[1) Opera Omnia V, pp. 385-94. From a letter Robert Boyle ]

Honored Sir,

I have so long deferred to send you my thoughts about the physical qualities we speak of that, did I not esteem myself 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 that I am not well satisfied myself in them; and 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 fancying. But because I am indebted to you, and yesterday met with a friend, Mr. Maulyverer, who told me he was going to London and intended to give you the trouble of a visit, I could not forbear to take the opportunity of conveying this to you by him. It being only an explication of qualities which you desire of me, I shall set down my apprehensions in the form of suppositions as follows. And first, I suppose that there is diffused through all places an etherial substance, capable of contraction and dilatation, strongly elastic, and, in a word, much like air in all respects, but far more subtle. 2. I suppose this ether pervades all gross bodies, but yet so as to stand rarer in their pores than in free spaces, and so much the rarer as their pores are less; and this I suppose (with others) to be the cause why light incident on those bodies is refracted toward the perpendicular, why two well-polished metals cohere in a receiver exhausted of air, why mercury stands sometimes up to the top of a glass pipe though much higher than thirty inches, and one of the main causes why the parts of all bodies cohere; also the cause of filtration and of the rising of water in small glass pipes above the surface of the stagnating water they are dipped into; for I suspect the ether may stand rarer, not only in the insensible pores of bodies, but even in the very sensible cavities of those pipes; and the same principle may cause menstruums to pervade with violence the pores of the bodies they dissolve, the surrounding ether, as well as the atmosphere, pressing them together. 3. I suppose the rarer ether within bodies and the denser without them not to be terminated in a mathematical superficies, but to grow gradually into one another; the external ether beginning to grow rarer and the internal to grow denser at some little distance from the superficies of the body, and running through all intermediate degrees of density in the intermediate spaces; and this may be the cause why light, in Grimaldo’s experiment, passing by the edge of a knife or other opaque body is turned aside and as it were refracted, and by that refraction makes several colors. 4. When two bodies moving toward one another come near together, I suppose the ether between them to grow rarer than before and the spaces of its graduated rarity to extend further from the superficies of the bodies toward one another, and this by reason that the ether cannot move and play up and down so freely in the straight passage between the bodies as it could before they came so near together. 5. Now, from the fourth supposition, it follows that when two bodies approaching one another come so near together as to make the ether between them begin to rarefy, they will begin to have a reluctance from being brought nearer together and an endeavor to recede from one another, which reluctance and endeavor will increase as they come nearer together, because thereby they cause the interjacent ether to rarefy more and more. But at length, when they come so near together that the excess of pressure of the external ether which surrounds the bodies, above that of the rarefied ether which is between them, is so great as to overcome the reluctance which the bodies have from being brought together, then will that excess of pressure drive them with violence together and make them adhere strongly to one another, as was said in the second supposition. ... But if the bodies come nearer together, so as to make the ether in midway line ... grow rarer than the surrounding ether, there will arise from the excess of density of the surrounding ether a compressure of the bodies toward one another which, when by the nearer approach of the bodies it becomes so great as to overcome the aforesaid endeavor the bodies have to recede from one another, they will then go toward one another and adhere together. And, on the contrary, if any power force them asunder to that distance where the endeavor to recede begins to overcome the endeavor to accede, they will again leap from one another. Now hence I conceive it is chiefly that a fly walks on water without wetting her feet, and consequently without touching the water; that two polished pieces of glass are not without pressure brought to contact, no, not though the one be plain, the other a little convex; that the particles of dust cannot by pressing be made to cohere, as they would do if they did but fully touch; that the particles of tinging substances and salts dissolved in water do not of their own accord concrete and fall to the bottom, but diffuse themselves all over the liquor and expand still more if you add more liquor to them. Also, that the particles of vapors, exhalations, and air do stand at a distance from one another and endeavor to recede as far from one another as the pressure of the incumbent atmosphere will let them; for I conceive the confused mass of vapors, air, and exhalations which we call the atmosphere to be nothing else but the particles of all sorts of bodies of which the earth consists, separated from one another and kept at a distance by the said principle. ... Nor does the size only, but the density of the particles also, conduce to the permanency of aerial substances; for the excess of density of the ether without such particles above that of the ether within them is still greater, which has made me sometimes think that the true permanent air may be of a metallic original, the particles of no substances being more dense than those of metals. This, I think, is also favored by experience, for I remember I once read in the Philosophical Transactions M. Huygens at Paris found that the air made by dissolving salt of tartar would in two or three days’ time condense and fall down again, but the air made by dissolving a metal continued without condensing or relenting in the least. If you consider, then, how by the continual fermentations made in the bowels of the earth there are aerial substances raised out of all kinds of bodies, all which together make the atmosphere, and that of all these the metallic are the most permanent, you will not perhaps think it absurd that the most permanent part of the atmosphere, which is the true air, should be constituted of these, especially since they are the heaviest of all other, and so must subside to the lower parts of the atmosphere and float upon the surface of the earth, and buoy up the lighter exhalations and vapors to float in greatest plenty above them. Thus, I say, it ought to be with the metallic exhalations raised in the bowels of the earth by the action of acid menstruums, and thus it is with the true permanent air; for this, as in reason it ought to be esteemed the most ponderous part of the atmosphere because the lowest, so it betrays its ponderosity by making vapors ascend readily in it, by sustaining mists and clouds of snow, and by buoying up gross and ponderous smoke. The air also is the most gross inactive part of the atmosphere, affording living things no nourishment, if deprived of the more tender exhalations and spirits that float in it; and what more inactive and remote from nourishment than metallic bodies? I shall set down one conjecture more, which came into my mind now as I was writing this letter; it is about the cause of gravity. For this end I will suppose ether to consist of parts differing from one another in sublety by indefinite degrees; that in the pores of bodies there is less of the grosser ether, in proportion to the finer, than in open spaces; and consequently that in the great body of the earth there is much less of the grosser ether, in proportion to the finer, than in the regions of the air; and that yet the grosser ether in air affects the upper regions of the earth, and the finer ether in the earth the lower regions of the air, in such a manner that from the top of the air to the surface of the earth, and again from the surface of the earth to the center thereof, the ether is insensibly finer and finer. Imagine now any body suspended in the air or lying on the earth, and the ether being by the hypothesis grosser in the pores which are in the upper parts of the body than in those which are in its lower parts, and that grosser ether being less apt to be lodged in those pores than the finer ether below, it will endeavor to get out and give way to the finer ether below, which cannot be without the bodies descending to make room above for it to go out into. From this supposed gradual subtlety of the parts of ether some things above might be further illustrated and made more intelligible; but by what has been said you will easily discern whether in these conjectures there be any degree of probability, which is all I aim at. For my own part, I have so little fancy to things of this nature that, had not your encouragement moved me to it, I should never, I think, have thus far set pen to paper about them. What is amiss, therefore, I hope you will the more easily pardon in Your most humble servant and honorer, ISAAC NEWTON Cambridge, February 28, 1678/9