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ELEMENTS of NATURAL PHILOSOPHY. - John Locke, The Works, vol. 2 An Essay concerning Human Understanding Part 2 and Other Writings 
The Works of John Locke in Nine Volumes, (London: Rivington, 1824 12th ed.). Vol. 2.
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ELEMENTS of NATURAL PHILOSOPHY.
Of Matter and Motion.
Matter is an extended solid substance; which being comprehended under distinct surfaces, makes so many particular distinct bodies.
Motion is so well known by the sight and touch, that to use words to give a clear idea of it, would be in vain.
Matter, or body, is indifferent to motion, or rest.
There is as much force required to put a body, which is in motion, at rest; as there is to set a body, which is at rest, into motion.
No parcel of matter can give itself either motion or rest, and therefore a body at rest will remain so eternally, except some external cause puts it in motion; and a body in motion will move eternally, unless some external cause stops it.
A body in motion will always move on in a straight line, unless it be turned out of it by some external cause; because a body can no more alter the determination of its motion, than it can begin it, alter or stop its motion itself.
The swiftness of motion is measured by distance of place and length of time wherein it is performed. For instance, if A and B, bodies of equal or different bigness, move each of them an inch in the same time; their motions are equally swift; but if A moves two inches, in the time whilst B is moving one inch; the motion of A is twice as swift as that of B.
The quantity of motion is measured by the swiftness of the motion, and the quantity of the matter moved, taken together. For instance, if A, a body equal to B, moves as swift as B; then it hath an equal quantity of motion. If A hath twice as much matter as B, and moves equally as swift, it hath double the quantity of motion; and so in proportion.
It appears, as far as human observation reaches, to be a settled law of nature, that all bodies have a tendency, attraction, or gravitation towards one another.
The same force, applied to two different bodies, produces always the same quantity of motion in each of them. For instance, let a boat which with its lading is one ton, be tied at a distance to another vessel, which with its lading is twenty-six tons; if the rope that ties them together be pulled, either in the less or bigger of these vessels, the less of the two, in their approach one to another, will move twenty-six feet, while the other moves but one foot.
Wherefore the quantity of matter in the earth being twenty-six times more than in the moon; the motion in the moon towards the earth, by the common force of attraction, by which they are impelled towards one another, will be twenty-six times as fast as in the earth; that is, the moon will move twenty-six miles towards the earth, for every mile the earth moves towards the moon.
Hence it is, that, in this natural tendency of bodies towards one another, that in the lesser is considered as gravitation; and that in the bigger as attraction; because the motion of the lesser body (by reason of its much greater swiftness) is alone taken notice of.
This attraction is the strongest, the nearer the attracting bodies are to each other; and, in different distances of the same bodies, is reciprocally in the duplicate proportion of those distances. For instance, if two bodies at a given distance attract each other with a certain force, at half the distance, they will attract each other with four times that force; at one third of the distance, with nine times that force; and so on.
Two bodies at a distance will put one another into motion by the force of attraction; which is inexplicable by us, though made evident to us by experience, and so to be taken as a principle in natural philosophy.
Supposing then the earth the sole body in the universe, and at rest; if God should create the moon, at the same distance that it is now from the earth; the earth and the moon would presently begin to move one towards another in a straight line by this motion of attraction or gravitation.
If a body, that by the attraction of another would move in a straight line towards it, receives a new motion any ways oblique to the first; it will no longer move in a straight line, according to either of those directions; but in a curve that will partake of both. And this curve will differ, according to the nature and quantity of the forces that concurred to produce it; as, for instance, in many cases it will be such a curve as ends where it began, or recurs into itself; that is, makes up a circle, or an ellipsis or oval very little differing from a circle.
Of the Universe.
To any one, who looks about him in the world, there are obvious several distinct masses of matter, separate from one another; some whereof have discernible motions. These are the sun, the fixt stars, the comets and the planets, amongst which this earth, which we inhabit, is one. All these are visible to our naked eyes.
Besides these, telescopes have discovered several fixt stars, invisible to the naked eye; and several other bodies moving about some of the planets; all which were invisible and unknown, before the use of perspective glasses were found.
The vast distances between these great bodies, are called intermundane spaces; in which though there may be some fluid matter, yet it is so thin and subtile, and there is so little of that in respect of the great masses that move in those spaces, that it is as much as nothing.
These masses of matter are either luminous, or opake or dark.
Luminous bodies, are such as give light of themselves; and such are the sun and fixt stars.
Dark or opake bodies are such as emit no light of themselves, though they are capable of reflecting of it, when it is cast upon them from other bodies; and such are the planets.
There are some opake bodies, as for instance the comets, which, besides the light that they may have from the sun, seem to shine with a light that is nothing else but an ascension, which they receive from the sun, in their near approaches to it, in their respective revolutions.
The fixt stars are called fixt, because they al wys keep the same distance one from another.
The sun, at the same distance from us that the fixt stars are, would have the appearance of one of the fixt stars.
Of our Solar System.
Our solar system consists of the sun, and the planets and comets moving about it.
The planets are bodies, which appear to us like stars; not that they are luminous bodies, that is, have light in themselves; but they shine by reflecting the light of the sun.
They are called planets from a Greek word, which signifies wandering; because they change their places, and do not always keep the same distance with one another, nor with the fixt stars, as the fixt stars do.
The planets are either primary, or secondary.
There are six primary planets, viz. Mercury, Venus, the Earth, Mars, Jupiter, and Saturn.
All these move round the sun, which is, as it were, the centre of their motions.
The secondary planets move round about other planets. Besides the moon, which moves about the earth; four moons move about Jupiter, and five about Saturn, which are called their satellites.
The middle distances of the primary planets from the sun are as follows:
The orbits of the planets, and their respective distances from the sun, and from one another, together with the orbit of a comet, may be seen in the figure of the solar system hereunto annexed.
The periodical times of each planet’s revolution about the sun are as follows:
The planets move round about the sun from west to east in the zodiac; or, to speak plainer, are always found amongst some of the stars of those constellations, which make the twelve signs of the zodiac.
The motion of the planets about the sun is not perfectly circular, but rather elliptical.
The reason of their motions in curve lines, is the attraction of the sun, or their gravitations towards the sun, (call it which you please); and an oblique or side-long impulse or motion.
These two motions or tendencies, the one always endeavouring to carry them in a straight line from the circle they move in, and the other endeavouring to draw them in a straight line to the sun, makes that curve line they revolve in.
The motion of the comets about the sun is in a very long slender oval: whereof one of the focuses is the centre of the sun, and the other very much beyond the sphere of Saturn.
The moon moves about the earth, as the earth doth about the sun. So that it hath the centre of its motion in the earth; as the earth hath the centre of its revolution in the sun, about which it moves.
The moon makes its synodical motion about the earth, in 29 days, 12 hours, and about 44 minutes.
It is full moon, when, the earth being between the sun and the moon, we see all the enlightened part of the moon: new moon, when, the moon being between us and the sun, its enlightened part is turned from us; and half moon, when the moon being in the quadratures, as the astronomers call it, we see but half the enlightened part.
An eclipse of the moon is, when the earth, being between the sun and the moon, hinders the light of the sun from falling upon, and being reflected by, the moon. If the light of the sun is kept off from the whole body of the moon, it is a total eclipse; if from a part only, it is a partial one.
An eclipse of the sun is, when the moon, being between the sun and the earth, hinders the light of the sun from coming to us. If the moon hides from us the whole body of the sun, it is a total eclipse; if not, a partial one.
Our solar system is distant from the fixt stars 20,000,000,000 semi-diameters of the earth; or, as Mr. Huygens expresses the distance, in his Cosmotheorosa : the fixt stars are so remote from the earth, that, if a cannon-bullet should come from one of the fixt stars with as swift a motion as it hath when it is shot out of the mouth of a cannon, it would be 700,000 years in coming to the earth.
This vast distance so much abates the attraction to those remote bodies, that its operation upon those of our system is not at all sensible, nor would draw away or hinder the return of any of our solar comets; though some of them should go so far from the sun, as not to make the revolution about it in less than 1000 years.
It is more suitable to the wisdom, power, and greatness of God, to think that the fixt stars are all of them suns, with systems of inhabitable planets moving about them, to whose inhabitants he displays the marks of his goodness as well as to us; rather than to imagine that those very remote bodies, so little useful to us, were made only for our sake.
Of the Earth, considered as a Planet.
The earth, by its revolution about the sun in 365 days, 5 hours, 49 minutes, makes that space of time we call a year.
The line, which the centre of the earth describes in its annual revolution about the sun, is called ecliptic.
The annual motion of the earth about the sun, is in the order of the signs of the zodiac; that is, speaking vulgarly, from west to east.
Besides this annual revolution of the earth about the sun in the ecliptic, the earth turns round upon its own axis in 24 hours.
The turning of the earth upon its own axis every 24 hours, whilst it moves round the sun in a year, we may conceive by the running of a bowl on a bowling-green; in which not only the centre of the bowl hath a progressive motion on the green; but the bowl in its going orward from one part of the green to another, turns round about its own axis.
The turning of the earth on its own axis, makes the difference of day and night; it being day in those parts of the earth which are turned towards the sun; and night in those parts which are in the shade, or turned from the sun.
The annual revolution of the earth in the ecliptic, is the cause of the different seasons, and of the several lengths of days and nights, in every part of the world, in the course of the year.
The reason of it, is the earth’s going round its own axis in the ecliptic, but at the same time keeping every where its axis equally inclined to the plane of the ecliptic, and parallel to itself. For the plane of the ecliptic inclining to the plane of the equator, 23 degrees and an half, makes that the earth, moving round in the ecliptic, hath sometimes one of its poles, and sometimes the other nearer the sun.
If the diameter of the sun be to the diameter of the earth, as 48 to 1, as by some it is accounted; then the disk of the sun, speaking “numero rotundo,” is above 2000 times bigger than the disk of the earth; and the globe of the sun is above 100,000 times bigger than the globe of the earth.
The distance of the earth’s orbit from the sun, is above 200,000 semi-diameters of the earth.
If a cannon-bullet should come from the sun, with the same velocity it hath when it is shot out of the mouth of a cannon, it would be 25 years in coming to the earth.
Of the Air and Atmosphere.
We have already considered the earth as a planet, or one of the great masses of matter moving about the sun; we shall now consider it as it is made up of its several parts, abstractedly from its diurnal and annual motions.
The exterior part of this our habitable world is the air or atmosphere; a light, thin fluid, or springy body, that encompasses the solid earth on all sides.
The height of the atmosphere, above the surface of the solid earth, is not certainly known; but that it doth reach but to a very small part of the distance betwixt the earth and the moon, may be concluded from the refraction of the rays coming from the sun, moon, and other luminous bodies.
Though considering that the air we are in, being near 1000 times lighter than water; and that the higher it is, the less it is compressed by the superior incumbent air, and so consequently being a springy body the thinner it is; and considering also that a pillar of air of any diameter is equal in weight to a pillar of quicksilver of the same diameter of between 29 and 30 inches height; we may infer that the top of the atmosphere is not very near the surface of the solid earth.
It may be concluded, that the utmost extent of the atmosphere reaches upwards, from the surface of the solid earth that we walk on, to a good distance above us; first, if we consider that a column of air of any given diameter is equiponderant to a column of quicksilver of between 29 and 30 inches height. Now quicksilver being near 14 times heavier than water, if air was as heavy as water, the atmosphere would be about 14 times higher than the column of quicksilver, i. e. about 35 feet.
Secondly, if we consider that air is 1000 times lighter than water, then a pillar of air equal in weight to a pillar of quicksilver of 30 inches high will be 35000 feet; whereby we come to know that the air or atmosphere is 35000 feet, i. e. near seven miles high.
Thirdly, if we consider that the air is a springy body, and that that, which is nearest the earth, is compressed by the weight of all the atmosphere that is above it, and rests perpendicularly upon it; we shall find that the air here, near the surface of the earth, is much denser and thicker than it is in the upper parts. For example, if upon a fleece of wool you lay another; the under one will be a little compressed by the weight of that which lies upon it; and so both of them by a third, and so on; so that, if 10000 were piled one upon another, the under one would by the weight of all the rest be very much compressed, and all the parts of it be brought abundantly closer together, than when there was no other upon it; and the next to that a little less compressed, the third a little less than the second, and so on till it came to the uppermost, which would be in its full expansion, and not compressed at all. Just so it is in the air; the higher you go in it, the less it is compressed, and consequently the less dense it is; and so the upper part being exceedingly thinner than the lower part, which we breathe in (which is that that is 1000 times lighter than water); the top of the atmosphere is probably much higher than the distance above assigned.
That the air near the surface of the earth will mightily expand itself, when the pressure of the incumbent atmosphere is taken off, may be abundantly seen in the experiments made by Mr. Boyle in his pneumatic engine. In his “Physico-mechanical Experiments,” concerning the air, he declares* it probable that the atmosphere may be several hundred miles high; which is easy to be admitted, when we consider what he proves in another part of the same treatise, viz. that the air here about the surface of the earth, when the pressure is taken from it, will dilate itself about 152 times.
The atmosphere is the scene of the meteors; and therein is collected the matter of rain, hail, snow, thunder, and lightning; and a great many other things observable in the air.
Of Meteors in general.
Besides the springy particles of pure air, the atmosphere is made up of several steams or minute particles of several sorts, rising from the earth and the waters, and floating in the air, which is a fluid body, and though much finer and thinner, may be considered in respect of its fluidity to be like water, and so capable, like other liquors, of having heterogeneous particles floating in it.
The most remarkable of them are, first, the particles of water raised into the atmosphere, chiefly by the heat of the sun, out of the sea and other waters, and the surface of the earth; from whence it falls in dew, rain, hail, and snow.
Out of the vapours rising from moisture, the clouds are principally made.
Clouds do not consist wholly of watery parts; for, besides the aqueous vapours that are raised into the air, there are also sulphureous and saline particles that are raised up, and in the clouds mixed with the aqueous particles, the effects whereof are sometimes very sensible; as particularly in lightning and thunder, when the sulphureous and nitrous particles firing break out with that violence of light and noise, which is observable in thunder, and very much resembles gunpowder.
That there are nitrous particles raised into the air is evident from the nourishment which rain gives to vegetables more than any other water; and also by the collection of nitre or salt-petre in heaps of earth, out of which it has been extracted, if they be exposed to the air, so as to be kept from rain; not to mention other efforts, wherein the nitrous spirit in the air shows itself.
Clouds are the greatest and most considerable of all the meteors, as furnishing matter and plenty to the earth. They consist of very small drops of water, and are elevated a good distance above the surface of the earth; for a cloud is nothing but a mist flying high in the air, as a mist is nothing but a cloud here below.
How vapours are raised into the air in invisible steams by the heat of the sun out of the sea, and moist parts of the earth, is easily understood; and there is a visible instance of it in ordinary distillations. But how these steams are collected into drops, which bring back the water again, is not so easy to determine.
To those that will carefully observe, perhaps it will appear probable, that it is by that, which the chymists call precipitation; to which it answers in all its parts.
The air may be looked on as a clear and pellucid menstruum, in which the insensible particles of dissolved matter float up and down, without being discerned, or troubling the pellucidity of the air; when on a sudden, as if it were by a precipitation, they gather into the very small but visible misty drops that make clouds.
This may be observed some times in a very clear sky; when, there not appearing any cloud, or any thing opake, in the whole horizon, one may see on a sudden clouds gather, and all the hemisphere overcast; which cannot be from the rising of the new aqueous vapours at that time, but from the precipitation of the moisture, that in invisible particles floated in the air, into very small, but very visible drops, which by a like cause being united into greater drops, they become too heavy to be sustained in the air, and so fall down in rain.
Hail seems to be the drops of rain frozen in their falling.
Snow is the small particles of water frozen before they unite into drops.
The regular figures, which branch out in flakes of snow, seem to show that there are some particles of salt mixed with the water, which makes them unite in certain angles.
The rainbow is reckoned one of the most remarkable meteors, though really it be no meteor at all; but the reflection of the sun-beams from the smallest drops of a cloud or mist, which are placed in a certain angle made by the concurrence of two lines, one drawn from the sun, and the other from the eye to these little drops in the cloud, which reflect the sun-beams; so that two people, looking upon a rainbow at the same time, do not see exactly the same rainbow.
Of Springs, Rivers, and the Sea.
Part of the water that falls down from the clouds, runs away upon the surface of the earth into channels, which convey it to the sea; and part of it is imbibed in the spungy shell of the earth, from whence sinking lower by degrees, it falls down into subterranean channels, and so under ground passes into the sea; or else, meeting with beds of rock or clay, it is hindred from sinking lower, and so breaks out in springs, which are most commonly in the sides, or at the bottom of hilly ground.
Springs make little rivulets; those united make brooks; and those coming together make rivers, which empty themselves into the sea.
The sea is a great collection of waters in the deep valleys of the earth. If the earth were all plain, and had not those deep hollows, the earth would be all covered with water; because the water being lighter than the earth, would be above the earth, as the air is above the water.
The most remarkable thing in the sea is that motion of the water called tides. It is a rising and falling of the water of the sea. The cause of this is the attraction of the moon, whereby the part of the water in the great ocean, which is nearest the moon, being most strongly attracted, is raised higher than the rest; and the part opposite to it on the contrary side, being least attracted, is also higher than the rest. And these two opposite rises of the surface of the water in the great ocean, following the motion of the moon from east to west, and striking against the large coasts of the continents that lie in its way; from thence rebounds back again, and so makes floods and ebbs in narrow seas, and rivers remote from the great ocean. Herein we also see the reason of the times of the tides, and why they so constantly follow the course of the moon.
Of several Sorts of Earth, Stones, Metals, Minerals, and other Fossils.
This solid globe we live upon is called the earth, though it contains in it a great variety of bodies, several whereof are not properly earth; which word, taken in a more limited sense, signifies such parts of this globe as are capable, being exposed to the air, to give rooting and nourishment to plants, so that they may stand and grow in it. With such earth as this, the greatest part of the surface of this globe is covered; and it is as it were the store-house, from whence all the living creatures of our world have originally their provisions; for from thence all the plants have their sustenance, and some few animals, and from these all the other animals.
Of earth, taken in this sense, there are several sorts, v. g. common mould, or garden earth, clay of several kinds, sandy soils.
Besides these, there is medicinal earth; as that which is called terra lemnia, bolus armena, and divers others.
After the several earths, we may consider the parts of the surface of this globe, which is barren; and such, for the most, are sand, gravel, chalk, and rocks, which produce nothing, where they have no earth mixt amongst them. Barren sands are of divers kinds, and consist of several little irregular stones without any earth; and of such there are great deserts to be seen in several parts of the world.
Besides these, which are most remarkable on the surface of the earth, there are found deeper, in this globe, many other bodies, which, because we discover by digging into the bowels of the earth, are called by one common name, fossils; under which are comprehended metals, minerals or half metals, stones of divers kinds, and sundry bodies that have the texture between earth and stone.
To begin with those fossils which come nearest the earth; under this head we may reckon the several sorts of oker, chalk, that which they call black-lead, and other bodies of this kind, which are harder than earth, but have not the consistency and hardness of perfect stone.
Next to these may be considered stones of all sorts; whereof there is almost an infinite variety. Some of the most remarkable, either for beauty or use, are these: marble of all kinds, porphyry, granate, free-stone, &c. flints, agates, cornelians, pebbles, under which kind come the precious stones, which are but pebbles of an excessive hardness, and when they are cut and polished, they have an extraordinary lustre. The most noted and esteemed are, diamonds, rubies, amethysts, emeralds, topazes, opals.
Besides these, we must not omit those which, though of not so much beauty, yet are of greater use, viz. loadstones, whetstones of all kinds, limestones, callamine, or lapis calaminaris; and abundance of others.
Besides these, there are found in the earth several sorts of salts, as eating or common salt, vitriol, sal gemma, and others.
The minerals, or semi-metals, that are dug out of the bowels of the earth, are antimony, cinnabar, zink, &c. to which may be added brimstone.
But the bodies of most use, that are sought for out of the depths of the earth, are the metals; which are distinguished from other bodies by their weight, fusibility, and malleableness; of which there are these sorts, gold, silver, copper, tin, lead, and, the most valuable of them all, iron; to which one may join that anomalous body quicksilver, or mercury.
He that desires to be more particularly informed concerning the qualities and properties of these subterraneous bodies, may consult natural historians and chymists.
What lies deeper towards the centre of the earth we know not, but a very little beneath the surface of this globe, and whatever we fetch from under ground, is only what is lodged in the shell of the earth.
All stones, metals, and minerals, are real vegetables; that is, grow organically from proper seeds, as well as plants.
Of Vegetables, or Plants.
Next to the earth itself, we may consider those that are maintained on its surface; which, though they are fastened to it, yet are very distinct from it; and those are the whole tribe of vegetables or plants. These may be divided into three sorts, herbs, shrubs, and trees.
Herbs are those plants whose stalks are soft, and have nothing woody in them, as grass, sowthistle, and hemlock. Shrubs and trees have all wood in them; but with this difference, that shrubs grow not to the height of trees, and usually spread into branches near the surface of the earth, whereas trees generally shoot up in one great stem or body, and then, at a good distance from the earth, spread into branches; thus gooseberries, and currants, are shrubs; oaks, and cherries, are trees.
In plants, the most considerable parts are these, the root, the stalk, the leaves, the flower, and the seed. There are very few of them that have not all these parts, though some there are that have no stalk; others that have no leaves; and others that have no flowers. But without seed or root I think there are none.
In vegetables, there are two things chiefly to be considered, their nourishment and propagation.
Their nourishment is thus: the small and tender fibres of the roots, being spread under ground, imbibe, from the moist earth, juice fit for their nourishment; this is conveyed by the stalk up into the branches, and leaves, through little, and, in some plants, imperceptible tubes, and from thence, by the bark, returns again to the root; so that there is in vegetables, as well as animals, a circulation of the vital liquor. By what impulse it is moved, is somewhat hard to discover. It seems to be from the difference of day and night, and other changes in the heat of the air; for the heat dilating, and the cold contracting those little tubes, supposing there be valves in them, it is easy to be conceived how the circulation is performed in plants, where it is not required to be so rapid and quick as in animals.
Nature has provided for the propagation of the species of plants several ways. The first and general is by seed. Besides this, some plants are raised from any part of the root set in the ground; others by new roots that are propagated from the old one, as in tulips; others by offsets; and in others, the branches set in the ground will take root and grow; and last of all, grafting and inoculation, in certain sorts, are known ways of propagation. All these ways of increasing plants make one good part of the skill of gardening; and from the books of gardeners may be best learnt.
There is another sort of creatures belonging to this our earth, rather as inhabitants than parts of it. They differ in this from plants, that they are not fixed to any one place, but have a freedom of motion up and down, and, besides, have sense to guide them in their motions.
Man and brute, divide all the animals of this our globe.
Brutes may be considered as either aerial, terrestrial, aquatic, or amphibious. I call those aerial, which have wings, wherewith they can support themselves in the air. Terrestrial, are those, whose only place of rest is upon the earth. Aquatic, are those, whose constant abode is upon the water. Those are called amphibious, which live freely in the air upon the earth, and yet are observed to live long upon the water, as if they were natural inhabitants of that element; though it be worth the examination to know, whether any of those creatures that live at their ease, and by choice, a good while or at any time upon the earth, can live a long time together perfectly under water.
Aerial animals may be subdivided into birds, and flies.
Fishes, which are the chief part of aquatic animals, may be divided into shell-fishes, scaly fishes, and those that have neither apparent scales nor shells.
And the terrestrial animals may be divided into quadrupeds or beasts, reptiles, which have many feet, and serpents, which have no feet at all.
Insects, which in their several changes belong to several of the before-mentioned divisions, may be considered together as one great tribe of animals. They are called insects, from a separation in the middle of their bodies, whereby they are, as it were, cut into two parts, which are joined together by a small ligature; as we see in wasps, common flies, and the like.
Besides all these, there are some animals that are not perfectly of these kinds, but placed, as it were, in the middle betwixt two of them, by something of both; as bats, which have something of beasts and birds in them.
Some reptiles of the earth, and some of aquatics, want one or more of the senses, which are in perfecter animals; as worms, oysters, cockles, &c.
Animals are nourished by food, taken in at the mouth, digested in the stomach, and thence by fit vessels distributed over the whole body, as is described in books of anatomy.
The greatest part of animals have five senses, viz. seeing, hearing, smelling, tasting, and feeling. These, and the way of nourishment of animals, we shall more particularly consider; because they are common to man with beasts.
The way of nourishment of animals, particularly of man, is by food taken in at the mouth, which being chewed there, is broken and mixed with the saliva, and thereby prepared for an easier and better digestion in the stomach.
When the stomach has performed its office upon the food, it protrudes it into the guts, by whose peristaltic motion it is gently conveyed along through the guts, and, as it passes, the chyle, which is the nutritive part, is separated from the excrementitious, by the lacteal veins; and from thence conveyed into the blood, with which it circulates till itself be concocted into blood. The blood, being by the vena cava brought into the right ventricle of the heart, by the contraction of that muscle, is driven through the arteria pulmonaris into the lungs; where the constantly inspired air mixing with it, enlivens it; and from thence being conveyed by the vena pulmonaris into the left ventricle of the heart, the contraction of the heart forces it out, and, by the arteries, distributes it into all parts of the body; from whence it returns by the veins into the right ventricle of the heart, to take the same course again. This is called the circulation of the blood; by which life and heat are communicated to every part of the body.
In the circulation of the blood, a good part of it goes up into the head; and by the brains are separated from it, or made out of it, the animal spirits; which, by the nerves, impart sense and motion to all parts of the body.
The instruments of motion are the muscles; the fibres whereof contracting themselves, move the several parts of the body.
This contraction of the muscles is, in some of them, by the direction of the mind, and in some of them without it; which is the difference between voluntary and involuntary motions, in the body.
Of the Five Senses.
The organ of seeing is the eye; consisting of variety of parts wonderfully contrived, for the admitting and refracting the rays of light; so that those that come from the same point of the object, and fall upon different parts of the pupil, are brought to meet again at the bottom of the eye, whereby the whole object is painted on the retina that is spread there.
That which immediately affects the sight, and produces in us that sensation which we call seeing, is light.
Light may be considered either, first, as it radiates from luminous bodies directly to our eyes; and thus we see luminous bodies themselves, as the sun, or a flame, &c.; or secondly, as it is reflected from other bodies; and thus we see a man, or a picture, by the rays of light reflected from them to our eyes.
Bodies, in respect of light, may be divided into three sorts; first, those that emit rays of light, as the sun and fixt stars; secondly, those that transmit the rays of light, as the air; thirdly, those that reflect the rays of light, as iron, earth, &c. The first are called luminous; the second pellucid; and the third opake.
The rays of light themselves are not seen; but by them the bodies, from which they originally come; as the sun, or a fixt star; or the bodies, from which they are reflected; as a horse, or a tulip. When the moon shines, we do not see the rays which come from the sun to the moon, but by them we see the moon, from whence they are reflected.
If the eye be placed in the medium, through which the rays pass to it, the medium is not seen at all; for instance, we do not see the air through which the rays come to our eyes. But if a pellucid body, through which the light comes, be at a distance from our eye, we see that body, as well as the bodies, from whence the rays come that pass through them to come to our eyes. For instance, we do not only see bodies through a pair of spectacles, but we see the glass itself. The reason whereof is, that pellucid bodies being bodies, the surfaces of which reflect some rays of light from their solid parts; these surfaces, placed at a convenient distance from the eye, may be seen by those reflected rays; as, at the same time, other bodies beyond those pellucid ones may be seen by the transmitted rays.
Opake bodies are of two sorts, specular, or not specular. Specular bodies, or mirrours, are such opake bodies, whose surfaces are polished; whereby they, reflecting the rays in the same order as they come from other bodies, show us their images.
The rays that are reflected from opake bodies, always bring with them to the eye the idea of colour; and this colour is nothing else, in the bodies, but a disposition to reflect to the eye more copiously one sort of rays than another. For particular rays are originally endowed with particular colours; some are red, others blue, others yellow, and others green, &c.
Every ray of light, as it comes from the sun, seems a bundle of all these several sorts of rays; and as some of them are more refrangible than others; that is, are more turned out of their course, in passing from one medium to another; it follows, that after such refraction they will be separated, and their distinct colour observed. Of these, the most refrangible are violet, and the least red; and the intermediate ones, in order, are indigo, blue, green, yellow, and orange. This separation is very entertaining, and will be observed with pleasure in holding a prism in the beams of the sun.
As all these rays differ in refrangibility, so they do in reflexibility; that is, in the property of being more easily reflected from certain bodies, than from others; and hence arise, as hath been said, all the colours of bodies; which are, in a manner, infinite, as an infinite number of compositions and proportions, of the original colours, may be imagined.
The whiteness of the sun’s light is compounded of all the original colours, mixed in a due proportion.
Whiteness, in bodies, is but a disposition to reflect all colours of light, nearly in the proportion they are mixed in the original rays; as, on the contrary, blackness is only a disposition to absorb or stifle, without reflection, most of the rays of every sort that fall on the bodies.
Light is successively propagated with an almost inconceivable swiftness; for it comes from the sun, to this our earth, in about seven or eight minutes of time, which distance is about 80,000,000 English miles.
Besides colour, we are supposed to see figure, but, in truth, that which we perceive when we see figure, as perceivable by sight, is nothing but the termination of colour.
Next to seeing, hearing is the most extensive of our senses. The ear is the organ of hearing, whose curious structure is to be learnt from anatomy.
That which is conveyed into the brain by the ear is called sound; though, in truth, till it come to reach and affect the perceptive part, it be nothing but motion.
The motion, which produces in us the perception of sound, is a vibration of the air, caused by an exceeding short, but quick, tremulous motion of the body, from which it is propagated; and therefore we consider and denominate them as bodies sounding.
That sound is the effect of such a short, brisk, vibrating motion of bodies, from which it is propagated, may be known from what is observed and felt in the strings of instruments, and the trembling of bells, as long as we perceive any sound come from them; for as soon as that vibration is stopt, or ceases in them, the perception ceases also.
The propagation of sound is very quick, but not approaching that of light. Sounds move about 1140 English feet in a second of time; and in seven or eight minutes of time, they move about one hundred English miles.
Smelling is another sense, that seems to be wrought on by bodies at a distance; though that which immediately affects the organ, and produces in us the sensation of any smell, are effluvia, or invisible particles, that coming from bodies at a distance, immediately affect the olfactory nerves.
Smelling bodies seem perpetually to send forth effluvia, or steams, without sensibly wasting at all. Thus a grain of musk will send forth odoriferous particles for scores of years together, without its being spent; whereby one would conclude that these particles are very small; and yet it is plain, that they are much grosser than the rays of light, which have a free passage through glass; and grosser also than the magnetic effluvia, which pass freely through all bodies, when those that produce smell will not pass through the thin membranes of a bladder, and many of them scarce ordinary white paper.
There is a great variety of smells, though we have but a few names for them; sweet, stinking, sour, rank, and musty, are almost all the denominations we have for odours; though the smell of a violet, and of musk, both called sweet, are as distinct as any two smells whatsoever.
Taste is the next sense to be considered.
The organ of taste is the tongue and palate.
Bodies that emit light, sounds, and smells, are seen, heard, and smelt at a distance; but bodies are not tasted, but by immediate application to the organ; for till our meat touch our tongues, or palates, we taste it not, how near soever it be.
It may be observed of tastes, that though there be a great variety of them, yet, as in smells, they have only some few general names; as sweet, bitter, sour, harsh, rank, and some few others.
The fifth and last of our senses is touch; a sense spread over the whole body, though it be most eminently placed in the ends of the fingers.
By this sense the tangible qualities of bodies are discerned; as hard, soft, smooth, rough, dry, wet, clammy, and the like.
But the most considerable of the qualities, that are perceived by this sense, are heat and cold.
The due temperament of those two opposite qualities, is the great instrument of nature, that she makes use of in most, if not all, her productions.
Heat is a very brisk agitation of the insensible parts of the object, which produces in us that sensation, from whence we denominate the object hot; so what in our sensation is heat, in the object is nothing but motion. This appears by the way whereby heat is produced; for we see that the rubbing of a brass nail upon a board will make it very hot, and the axle-trees of carts and coaches are often hot, and sometimes to a degree, that it sets them on fire, by the rubbing of the nave of the wheel upon it.
On the other side, the utmost degree of cold is the cessation of that motion of the insensible particles, which to our touch is heat.
Bodies are denominated hot and cold in proportion to the present temperament of that part of our body to which they are applied; so that feels hot to one, which seems cold to another; nay, the same body, felt by the two hands of the same man, may at the same time appear hot to the one, and cold to the other: because the motion of the insensible particles of it may be more brisk than that of the particles of the other.
Besides the objects before-mentioned, which are peculiar to each of our senses, as light, and colour of the sight; sound of hearing; odours of smelling; savours of tasting; and tangible qualities of the touch; there are two others that are common to all the senses; and those are pleasure and pain, which they may receive by and with their peculiar objects. Thus, too much light offends the eye; some sounds delight, and others grate the ear; heat in a certain degree is very pleasant, which may be augmented to the greatest torment; and so the rest.
These five senses are common to beasts with men; nay, in some of them, some brutes exceed mankind. But men are endowed with other faculties, which far excel any thing that is to be found in the other animals in this our globe.
Memory also brutes may be supposed to have, as well as men.
Of the Understanding of Man.
The understanding of man does so surpass that of brutes, that some are of opinion brutes are mere machines, without any manner of perception at all. But letting this opinion alone, as ill-grounded, we will proceed to the consideration of human understanding, and the distinct operations thereof.
The lowest degree of it consists in perception, which we have before in part taken notice of, in our discourse of the senses. Concerning which it may be convenient farther to observe, that, to conceive a right notion of perception, we must consider the distinct objects of it, which are simple ideas; v. g. such as are those signified by these words, scarlet, blue, sweet, bitter, heat, cold, &c. from the other objects of our senses; to which we may add the internal operations of our minds, as the objects of our own reflection, such as are thinking, willing, &c.
Out of these simple ideas are made, by putting them together, several compounded or complex ideas; as those signified by the words pebble, marygold, horse.
The next thing the understanding doth in its progress to knowledge, is to abstract its ideas, by which abstraction they are made general.
A general idea is an idea in the mind, considered there as separated from time and place; and so capable to represent any particular being that is conformable to it. Knowledge, which is the highest degree of the speculative faculties, consists in the perception of the truth of affirmative, or negative, propositions.
This perception is either immediate, or mediate. Immediate perception of the agreement, or disagreement, of two ideas, is when, by comparing them together in our minds, we see, or, as it were, behold, their agreement, or disagreement. This therefore is called intuitive knowledge. Thus we see that red is not green; that the whole is bigger than a part; and that two and two are equal to four.
The truth of these, and the like propositions, we know by a bare simple intuition of the ideas themselves, without any more ado; and such propositions are called self-evident.
The mediate perception of the agreement, or disagreement, of two ideas, is when, by the intervention of one or more other ideas, their agreement, or disagreement, is shown. This is called demonstration, or rational knowledge. For instance: The inequality of the breadth of two windows, or two rivers, or any two bodies that cannot be put together, may be known by the intervention of the same measure, applied to them both; and so it is in our general ideas, whose agreement or disagreement may be often shown by the intervention of some other ideas, so as to produce demonstrative knowledge; where the ideas in question cannot be brought together, and immediately compared, so as to produce intuitive knowledge.
The understanding doth not know only certain truth; but also judges of probability, which consists in the likely agreement, or disagreement, of ideas.
The assenting to any proposition as probable is called opinion or belief.
We have hitherto considered the great and visible parts of the universe, and those great masses of matter, the stars, planets, and particularly this our earth, together with the inanimate parts, and animate inhabitants of it; it may be now fit to consider what these sensible bodies are made of, and that is of unconceivably small bodies, or atoms, out of whose various combinations bigger moleculæ are made: and so, by a greater and greater composition, bigger bodies; and out of these the whole material world is constituted.
By the figure, bulk, texture, and motion, of these small and insensible corpuscles, all the phænomena of bodies may be explained.
[a ]Christiani Huygenii ΚΟΣΜΟΘΕΩΡΟΣ, sive de terris cœlestibus earumque ornatu, conjecturæ, &c. p. m. 137.
[* ]New experiments Physico-mechanical, touching the spring of the air, and its effects; (made for the most part in a new pneumatical engine) written ..... by the honourable Robert Boyle, Esq.; experiment xxxvi. p. 155. Oxford, 1662, in 4to.