Opticks: or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light

QUERIE 31. Have not the small particles of bodies certain powers, virtues, or forces, by which they act at a distance, not only upon the rays of light for reflecting, refracting, and inflecting them, but also upon one another for producing a great part of the phenomena of Nature? For it’s well known that bodies act one upon another by the attractions of gravity, magnetism, and electricity; and these instances shew the tenor and course of Nature, and make it not improbable but that there may be more attractive powers than these. For Nature is very consonant and conformable to herself. How these attractions may be performed I do not here consider. What I call attraction may be performed by impulse, or by some other means unknown to me. I use that word here to signify only in general any force by which bodies tend towards one another, whatsoever be the cause. For we must learn from the phenomena of Nature what bodies attract one another, and what are the laws and properties of the attraction, before we enquire the cause by which the attraction is performed. The attractions of gravity, magnetism, and electricity reach to very sensible distances, and so have been observed by vulgar eyes, and there may be others which reach to so small distances as hitherto escape observation; and perhaps electrical attraction may reach to such small distances, even without being excited by friction.
For when salt of tartar runs per deliquium, is not this done by an attraction between the particles of the salt of tartar, and the particles of the water which float in the air in the form of vapours? And why does not common salt, or saltpetre, or vitriol, run per deliquium, but for want of such an attraction? Or why does not salt of tartar draw more water out of the air than in a certain proportion to its quantity, but for want of an attractive force after it is satiated with water? And whence is it but from this attractive power that water which alone distils with a gentle luke-warm heat, will not distil from salt of tartar without a great heat? And is it not from the like attractive power between the particles of oil of vitriol and the particles of water, that oil of vitriol draws to it a good quantity of water out of the air, and after it is satiated draws no more, and in distillation lets go the water very difficultly? And when water and oil of vitriol poured successively into the same vessel grow very hot in the mixing, does not this heat argue a great motion in the parts of the liquors? And does not this motion argue that the parts of the two liquors in mixing coalesce with violence, and by consequence rush towards one another with an accelerated motion? And when aqua fortis, or spirit of vitriol poured upon filings of iron dissolves the filings with a great heat and ebullition, is not this heat and ebullition effected by a violent motion of the parts, and does not that motion argue that the acid parts of the liquor rush towards the parts of the metal with violence, and run forcibly into its pores till they get between its outmost particles, and the main mass of the metal, and surrounding those particles loosen them from the main mass, and set them at liberty to float off into the water? And when the acid particles, which alone would distil with an easy heat, will not separate from the particles of the metal without a very violent heat, does not this confirm the attraction between them?
When spirit of vitriol poured upon common salt or saltpetre makes an ebullition with the salt, and unites with it, and in distillation the spirit of the common salt or saltpetre comes over much easier than it would do before, and the acid part of the spirit of vitriol stays behind, does not this argue that the fixed alkali of the salt attracts the acid spirit of the vitriol more strongly than its own spirit, and not being able to hold them both, lets go its own? And when oil of vitriol is drawn off from its weight of nitre, and from both the ingredients a compound spirit of nitre is distilled, and two parts of this spirit are poured on one part of oil of cloves or caraway seeds, or of any ponderous oil of vegetable or animal substances, or oil of turpentine thickened with a little balsam of sulphur, and the liquors grow so very hot in mixing, as presently to send up a burning flame—does not this very great and sudden heat argue that the two liquors mix with violence, and that their parts in mixing run towards one another with an accelerated motion, and clash with the greatest force? And is it not for the same reason that well-rectified spirit of wine poured on the same compound spirit flashes; and that the pulvis fulminans, composed of sulphur, nitre, and salt of tartar, goes off with a more sudden and violent explosion than gunpowder, the acid spirits of the sulphur and nitre rushing towards one another, and towards the salt of tartar, with so great a violence as by the shock to turn the whole at once into vapour and flame? Where the dissolution is slow, it makes a slow ebullition and a gentle heat; and where it is quicker, it makes a greater ebullition with more heat; and where it is done at once, the ebullition is contracted into a sudden blast or violent explosion, with a heat equal to that of fire and flame. So when a drachm of the above-mentioned compound spirit of nitre was poured upon half a drachm of oil of caraway seeds in vacuo, the mixture immediately made a flash like gunpowder, and burst the exhausted receiver, which was a glass six inches wide and eight inches deep. And even the gross body of sulphur powdered, and with an equal weight of iron filings and a little water made into paste, acts upon the iron, and in five or six hours grows too hot to be touched, and emits a flame. And by these experiments compared with the great quantity of sulphur with which the Earth abounds, and the warmth of the interior parts of the Earth, and hot springs, and burning mountains, and with damps, mineral coruscations, earthquakes, hot suffocating exhalations, hurricanes, and spouts, we may learn that sulphureous steams abound in the bowels of the Earth and ferment with minerals, and sometimes take fire with a sudden coruscation and explosion; and, if pent up in subterraneous caverns, burst the caverns with a great shaking of the Earth, as in springing of a mine. And then the vapour generated by the explosion, expiring through the pores of the Earth, feels hot and suffocates, and makes tempests and hurricanes, and sometimes causes the land to slide, or the sea to boil, and carries up the water thereof in drops, which by their weight fall down again in spouts. Also some sulphureous steams, at all times when the Earth is dry, ascending into the air, ferment there with nitrous acids, and sometimes taking fire cause lightning and thunder, and fiery meteors. For the air abounds with acid vapours fit to promote fermentations, as appears by the rusting of iron and copper in it, the kindling of fire by blowing, and the beating of the heart by means of respiration. Now, the above-mentioned motions are so great and violent as to shew that in fermentations the particles of bodies which almost rest are put into new motions by a very potent principle, which acts upon them only when they approach one another, and causes them to meet and clash with great violence, and grow hot with the motion, and dash one another into pieces, and vanish into air and vapour and flame.
When salt of tartar per deliquium, being poured into the solution of any metal, precipitates the metal and makes it fall down to the bottom of the liquor in the form of mud, does not this argue that the acid particles are attracted more strongly by the salt of tartar than by the metal, and by the stronger attraction go from the metal to the salt of tartar? And so when a solution of iron in aqua fortis dissolves the lapis calaminaris, and lets go the iron, or a solution of copper dissolves iron immersed in it and lets go the copper, or a solution of silver dissolves copper and lets go the silver, or a solution of mercury in aqua fortis being poured upon iron, copper, tin, or lead, dissolves the metal and lets go the mercury—does not this argue that the acid particles of the aqua fortis are attracted more strongly by the lapis calaminaris than by iron, and more strongly by iron than by copper, and more strongly by copper than by silver, and more strongly by iron, copper, tin, and lead, than by mercury? And is it not for the same reason that iron requires more aqua fortis to dissolve it than copper, and copper more than the other metals; and that, of all metals, iron is dissolved most easily, and is most apt to rust; and, next after iron, copper?
When oil of vitriol is mixed with a little water, or is run per deliquium, and in distillation the water ascends difficultly, and brings over with it some part of the oil of vitriol in the form of spirit of vitriol, and this spirit (being poured upon iron, copper, or salt of tartar) unites with the body and lets go the water —doth not this shew that the acid spirit is attracted by the water, and more attracted by the fixed body than by the water, and therefore lets go the water to close with the fixed body? And is it not for the same reason that the water and acid spirits which are mixed together in vinegar, aqua fortis, and spirit of salt, cohere and rise together in distillation; but if the menstruum be poured on salt of tartar, or on lead, or iron, or any fixed body which it can dissolve, the acid by a stronger attraction adheres to the body, and lets go the water? And is it not also from a mutual attraction that the spirits of soot and sea-salt unite and compose the particles of sal-ammoniac, which are less volatile than before, because grosser and freer from water; and that the particles of sal-ammoniac in sublimation carry up the particles of antimony, which will not sublime alone; and that the particles of mercury uniting with the acid particles of spirit of salt compose mercury sublimate and with the particles of sulphur, compose cinnabar; and that the particles of spirit of wine and spirit of urine well rectified unite and, letting go the water which dissolved them, compose a consistent body; and that in subliming cinnabar from salt of tartar, or from quicklime, the sulphur by a stronger attraction of the salt or lime lets go the mercury, and stays with the fixed body; and that when mercury sublimate is sublimed from antimony, or from regulus of antimony, the spirit of salt lets go the mercury, and unites with the antimonial metal which attracts it more strongly, and stays with it till the heat be great enough to make them both ascend together, and then carries up the metal with it in the form of a very fusible salt called butter of antimony, although the spirit of salt alone be almost as volatile as water, and the antimony alone as fixed as lead?
When aqua fortis dissolves silver and not gold, and aqua regia dissolves gold and not silver, may it not be said that aqua fortis is subtile enough to penetrate gold as well as silver, but wants the attractive force to give it entrance; and that aqua regia is subtile enough to penetrate silver as well as gold, but wants the attractive force to give it entrance? For aqua regia is nothing else than aqua fortis mixed with some spirit of salt, or with sal-ammoniac; and even common salt dissolved in aqua fortis enables the menstruum to dissolve gold, though the salt be a gross body. When, therefore, spirit of salt precipitates silver out of aqua fortis, is it not done by attracting and mixing with the aqua fortis, and not attracting, or perhaps repelling silver? And when water precipitates antimony out of the sublimate of antimony and sal-ammoniac, or out of butter of antimony, is it not done by its dissolving, mixing with, and weakening the sal-ammoniac or spirit of salt, and its not attracting, or perhaps repelling, the antimony? And is it not for want of an attractive virtue between the parts of water and oil, of quick-silver and antimony, of lead and iron, that these substances do not mix; and by a weak attraction, that quick-silver and copper mix difficultly; and from a strong one, that quick-silver and tin, antimony and iron, water and salts, mix readily? And, in general, is it not from the same principle that heat congregates homogeneal bodies, and separates heterogeneal ones?
When arsenic with soap gives a regulus, and with mercury sublimate a volatile fusible salt, like butter of antimony, doth not this shew that arsenic, which is a substance totally volatile, is compounded of fixed and volatile parts, strongly cohering by a mutual attraction, so that the volatile will not ascend without carrying up the fixed? And so, when an equal weight of spirit of wine and oil of vitriol are digested together, and in distillation yield two fragrant and volatile spirits which will not mix with one another, and a fixed black earth remains behind—doth not this shew that oil of vitriol is composed of volatile and fixed parts strongly united by attraction, so as to ascend together in form of a volatile, acid, fluid salt, until the spirit of wine attracts and separates the volatile parts from the fixed? And, therefore, since oil of sulphur per campanam is of the same nature with oil of vitriol, may it not be inferred that sulphur is also a mixture of volatile and fixed parts so strongly cohering by attraction as to ascend together in sublimation? By dissolving flowers of sulphur in oil of turpentine, and distilling the solution, it is found that sulphur is composed of an inflammable thick oil or fat bitumen, an acid salt, a very fixed earth, and a little metal. The three first were found not much unequal to one another, the fourth in so small a quantity as scarce to be worth considering. The acid salt, dissolved in water, is the same with oil of sulphur per campanam, and abounding much in the bowels of the Earth, and particularly in marcasites—unites itself to the other ingredients of the marcasite, which are, bitumen, iron, copper, and earth, and with them compounds alum, vitriol, and sulphur. With the earth alone it compounds alum; with the metal alone, or metal and earth together, it compounds vitriol; and with the bitumen and earth it compounds sulphur. Whence it comes to pass that marcasites abound with those three minerals. And is it not from the mutual attraction of the ingredients that they stick together for compounding these minerals, and that the bitumen carries up the other ingredients of the sulphur, which without it would not sublime? And the same question may be put concerning all, or almost all, the gross bodies in Nature. For all the parts of animals and vegetables are composed of substances volatile and fixed, fluid and solid, as appears by their analysis; and so are salts and minerals, so far as chemists have been hitherto able to examine their composition.
When mercury sublimate is re-sublimed with fresh mercury, and becomes mercurius dulcis, which is a white, tasteless earth scarce dissolvable in water, and mercurius dulcis re-sublimed with spirit of salt returns into mercury sublimate; and when metals corroded with a little acid turn into rust, which is an earth tasteless and indissolvable in water, and this earth imbibed with more acid becomes a metallic salt; and when some stones, as spar of lead, dissolved in proper menstruums become salts—do not these things shew that salts are dry earth and watery acid united by attraction, and that the earth will not become a salt without so much acid as makes it dissolvable in water? Do not the sharp and pungent tastes of acids arise from the strong attraction whereby the acid particles rush upon and agitate the particles of the tongue? And when metals are dissolved in acid menstruums, and the acids in conjunction with the metal act after a different manner, so that the compound has a different taste much milder than before, and sometimes a sweet one—is it not because the acids adhere to the metallic particles, and thereby lose much of their activity? And if the acid be in too small a proportion to make the compound dissolvable in water, will it not by adhering strongly to the metal become unactive and lose its taste, and the compound be a tasteless earth? For such things as are not dissolvable by the moisture of the tongue, act not upon the taste.
As gravity makes the sea flow round the denser and weightier parts of the globe of the Earth, so the attraction may make the watery acid flow round the denser and compacter particles of earth for composing the particles of salt. For otherwise the acid would not do the office of a medium between the earth and common water, for making salts dissolvable in the water; nor would salt of tartar readily draw off the acid from dissolved metals, nor metals the acid from mercury. Now, as in the great globe of the Earth and sea, the densest bodies by their gravity sink down in water, and always endeavour to go towards the centre of the globe; so in particles of salt the densest matter may always endeavour to approach the centre of the particle: so that a particle of salt may be compared to a chaos, being dense, hard, dry, and earthy in the centre; and rare, soft, moist, and watery in the circumference. And hence it seems to be that salts are of a lasting nature, being scarce destroyed unless by drawing away their watery parts by violence, or by letting them soak into the pores of the central earth by a gentle heat in putrefaction, until the earth be dissolved by the water, and separated into smaller particles, which by reason of their smallness make the rotten compound appear of a black colour. Hence also it may be that the parts of animals and vegetables preserve their several forms, and assimilate their nourishment; the soft and moist nourishment easily changing its texture by a gentle heat and motion till it becomes like the dense, hard, dry, and durable earth in the centre of each particle. But when the nourishment grows unfit to be assimilated, or the central earth grows too feeble to assimilate it, the motion ends in confusion, putrefaction, and death.
If a very small quantity of any salt or vitriol be dissolved in a great quantity of water, the particles of the salt or vitriol will not sink to the bottom, though they be heavier in species than the water, but will evenly diffuse themselves into all the water, so as to make it as saline at the top as at the bottom. And does not this imply that the parts of the salt or vitriol recede from one another, and endeavour to expand themselves, and get as far asunder as the quantity of water, in which they float, will allow? And does not this endeavour imply that they have a repulsive force by which they fly from one another, or, at least, that they attract the water more strongly than they do one another? For as all things ascend in water which are less attracted than water, by the gravitating power of the Earth; so all the particles of salt which float in water, and are less attracted than water by any one particle of salt, must recede from that particle, and give way to the more attracted water.
When any saline liquor is evaporated to a cuticle and let cool, the salt concretes in regular figures; which argues that the particles of the salt, before they concreted, floated in the liquor at equal distances in rank and file, and by consequence that they acted upon one another by some power which at equal distances is equal, at unequal distances unequal. For by such a power they will range themselves uniformly, and without it they will float irregularly, and come together as irregularly. And since the particles of island crystal act all the same way upon the rays of light for causing the unusual refraction, may it not be supposed that, in the formation of this crystal, the particles not only ranged themselves in rank and file for concreting in regular figures, but also by some kind of polar virtue turned their homogeneal sides the same way.
The parts of all homogeneal hard bodies which fully touch one another stick together very strongly. And for explaining how this may be, some have invented hooked atoms, which is begging the question; and others tell us that bodies are glued together by rest (that is, by an occult quality, or rather by nothing); and others, that they stick together by conspiring motions (that is, by relative rest amongst themselves). I had rather infer from their cohesion that their particles attract one another by some force, which in immediate contact is exceeding strong, at small distances performs the chemical operations above mentioned, and reaches not far from the particles with any sensible effeet.
All bodies seem to be composed of hard particles, for otherwise fluids would not congeal; as water, oils, vinegar, and spirit or oil of vitriol do by freezing; mercury by fumes of lead; spirit of nitre and mercury by dissolving the mercury and evaporating the phlegm; spirit of wine and spirit of urine by dephlegming and mixing them; and spirit of urine and spirit of salt by subliming them together to make sal-ammoniac. Even the rays of light seem to be hard bodies; for otherwise they would not retain different properties in their diflerent sides. And, therefore, hardness may be reckoned the property of all uncompounded matter. At least, this seems to be as evident as the universal impenetrability of matter. For all bodies, so far as experience reaches, are either hard, or may be hardened; and we have no other evidence of universal impenetrability, besides a large experience without an experimental exception. Now, if compound bodies are so very hard as we find some of them to be, and yet are very porous, and consist of parts which are only laid together, the simple particles which are void of pores, and were never yet divided, must be much harder. For such hard particles, being heaped up together, can scarce touch one another in more than a few points, and therefore must be separable by much less force than is requisite to break a solid particle whose parts touch in all the space between them, without any pores or interstices to weaken their cohesion. And how such very hard particles, which are only laid together and touch only in a few points, can stick together, and that so firmly as they do, without the assistance of something which causes them to be attracted or pressed towards one another, is very difficult to conceive.
The same thing I infer also from the cohering of two polished marbles in vacuo, and from the standing of quick-silver in the barometer at the height of 50, 60 or 70 inches, or above, whenever it is well-purged of air and carefully poured in, so that its parts be everywhere contiguous both to one another and to the glass. The atmosphere by its weight presses the quick-silver into the glass, to the height of 29 or 30 inches. And some other agent raises it higher, not by pressing it into the glass, but by making its parts stick to the glass, and to one another. For upon any discontinuation of parts, made either by bubbles or by shaking the glass, the whole mercury falls down to the height of 29 or 30 inches.
And of the same kind with these experiments are those that follow: If two plane polished plates of glass (suppose two pieces of a polished looking-glass) be laid together, so that their sides be parallel and at a very small distance from one another, and then their lower edges be dipped into water, the water will rise up between them. And the less the distance of the glasses is, the greater will be the height to which the water will rise. If the distance be about the hundredth part of an inch, the water will rise to the height of about an inch; and if the distance be greater or less in any proportion, the height will be reciprocally proportional to the distance very nearly. For the attractive force of the glasses is the same, whether the distance between them be greater or less; and the weight of the water drawn up is the same, if the height of it be reciprocally proportional to the distance of the glasses. And in like manner, water ascends between two marbles polished plane, when their polished sides are parallel, and at a very little distance from one another. And if slender pipes of glass be dipped at one end into stagnating water, the water will rise up within the pipe, and the height to which it rises will be reciprocally proportional to the diameter of the cavity of the pipe, and will equal the height to which it rises between two planes of glass, if the semi-diameter of the cavity of the pipe be equal to the distance between the planes, or thereabouts. And these experiments succeed after the same manner in vacuo as in the open air (as hath been tried before the Royal Society) and therefore are not influenced by the weight or pressure of the atmosphere.
And if a large pipe of glass be filled with sifted ashes well pressed together in the glass, and one end of the pipe be dipped into stagnating water, the water will rise up slowly in the ashes, so as in the space of a week or fortnight to reach up within the glass to the height of 30 or 40 inches above the stagnating water. And the water rises up to this height by the action only of those particles of the ashes which are upon the surface of the elevated water; the particles which are within the water attracting or repelling it as much downwards as upwards. And, therefore, the action of the particles is very strong. But the particles of the ashes being not so dense and close together as those of glass, their action is not so strong as that of glass, which keeps quick-silver suspended to the height of 60 or 70 inches, and, therefore, acts with a force which would keep water suspended to the height of above 60 feet.
By the same principle, a sponge sucks in water, and the glands in the bodies of animals, according to their several natures and dispositions, suck in various juices from the blood.
If two plane polished plates of glass three or four inches broad, and twenty or twenty-five long, be laid one of them parallel to the horizon, the other upon the first, so as at one of their ends to touch one another, and contain an angle of about 10 or 15 minutes, and the same be first moistened on their inward sides with a clean cloth dipped into oil of oranges or spirit of turpentine, and a drop or two of the oil or spirit be let fall upon the lower glass at the other; so soon as the upper glass is laid down upon the lower, so as to touch it at one end as above, and to touch the drop at the other end, making with the lower glass an angle of about 10 or 15 minutes, the drop will begin to move towards the concourse of the glasses, and will continue to move with an accelerated motion till it arrives at that concourse of the glasses. For the two glasses attract the drop, and make it run that way towards which the attractions incline. And if when the drop is in motion you lift up that end of the glasses where they meet, and towards which the drop moves, the drop will ascend between the glasses, and therefore is attracted. And as you lift up the glasses more and more, the drop will ascend slower and slower, and at length rest, being then carried downward by its weight, as much as upwards by the attraction. And by this means you may know the force by which the drop is attracted at all distances from the concourse of the glasses.
Now, by some experiments of this kind (made by Mr. Hauksbee), it has been found that the attraction is almost reciprocally in a duplicate proportion of the distance of the middle of the drop from the concourse of the glasses, viz., reciprocally in a simple proportion, by reason of the spreading of the drop, and its touching each glass in a larger surface; and again reciprocally in a simple proportion, by reason of the attractions growing stronger within the same quantity of attracting surface. The attraction, therefore, within the same quantity of attracting surface, is reciprocally as the distance between the glasses. And, therefore, where the distance is exceeding small, the attraction must be exceeding great. By the Table in the second part of the second book, wherein the thicknesses of coloured plates of water between two glasses are set down, the thickness of the plate where it appears very black is three-eighths of the ten hundred thousandth part of an inch. And where the oil of oranges between the glasses is of this thickness, the attraction collected by the foregoing rule seems to be so strong as, within a circle of an inch in diameter, to suffice to hold up a weight equal to that of a cylinder of water of an inch in diameter, and two or three furlongs in length. And where it is of a less thickness, the attraction may be proportionally greater, and continue to increase, until the thickness do not exceed that of a single particle of the oil. There are, therefore, agents in nature able to make the particles of bodies stick together by very strong attractions. And it is the business of experimental philosophy to find them out.
Now, the smallest particles of matter may cohere by the strongest attractions, and compose bigger particles of weaker virtue; and many of these may cohere and compose bigger particles whose virtue is still weaker, and so on for divers successions, until the progression end in the biggest particles on which the operations in chemistry, and the colours of natural bodies depend, and which by cohering compose bodies of a sensible magnitude. If the body is compact, and bends or yields inward to pression—without any sliding of its parts, it is hard and elastic, returning to its figure with a force rising from the mutual attraction of its parts. If the parts slide upon one another, the body is malleable or soft. If they slip easily, and are of a fit size to be agitated by heat, and the heat is big enough to keep them in agitation, the body is fluid; and if it be apt to stick to things, it is humid; and the drops of every fluid affect a round figure by the mutual attraction of their parts, as the globe of the Earth and sea affects a round figure by the mutual attraction of its parts by gravity.
Since metals dissolved in acids attract but a small quantity of the acid, their attractive force can reach but to a small distance from them. And as in algebra, where affirmative quantities vanish and cease, there negative ones begin; so in mechanics, where attraction ceases, there a repulsive virtue ought to succeed. And that there is such a virtue seems to follow from the reflexions and inflexions of the rays of light. For the rays are repelled by bodies in both these cases, without the immediate contact of the reflecting or inflecting body. It seems also to follow from the emission of light, the ray so soon as it is shaken off from a shining body by the vibrating motion of the parts of the body, and gets beyond the reach of attraction, being driven away with exceeding great velocity. For that force which is sufficient to turn it back in reflexion may be sufficient to emit it. It seems also to follow from the production of air and vapour. The particles when they are shaken off from bodies by heat or fermentation, so soon as they are beyond the reach of the attraction of the body, receding from it, and also from one another with great strength, and keeping at a distance, so as sometimes to take up above a million of times more space than they did before in the form of a dense body. Which vast contraction and expansion seems unintelligible, by feigning the particles of air to be springy and ramous, or rolled up like hoops, or by any other means than a repulsive power. The particles of fluids which do not cohere too strongly, and are of such a smallness as renders them most susceptible to those agitations which keep liquors in a Fluor, are most easily separated and rarefied into vapour, and in the language of the chemists they are volatile, rarefying with an easy heat, and condensing with cold. But those which are grosser, and so less susceptible of agitation, or cohere by a stronger attraction, are not separated without a stronger heat, or perhaps not without fermentation. And these last are the bodies which chemists call fixed, and being rarefied by fermentation become true, permanent air; those particles receding from one another with the greatest force, and being most difficultly brought together, which upon contact cohere most strongly. And because the particles of permanent air are grosser, and arise from denser substances than those of vapours, thence it is that true air is more ponderous than vapour, and that a moist atmosphere is lighter than a dry one, quantity for quantity. From the same repelling power it seems to be that flies walk upon the water without wetting their feet; and that the object-glasses of long telescopes lie upon one another without touching; and that dry powders are difficultly made to touch one another so as to stick together, unless by melting them, or wetting them with water, which by exhaling may bring them together; and that two polished marbles, which by immediate contact stick together, are difficultly brought so close together as to stick.
And thus Nature will be very conformable to herself and very simple, performing all the great motions of the heavenly bodies by the attraction of gravity which intercedes those bodies, and almost all the small ones of their particles by some other attractive and repelling powers which intercede the particles. The vis inertiae is a passive principle by which bodies persist in their motion or rest, receive motion in proportion to the force impressing it, and resist as much as they are resisted. By this principle alone there never could have been any motion in the world. Some other principle was necessary for putting bodies into motion; and now they are in motion, some other principle is necessary for conserving the motion. For from the various composition of two motions, ’tis very certain that there is not always the same quantity of motion in the world. For if two globes, joined by a slender rod, revolve about their common centre of gravity with a uniform motion, while that centre moves on uniformly in a right line drawn in the plane of their circular motion, the sum of the motions of the two globes, as often as the globes are in the right line described by their common centre of gravity, will be bigger than the sum of their motions when they are in a line perpendicular to that right line. By this instance it appears that motion may be got or lost. But by reason of the tenacity of fluids, and attrition of their parts, and the weakness of elasticity in solids, motion is much more apt to be lost than got, and is always upon the decay. For bodies which are either absolutely hard, or so soft as to be void of elasticity, will not rebound from one another. Impenetrability makes them only stop. If two equal bodies meet directly in vacuo, they will by the laws of motion stop where they meet, and lose all their motion, and remain in rest, unless they be elastic, and receive new motion from their spring. If they have so much elasticity as suffices to make them re-bound with a quarter, or half, or three-quarters of the force with which they come together, they will lose three-quarters, or half, or a quarter of their motion. And this may be tried by letting two equal pendulums fall against one another from equal heights. If the pendulums be of lead or soft clay, they will lose all or almost all their motions; if of elastic bodies they will lose all but what they recover from their elasticity. If it be said that they can lose no motion but what they communicate to other bodies, the consequence is, that in vacuo they can lose no motion, but when they meet they must go on and penetrate one another’s dimensions. If three equal round vessels be filled (the one with water, the other with oil, the third with molten pitch), and the liquors be stirred about alike to give them a vortical motion, the pitch by its tenacity will lose its motion quickly, the oil being less tenacious will keep it longer, and the water being less tenacious will keep it longest, but yet will lose it in a short time. Whence it is easy to understand that if many contiguous vortices of molten pitch were each of them as large as those which some suppose to revolve about the Sun and fixed stars, yet these and all their parts would, by their tenacity and stiffness, communicate their motion to one another till they all rested among themselves. Vortices of oil or water, or some fluider matter, might continue longer in motion; but unless the matter were void of all tenacity and attrition of parts, and communication of motion (which is not to be supposed), the motion would constantly decay. Seeing, therefore, the variety of motion which we find in the world is always decreasing, there is a necessity of conserving and recruiting it by active principles, such as are the cause of gravity, by which planets and comets keep their motions in their orbs, and bodies acquire great motion in falling; and the cause of fermentation, by which the heart and blood of animals are kept in perpetual motion and heat; the inward parts of the earth are constantly warmed, and in some places grow very hot; bodies burn and shine, mountains take fire, the caverns of the earth are blown up, and the Sun continues violently hot and lucid, and warms all things by his light. For we meet with very little motion in the world, besides what is owing to these active principles. And if it were not for these principles, the bodies of the earth, planets, comets, Sun, and all things in them, would grow cold and freeze, and become inactive masses; and all putrefaction, generation, vegetation and life would cease, and the planets and comets would not remain in their orbs.
All these things being considered, it seems probable to me that God in the beginning formed matter in solid, massy, hard, impenetrable, moveable particles, of such sizes and figures, and with such other properties, and in such proportion to space, as most conduced to the end for which he formed them; and that these primitive particles being solids, are incomparably harder than any porous bodies compounded of them; even so very hard as never to wear or break in pieces; no ordinary power being able to divide what God himself made one in the first creation. While the particles continue entire, they may compose bodies of one and the same nature and texture in all ages; but should they wear away, or break in pieces, the nature of things depending on them would be changed. Water and earth, composed of old worn particles and fragments of particles, would not be of the same nature and texture now, with water and earth composed of entire particles in the beginning. And, therefore, that Nature may be lasting, the changes of corporeal things are to be placed only in the various separations and new associations and motions of these permanent particles; compound bodies being apt to break, not in the midst of solid particles, but where those particles are laid together, and only touch in a few points.
It seems to me, further, that these particles have not only a vis inertiae, accompanied with such passive laws of motion as naturally result from that force, but also that they are moved by certain active principles, such as is that of gravity, and that which causes fermentation, and the cohesion of bodies. These principles I consider, not as occult qualities, supposed to result from the specific forms of things, but as general laws of nature, by which the things themselves are formed; their truth appearing to us by phenomena, though their causes be not yet discovered. For these are manifest qualities, and their causes only are occult. And the Aristotelians gave the name of occult qualities, not to manifest qualities, but to such qualities only as they supposed to lie hid in bodies, and to be the unknown causes of manifest effects. Such as would be the causes of gravity, and of magnetic and electric attractions, and of fermentations, if we should suppose that these forces or actions arose from qualities unknown to us, and incapable of being discovered and made manifest. Such occult qualities put a stop to the improvement of natural philosophy, and therefore of late years have been rejected. To tell us that every species of things is endowed with an occult specific quality by which it acts and produces manifest effects, is to tell us nothing; but to derive two or three general principles of motion from phenomena, and afterwards to tell us how the properties and actions of all corporeal things follow from those manifest principles, would be a very great step in philosophy, though the causes of those principles were not yet discovered. And, therefore, I scruple not to propose the principles of motion above mentioned, they being of very general extent, and leave their causes to be found out.
Now, by the help of these principles, all material things seem to have been composed of the hard and solid particles above mentioned, variously associated in the first creation by the counsel of an intelligent agent. For it became Him who created them to set them in order. And if He did so, it’s unphilosophical to seek for any other origin of the world, or to pretend that it might arise out of a chaos by the mere laws of Nature; though, being once formed, it may continue by those laws for many ages. For while comets move in very eccentric orbs in all manner of positions, blind fate could never make all the planets move one and the same way in orbs concentric, some inconsiderable irregularities excepted, which may have risen from the mutual actions of comets and planets upon one another, and which will be apt to increase, till this system wants a reformation. Such a wonderful uniformity in the planetary system must be allowed the effect of choice. And so must the uniformity in the bodies of animals, they having generally a right and a left side shaped alike, and on either side of their bodies two legs behind, and either two arms, or two legs, or two wings before upon their shoulders, and between their shoulders a neck running down into a backbone, and a head upon it; and in the head two ears, two eyes, a nose, a mouth, and a tongue, alike situated. Also the first contrivance of those very artificial parts of animals, the eyes, ears, brain, muscles, heart, lungs, midriff, glands, larynx, hands, wings, swimming bladders, natural spectacles, and other organs of sense and motion; and the instinct of brutes and insects can be the effect of nothing else than the wisdom and skill of a powerful, ever-living agent, who being in all places, is more able by His will to move the bodies within His boundless uniform sensorium, and thereby to form and reform the parts of the Universe, than we are by our will to move the parts of our own bodies. And yet we are not to consider the world as the body of God, or the several parts thereof as the parts of God. He is a uniform Being, void of organs, members or parts, and they are his creatures subordinate to him, and subservient to His will; and He is no more the soul of them than the soul of man is the soul of the species of things carried through the organs of sense into the place of its sensation, where it perceives them by means of its immediate presence, without the intervention of any third thing. The organs of sense are not for enabling the soul to perceive the species of things in its sensorium, but only for conveying them thither; and God has no need of such organs, He being everywhere present to the things themselves. And since space is divisible in infinitum, and matter is not necessarily in all places, it may be also allowed that God is able to create particles of matter of several sizes and figures, and in several proportions to space, and perhaps of different densities and forces, and thereby to vary the laws of Nature, and make worlds of several sorts in several parts of the Universe. At least, I see nothing of contradiction in all this.
As in mathematics, so in natural philosophy, the investigation of difficult things by the method of analysis, ought ever to precede the method of composition. This analysis consists in making experiments and observations, and in drawing general conclusions from them by induction, and admitting of no objections against the conclusions but such as are taken from experiments, or other certain truths. For hypotheses are not to be regarded in experimental philosophy. And although the arguing from experiments and observations by induction be no demonstration of general conclusions, yet it is the best way of arguing which the nature of things admits of, and may be looked upon as so much the stronger, by how much the induction is more general. And if no exception occur from phenomena, the conclusion may be pronounced generally. But if at any time afterwards any exception shall occur from experiments, it may then begin to be pronounced with such exceptions as occur. By this way of analysis we may proceed from compounds to ingredients, and from motions to the forces producing them; and, in general, from effects to their causes, and from particular causes to more general ones, till the argument end in the most general. This is the method of analysis; and the synthesis consists in assuming the causes discovered, and established as principles, and by them explaining the phenomena proceeding from them, and proving the explanations.
In the two first books of these Optics, I proceeded by this analysis to discover and prove the original differences of the rays of light in respect of refrangibility, reflexibility, and colour, and their alternate fits of easy reflexion and easy transmission, and the properties of bodies, both opaque and pellucid, on which their reflexions and colours depend. And these discoveries, being proved, may be assumed in the method of composition for explaining the phenomena arising from them, an instance of which method I gave in the end of the first book. In this third book I have only begun the analysis of what remains to be discovered about light and its effects upon the frame of Nature, hinting several things about it, and leaving the hints to be examined and improved by the further experiments and observations of such as are inquisitive. And if natural philosophy in all its parts, by pursuing this method, shall at length be perfected, the bounds of moral philosophy will be also enlarged. For so far as we can know by natural philosophy what is the First Cause, what power He has over us, and what benefits we recieve from Him, so far our duty towards Him, as well as that towards one another, will appear to us by the light of Nature. And no doubt, if the worship of false gods had not blinded the heathen, their moral philosophy would have gone farther than to the four cardinal virtues; and instead of teaching the transmigration of souls, and to worship the Sun and Moon, and dead heroes, they would have taught us to worship our true Author and Benefactor, as their ancestors did under the government of Noah and his sons before they corrupted themselves.