to ye particles of ye metal, (suppose with particles of salt of Tartar:) then so soon as they strike on one another in ye liquor, ye {illeg}|s|aline particles \will adhere to those/ more firmly then to ye metalline ones, & by degrees be wrought of from those to enclose these. Suppose A a metalline particle enclosed wth saline ones of spirit of Nitre, & E a particle of salt of Tartar contiguous to two of ye 
particles of spirt of nitre b & c, & suppose ye particle E is impelled by any motion towards d so as to roll about ye particle c till it touch ye particle d: the particle b adhering more firmly to E then to A, will be forced off from A. And by ye same means ye particle E as it rolls about A will tear of ye rest of ye saline particles from A, one after another, till it has got them all or almost all about it self. And when ye metallic particles are thus divested of ye nitrous ones which as a mediator between them & ye water held them floting in it: the Alcalizate ones crouding for ye room ye metallic ones took up before, will press these towards one another & make them come more easily together: so yt by the motion they continually have in ye water they shall be made to strike on one another, & then by means of the principle in ye second supposition they will cohere & grow{illeg} into clusters, & fall down by their weight to ye bottom, wch is called precipitation.
In ye solution of metals, wh{illeg}|e|n a particle is loosing from ye body, so soon as it gets to that distance from it where ye principle of receding described in ye 4th & 5t suppositions begins to overcome ye principle of acceding described in ye second conclusion supposition: the receding of ye particle will be thereby accelerated, so yt ye particle shall as were wth violence leap from ye body, & putting ye liquor into a brisk agitation, beget & promote yt heat we often find to be caused in solutions of Metals. And if any particle happen to leap of thus from ye body before it be surrounded with water, or to leap of wth that smartness as to get loos from ye water: the water by the principle in ye 4th & 5t suppositions, will be kept of from ye particle & stand round about it like a spherically hollow arch, not being able to come to a full contact with it any more. And severall of these particles afterwards gathering into a cluster, so as by ye same principle to stand at a distance from one another without any water between them, will compose a buble. Whence I suppose it is yt in brisk solutions there usually happens an ebullition.
This is one way of transmuting gross compact substances into aereal ones. Another way is by heat. For as fast as ye motion of heat can shake off y{illeg}|e| particles of water from ye surface of it: those particles by ye said principle will Rote up & down in ye air at a distance both from one another & from ye particles of air, & make yt substance we call vapor. Thus I suppose it is when ye particles of a body are very small (as I suppose those of water are) so yt ye action of h{illeg}|e|at alone may be sufficient to shake them asunder. But if ye particles be much larger, they then require the greater force of dissolving Menstruums to separate them, unless by any means the particles can be first broken into smaller ones. For ye most fixed bodies, even Gold it self, some have said will become volatile only by breaking their parts smaller. Thus may ye volatility & fixedness of bodies depend on ye diffe{illeg}t|re|nt sizes of their parts.
And on ye same difference of size may depend the more or less permanency of aereal substances in their state of rarefaction. To understand this let us suppose ABCD to be a large pie{illeg}|c|e of any metal, EFGH ye limit of ye interior uniform æther, & K 
a part of ye metal neare \at/ ye edge superficies of ye metal AB. If this part or particle K be so little yt it reaches not to ye limit EF, its plain that ye æther at its center must be less rare then if ye particle were greater, for were it greater, its center would be further from the superficies AB, that is, in a place where ye æther (by supposition) is rarer. The less ye particle K therefore, ye denser ye æther at its center, because its center comes nearer to ye edge AB where ye æther is de{illeg}|ns|er then within ye limit EFGH. And if ye particle were divided from ye body & removed to a distance from it where ye æther is still denser, the æther within it must proportionally grow denser. If you consider this you may apprehend how by diminishing the particle, ye rarity of ye æther within it will be diminished, till between the density of ye æther wthout & ye density of ye æther within it there be little difference, that is till ye cause be almost taken away wch should keep this & other such pa{illeg}|r|ticles at a distance from one another. For that cause, explained in ye 4th & 5t suppositions, was ye excess of of density of ye external æther above yt of ye internal. This may be ye reason \then/ why the small particles of vapors easily come together & are reduced back into water unless ye heat wch keeps them in agitation be so great as to dissipate them as faster their \as/ they come together: but ye grosser particles of exhalations raised by fermentation keep their aerial form more obstinately, because the æther within them is rarer.
Nor does ye size \only/ but ye density of ye particles \also/ conduce to to {sic} ye permanency of aereal substances. For ye excess of density of ye æther wthout such particles above yt of ye æther wthin them is \still/ greater. Which has made me sometimes think that ye true permanent Air may be of a metallic original: the particles of no substances being more dense then those of metals. This I think is also favoured by experience for I remember I once read in ye P. Transactions how M. Hugens at Paris found that ye air made by dissolving salt of Tartar would in two or three days d|t|ime condense & fall down again, but ye air made by dissolving a metal continued permanent wthout condensing or relenting in ye least. If you consider then how by the continual fermentations made in ye bowels of ye earth there are aereal substances raised out of all kinds of bodies, all wch together make ye Atmosphere & that of all these ye m{illeg}|et|allic are ye most permanent, you will not perhaps think it absurd that ye most permanent part of ye Atmosphere, wch is ye true air, should be constituted of these: especially since they are ye he{illeg}|a|viest {illeg}|o|f all other & so must subside to ye lower parts of ye Atmosphere & float upon ye surface of ye earth, & buoy up ye lighter exhalation & vapours to float in gr{illeg}|e|atest plenty above them. Thus I say it ought to be wth ye metallic exhalations raised in ye bowels of ye earth by ye action of acid menstruums, & thus it is wth ye true permanent air. For this as in reason it ought to be esteemed ye most ponderous part of ye Atmosphere because ye lowest: so it betrays its ponderosity by making vapors ascend readily in it, by susteining mists & clouds of snow, & by buoying up gross & ponderous smoke. The air also is ye most gross unactive part of ye Atmosphere affording living things no nourishment if deprived of ye more tender exhalations & spirits yt flote in it: & what more unactive & remote from nourishment then metallick bodies.
I shal set down one conjecture more which came into my mind now as I was writing this letter. It is about ye cause of gravity. For this end I will suppose æther to consist of parts differing from one another in subtilty by indefinite degrees: {illeg} That in ye pores of bodies there is less of ye finer grosser æther in proportion to ye finer then in open spaces, & consequently that in ye gre{illeg}|a|t body of ye earth there is much less of ye grosser æther in proportion to ye finer then in ye regions of ye air: & that yet ye grosser æther in ye Air affects ye upper regions of ye earth & ye finer æther in ye earth ye lower regions of ye air, in such a manner yt from ye top of ye air to ye surface of ye earth & again from ye surface of ye earth to ye center thereof the æther is insensibly finer & finer. Imagin now any body suspended in ye air or lying on ye earth: & ye æther being by the Hypothesis grosser in ye pores wch are in ye u{illeg}|p|per parts of ye body then in those wch are in its lower parts, & that grosser æther being less apt to be l{illeg}g lodged in those pores then ye finer æther below, it will endeavour to get out & give way to ye finer æther below, wch cannot be wthout ye bodies descending to make room above for it to go out into.
From this supposed gradual subtilty of ye parts of æther some things above might be further illustrated & made more intelligible, but by what has been said you will easily {illeg} discern whether in {illeg} these conjectures there be any degree of probability, wch is all I aim at. For my own part I have {so} little fansy to things of this nature that had not yor encouragement moved me to it, I should never I think have thus far set pe{illeg}|n| to paper about them. What's amiss therefore I hope you will ye more easily pardon in
Yor most humble Servant & honourer
Is. Newton.
|Philosophical Trc frō Mr Isaac Newton. Cambr. Feb. 28. 1678/9.|
For ye Honble Robert Boyle Esqꝫ
