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CONVERSATIONS
ON
NATURAL PHILOSOPHY,
IN WHICH
THE ELEMENTS OF THAT SCIENCE
ARE FAMILIARLY EXPLAINED.
_Illustrated with Plates._
BY THE AUTHOR OF CONVERSATIONS ON CHEMISTRY, &c.
WITH CORRECTIONS, IMPROVEMENTS, AND CONSIDERABLE ADDITIONS
IN THE BODY OF THE WORK;
_Appropriate Questions, and a Glossary:_
BY DR. THOMAS P. JONES,
PROFESSOR OF MECHANICS, IN THE FRANKLIN INSTITUTE
OF THE STATE OF PENNSYLVANIA.
Philadelphia:
Published and Sold by John Grigg,
No. 9 North Fourth Street.
Stereotyped by L. Johnson.
1826.
_Eastern District of Pennsylvania, to wit:_
Be it remembered, that, on the twenty-fourth day of April, in the
Fiftieth year of the Independence of the United States of America, A. D.
1826, John Grigg, of the said District, hath deposited in this office
the title of a book, the right whereof he claims as proprietor, in the
words following, to wit:
"Conversations on Natural Philosophy, in which the Elements of
that Science are familiarly explained. Illustrated with Plates.
By the Author of Conversations on Chemistry, &c. With
Corrections, Improvements, and considerable Additions, in the
Body of the Work; appropriate Questions, and a Glossary: By Dr.
Thomas P. Jones, Professor of Mechanics, in the Franklin
Institute, of the State of Pennsylvania."
In conformity to the Act of the Congress of the United States, entitled
"An Act for the Encouragement of Learning, by securing the Copies of
Maps, Charts, and Books, to the Authors and Proprietors of such Copies,
during the times therein mentioned;"--And also to the Act, entitled, "An
Act supplementary to an Act, entitled, 'An Act for the Encouragement of
Learning, by securing the Copies of Maps, Charts, and Books, to the
Authors and Proprietors of such Copies during the times therein
mentioned,' and extending the benefits thereof to the arts of designing,
engraving, and etching, historical and other prints."
D. CALDWELL,
Clerk of the Eastern District of Pennsylvania.
PREFACE.
Notwithstanding the great number of books which are written, expressly
for the use of schools, and which embrace every subject on which
instruction is given, it is a lamentable fact, that the catalogue of
those which are well adapted to the intended purpose, is a very short
one. Almost all of them have been written, either by those who are
without experience as teachers, or by teachers, deficient in a competent
knowledge of the subjects, on which they treat. Every intelligent
person, who has devoted himself to the instruction of youth, must have
felt and deplored, the truth of these observations.
In most instances, the improvement of a work already in use, will be
more acceptable, than one of equal merit would be, which is entirely
new; the introduction of a book into schools, being always attended with
some difficulty.
The "Conversations on Chemistry," written by Mrs. Marcet, had obtained a
well-merited celebrity, and was very extensively adopted as a
school-book, before the publication of her "Conversations on Natural
Philosophy." This, also, has been much used for the same purpose; but,
the observation has been very general, among intelligent teachers, that,
in its execution, it is very inferior to the former work.
The editor of the edition now presented to the public, had undertaken to
add to the work, questions, for the examination of learners; and notes,
where he deemed them necessary. He soon found, however, that the latter
undertaking would be a very unpleasant one, as he must have pointed out
at the bottom of many of the pages, the defects and mistakes in the
text; whilst numerous modes of illustration, or forms of expression,
which his experience as a teacher, had convinced him would not be clear
to the learner, must, of necessity, have remained unaltered. He
therefore determined to revise the whole work, and with the most perfect
freedom, to make such alterations in the body of it, as should, in his
opinion, best adapt it to the purpose for which it was designed. Were
the book, as it now stands, carefully compared with the original, it
would be found, that, in conformity with this determination, scarcely a
page of the latter, remains unchanged. Verbal alterations have been
made, errors, in points of fact, have been corrected; and new modes of
illustration have been introduced, whenever it was thought that those
already employed, could be improved; or when it was known, that, from
local causes, they are not familiar, in this country.
The editor feels assured, that, in performing this task, he has rendered
the book more valuable to the teacher, and more useful to the pupil; and
he doubts not that the intelligent author of it, would prefer the mode
which has been adopted, to that which was at first proposed.
The judicious teacher will, of course, vary the questions according to
circumstances; and those who may not employ them at all, as questions,
will still find them useful, in directing the pupil to the most
important points, in every page.
The Glossary has been confined to such terms of science as occur in the
work; and is believed to include all those, of which a clear definition
cannot be found in our common dictionaries.
CONTENTS.
CONVERSATION I.
ON GENERAL PROPERTIES OF BODIES. 9
INTRODUCTION. General Properties of Bodies. Impenetrability.
Extension. Figure. Divisibility. Inertia. Attraction.
Attraction of Cohesion. Density. Rarity. Heat. Attraction of
Gravitation.
CONVERSATION II.
ON THE ATTRACTION OF GRAVITY. 22
Attraction of Gravitation, continued. Of Weight. Of the Fall of
Bodies. Of the Resistance of the Air. Of the Ascent of Light
Bodies.
CONVERSATION III.
ON THE LAWS OF MOTION. 32
Of Motion. Of the Inertia of Bodies. Of Force to produce
Motion. Direction of Motion. Velocity, absolute and relative.
Uniform Motion. Retarded Motion. Accelerated Motion. Velocity
of Falling Bodies. Momentum. Action and Reaction equal.
Elasticity of Bodies. Porosity of Bodies. Reflected Motion.
Angles of Incidence and Reflection.
CONVERSATION IV.
ON COMPOUND MOTION. 46
Compound Motion, the result of two opposite forces. Of
Curvilinear Motion, the result of two forces. Centre of Motion,
the point at rest, while the other parts of the body move round
it. Centre of Magnitude, the middle of a body. Centripetal
Force, that which impels a body towards a fixed central point.
Centrifugal Force, that which impels a body to fly from the
centre. Fall of Bodies in a Parabola. Centre of Gravity, the
point about which the parts balance each other.
CONVERSATION V.
ON THE MECHANICAL POWERS. 54
Of the Power of Machines. Of the Lever in general. Of the Lever
of the first kind, having the Fulcrum between the power and the
weight. Of the Lever of the second kind, having the Weight
between the power and the fulcrum. Of the Lever of the third
kind, having the Power between the fulcrum and the weight. Of
the Pulley. Of the Wheel and Axle. Of the Inclined Plane. Of
the Wedge. Of the Screw.
CONVERSATION VI.
ASTRONOMY.
CAUSES OF THE MOTION OF THE HEAVENLY BODIES. 70
Of the Earth's annual motion. Of the Planets, and their motion.
Of the Diurnal motion of the Earth and Planets.
CONVERSATION VII.
ON THE PLANETS. 80
Of the Satellites and Moons. Gravity diminishes as the Square
of the Distance. Of the Solar System. Of Comets.
Constellations, signs of the Zodiac. Of Copernicus, Newton, &c.
CONVERSATION VIII.
ON THE EARTH. 91
Of the Terrestrial Globe. Of the Figure of the Earth. Of the
Pendulum. Of the Variation of the Seasons, and of the Length of
Days and Nights. Of the Causes of the Heat of Summer. Of Solar,
Siderial, and Equal or Mean Time.
CONVERSATION IX.
ON THE MOON. 108
Of the Moon's Motion. Phases of the Moon. Eclipses of the Moon.
Eclipses of Jupiter's Moons. Of Latitude and Longitude. Of the
Transits of the inferior Planets. Of the Tides.
CONVERSATION X.
HYDROSTATICS.
ON THE MECHANICAL PROPERTIES OF FLUIDS. 118
Definition of a Fluid. Distinction between Fluids and Liquids.
Of Non-Elastic Fluids, scarcely susceptible of Compression. Of
the Cohesion of Fluids. Of their Gravitation. Of their
Equilibrium. Of their Pressure. Of Specific Gravity. Of the
Specific Gravity of Bodies heavier than Water. Of those of the
same weight as Water. Of those lighter than Water. Of the
Specific Gravity of Fluids.
CONVERSATION XI.
OF SPRINGS, FOUNTAINS, &c. 128
Of the Ascent of Vapour and the Formation of Clouds. Of the
Formation and Fall of Rain, &c. Of the Formation of Springs. Of
Rivers and Lakes. Of Fountains.
CONVERSATION XII.
PNEUMATICS.
ON THE MECHANICAL PROPERTIES OF AIR. 136
Of the Spring or Elasticity of the Air. Of the Weight of the
Air. Experiments with the Air Pump. Of the Barometer. Mode of
Weighing Air. Specific Gravity of Air. Of Pumps. Description of
the Sucking Pump. Description of the Forcing Pump.
CONVERSATION XIII.
ON WIND AND SOUND. 146
Of Wind in General. Of the Trade Wind. Of the Periodical Trade
Winds. Of the Aerial Tides. Of Sound in General. Of Sonorous
Bodies. Of Musical Sounds. Of Concord or Harmony, and Melody.
CONVERSATION XIV.
ON OPTICS. 157
Of Luminous, Transparent, and Opaque Bodies. Of the Radiation
of Light. Of Shadows. Of the Reflection of Light. Opaque Bodies
seen only by Reflected Light. Vision Explained. Camera Obscura.
Image of Objects on the Retina.
CONVERSATION XV.
OPTICS--_continued._
OF THE ANGLE OF VISION, AND REFLECTION OF MIRRORS. 168
Angle of Vision. Reflection of Plain Mirrors. Reflection of
Convex Mirrors. Reflection of Concave Mirrors.
CONVERSATION XVI.
ON REFRACTION AND COLOURS. 179
Transmission of Light by Transparent Bodies. Refraction.
Refraction by the Atmosphere. Refraction by a Lens. Refraction
by the Prism. Of Colour from the Rays of Light. Of the Colours
of Bodies.
CONVERSATION XVII.
ON THE STRUCTURE OF THE EYE, AND OPTICAL INSTRUMENTS. 195
Description of the Eye. Of the Image on the Retina. Refraction
by the Humours of the Eye. Of the use of Spectacles. Of the
Single Microscope. Of the Double Microscope. Of the Solar
Microscope. Magic Lanthorn. Refracting Telescope. Reflecting
Telescope.
GLOSSARY, 205
CONVERSATION I.
ON GENERAL PROPERTIES OF BODIES.
INTRODUCTION. GENERAL PROPERTIES OF BODIES. IMPENETRABILITY. EXTENSION.
FIGURE. DIVISIBILITY. INERTIA. ATTRACTION. ATTRACTION OF COHESION.
DENSITY. RARITY. HEAT. ATTRACTION OF GRAVITATION.
EMILY.
I must request your assistance, my Dear Mrs. B., in a charge which I
have lately undertaken: it is that of instructing my youngest sister, a
task, which I find proves more difficult than I had at first imagined. I
can teach her the common routine of children's lessons tolerably well;
but she is such an inquisitive little creature, that she is not
satisfied without an explanation of every difficulty that occurs to her,
and frequently asks me questions which I am at a loss to answer. This
morning, for instance, when I had explained to her that the world was
round like a ball, instead of being flat as she had supposed, and that
it was surrounded by the air, she asked me what supported it. I told her
that it required no support; she then inquired why it did not fall as
every thing else did? This I confess perplexed me; for I had myself been
satisfied with learning that the world floated in the air, without
considering how unnatural it was that so heavy a body, bearing the
weight of all other things, should be able to support itself.
_Mrs. B._ I make no doubt, my dear, but that I shall be able to explain
this difficulty to you; but I believe that it would be almost impossible
to render it intelligible to the comprehension of so young a child as
your sister Sophia. You, who are now in your thirteenth year, may, I
think, with great propriety, learn not only the cause of this particular
fact, but acquire a general knowledge of the laws by which the natural
world is governed.
_Emily._ Of all things, it is what I should most like to learn; but I
was afraid it was too difficult a study even at my age.
_Mrs. B._ Not when familiarly explained: if you have patience to attend,
I will most willingly give you all the information in my power. You may
perhaps find the subject rather dry at first; but if I succeed in
explaining the laws of nature, so as to make you understand them, I am
sure that you will derive not only instruction, but great amusement from
that study.
_Emily._ I make no doubt of it, Mrs. B.; and pray begin by explaining
why the earth requires no support; for that is the point which just now
most strongly excites my curiosity.
_Mrs. B._ My dear Emily, if I am to attempt to give you a general idea
of the laws of nature, which is no less than to introduce you to a
knowledge of the science of natural philosophy, it will be necessary for
us to proceed with some degree of regularity. I do not wish to confine
you to the systematic order of a scientific treatise, but if we were
merely to examine every vague question that may chance to occur, our
progress would be but very slow. Let us, therefore, begin by taking a
short survey of the general properties of bodies, some of which must
necessarily be explained before I can attempt to make you understand why
the earth requires no support.
When I speak of _bodies_, I mean substances, of whatever nature, whether
solid or fluid; and _matter_ is the general term used to denote the
substance, whatever its nature be, of which the different bodies are
composed. Thus, the wood of which this table is made, the water with
which this glass is filled, and the air which we continually breathe,
are each of them _matter_.
_Emily._ I am very glad you have explained the meaning of the word
matter, as it has corrected an erroneous conception I had formed of it:
I thought that it was applicable to solid bodies only.
_Mrs. B._ There are certain properties which appear to be common to all
bodies, and are hence called the _essential or inherent properties_ of
bodies; these are _Impenetrability_, _Extension_, _Figure_,
_Divisibility_, _Inertia_ and _Attraction_. These are also called the
general properties of bodies, as we do not suppose any body to exist
without them.
By _impenetrability_ is meant the property which bodies have of
occupying a certain space, so that where one body is, another can not
be, without displacing the former; for two bodies can not exist in the
same place at the same time. A liquid may be more easily removed than a
solid body; yet it is not the less substantial, since it is as
impossible for a liquid and a solid to occupy the same space at the same
time, as for two solid bodies to do so. For instance, if you put a spoon
into a glass full of water, the water will flow over to make room for
the spoon.
_Emily._ I understand this perfectly. Liquids are in reality as
substantial or as impenetrable as solid bodies, and they appear less so,
only because they are more easily displaced.
_Mrs. B._ The air is a fluid differing in its nature from liquids, but
no less impenetrable. If I endeavour to fill this phial by plunging it
into this bason of water, the air, you see, rushes out of the phial in
bubbles, in order to make way for the water, for the air and the water
can not exist together in the same space, any more than two hard bodies;
and if I reverse this goblet, and plunge it perpendicularly into the
water, so that the air will not be able to escape, the water will no
longer be able to fill the goblet.
_Emily._ But it rises some way into the glass.
_Mrs. B._ Because the water compresses or squeezes the air into a
smaller space in the upper part of the glass; but, as long as it remains
there, no other body can occupy the same place.
_Emily._ A difficulty has just occurred to me, with regard to the
impenetrability of solid bodies; if a nail is driven into a piece of
wood, it penetrates it, and both the wood and the nail occupy the same
space that the wood alone did before?
_Mrs. B._ The nail penetrates between the particles of the wood, by
forcing them to make way for it; for you know that not a single atom of
wood can remain in the space which the nail occupies; and if the wood is
not increased in size by the addition of the nail, it is because wood is
a porous substance, like sponge, the particles of which may be
compressed or squeezed closer together; and it is thus that they make
way for the nail.
We may now proceed to the next general property of bodies, _extension_.
A body which occupies a certain space must necessarily have extension;
that is to say, _length_, _breadth_ and _depth_ or thickness; these are
called the dimensions of extension: can you form an idea of any body
without them?
_Emily._ No; certainly I can not; though these dimensions must, of
course vary extremely in different bodies. The length, breadth and depth
of a box, or of a thimble, are very different from those of a walking
stick, or of a hair.
But is not height also a dimension of extension?
_Mrs B._ Height and depth are the same dimension, considered in
different points of view; if you measure a body, or a space, from the
top to the bottom, you call it depth; if from the bottom upwards, you
call it height; thus the depth and height of a box are, in fact, the
same thing.
_Emily._ Very true; a moment's consideration would have enabled me to
discover that; and breadth and width are also the same dimension.
_Mrs. B._ Yes; the limits of extension constitute _figure_ or shape. You
conceive that a body having length, breadth and depth, can not be
without form, either symmetrical or irregular?
_Emily._ Undoubtedly; and this property admits of almost an infinite
variety.
_Mrs. B._ Nature has assigned regular forms to many of her productions.
The natural form of various mineral substances is that of crystals, of
which there is a great variety. Many of them are very beautiful, and no
less remarkable by their transparency or colour, than by the perfect
regularity of their forms, as may be seen in the various museums and
collections of natural history. The vegetable and animal creation
appears less symmetrical, but is still more diversified in figure than
the mineral kingdom. Manufactured substances assume the various
arbitrary forms which the art of man designs for them; and an infinite
number of irregular forms are produced by fractures and by the
dismemberment of the parts of bodies.
_Emily._ Such as a piece of broken china, or glass?
_Mrs. B._ Or the masses and fragments of stone, and other mineral
substances, which are dug out of the earth, or found upon its surface;
many of which, although composed of minute crystals, are in the lump of
an irregular form.
We may now proceed to _divisibility_; that is to say, a susceptibility
of being divided into an indefinite number of parts. Take any small
quantity of matter, a grain of sand for instance, and cut it into two
parts; these two parts might be again divided, had we instruments
sufficiently fine for the purpose; and if by means of pounding,
grinding, and other similar methods, we carry this division to the
greatest possible extent, and reduce the body to its finest imaginable
particles, yet not one of the particles will be destroyed, but will each
contain as many halves and quarters, as did the whole grain.
The dissolving of a solid body in a liquid, affords a very striking
example of the extreme divisibility of matter; when you sweeten a cup of
tea, for instance, with what minuteness the sugar must be divided to be
diffused throughout the whole of the liquid.
_Emily._ And if you pour a few drops of red wine into a glass of water,
they immediately tinge the whole of the water, and must therefore be
diffused throughout it.
_Mrs. B._ Exactly so; and the perfume of this lavender water will be
almost as instantaneously diffused throughout the room, if I take out
the stopper.
_Emily._ But in this case it is only the perfume of the lavender, and
not the water itself that is diffused in the room.
_Mrs. B._ The odour or smell of a body is part of the body itself, and
is produced by very minute particles or exhalations which escape from
the odoriferous bodies. It would be impossible that you should smell the
lavender water, if particles of it did not come in actual contact with
your nose.
_Emily._ But when I smell a flower, I see no vapour rise from it; and
yet I perceive the smell at a considerable distance.
_Mrs. B._ You could, I assure you, no more smell a flower, the
odoriferous particles of which did not touch your nose, than you could
taste a fruit, the flavoured particles of which did not come in contact
with your tongue.
_Emily._ That is wonderful indeed; the particles then, which exhale from
the flower and from the lavender water, are, I suppose, too small to be
visible?
_Mrs. B._ Certainly: you may form some idea of their extreme minuteness,
from the immense number which must have escaped in order to perfume the
whole room; and yet there is no sensible diminution of the liquid in the
phial.
_Emily._ But the quantity must really be diminished?
_Mrs. B._ Undoubtedly; and were you to leave the bottle open a
sufficient length of time, the whole of the water would evaporate and
disappear. But though so minutely subdivided as to be imperceptible to
any of our senses, each particle would continue to exist; for it is not
within the power of man to destroy a single particle of matter: nor is
there any reason to suppose that in nature an atom is ever annihilated.
_Emily._ Yet, when a body is burnt to ashes, part of it, at least,
appears to be effectually destroyed: look how small is the residue of
ashes in the fire place, from all the fuel which has been consumed
within it.
_Mrs. B._ That part of the fuel, which you suppose to be destroyed,
evaporates in the form of smoke, and vapour, and air, whilst the
remainder is reduced to ashes. A body, in burning, undergoes no doubt
very remarkable changes; it is generally subdivided; its form and colour
altered; its extension increased: but the various parts, into which it
has been separated by combustion, continue in existence, and retain all
the essential properties of bodies.
_Emily._ But that part of a burnt body which evaporates in smoke has no
figure; smoke, it is true, ascends in columns into the air, but it is
soon so much diffused as to lose all form; it becomes indeed invisible.
_Mrs. B._ Invisible, I allow; but we must not imagine that what we no
longer see no longer exists. Were every particle of matter that becomes
invisible annihilated, the world itself would in the course of time be
destroyed. The particles of smoke, when diffused in the air, continue
still to be particles of matter as well as when more closely united in
the form of coals: they are really as substantial in the one state as in
the other, and equally so when by their extreme subdivision they become
invisible. No particle of matter is ever destroyed: this is a principle
you must constantly remember. Every thing in nature decays and corrupts
in the lapse of time. We die, and our bodies moulder to dust; but not a
single atom of them is lost; they serve to nourish the earth, whence,
while living, they drew their support.
The next essential property of matter is called _inertia_ or inactivity;
this word expresses the resistance which matter makes to a change from a
state of rest, to that of motion, or from a state of motion to that of
rest. Bodies are equally incapable of changing their actual state,
whether it be of motion or of rest. You know that it requires force to
put a body which is at rest in motion; an exertion of strength is also
requisite to stop a body which is already in motion. The resistance of
the body to a change of state, in either case, arises from its
_inertia_.
_Emily._ In playing at base-ball I am obliged to use all my strength to
give a rapid motion to the ball; and when I have to catch it, I am sure
I feel the resistance it makes to being stopped. But if I did not catch
it, it would soon fall to the ground and stop of itself.
_Mrs. B._ Matter being inert it is as incapable of stopping of itself as
it is of putting itself into motion: when the ball ceases to move,
therefore, it must be stopped by some other cause or power; but as it is
one with which you are yet unacquainted, we can not at present
investigate its effects.
The last property which appears to be common to all bodies is
_attraction_. All bodies consist of infinitely small particles of
matter, each of which possesses the power of attracting or drawing
towards it, and uniting with any other particle sufficiently near to be
within the influence of its attraction; but in minute particles this
power extends to so very small a distance around them, that its effect
is not sensible, unless they are (or at least appear to be) in contact;
it then makes them stick or adhere together, and is hence called the
_attraction of cohesion_. Without this power, solid bodies would fall in
pieces, or rather crumble to atoms.
_Emily._ I am so much accustomed to see bodies firm and solid, that it
never occurred to me that any power was requisite to unite the particles
of which they are composed. But the attraction of cohesion does not, I
suppose, exist in liquids; for the particles of liquids do not remain
together so as to form a body, unless confined in a vessel?
_Mrs. B._ I beg your pardon; it is the attraction of cohesion which
holds this drop of water suspended at the end of my finger, and keeps
the minute watery particles of which it is composed united. But as this
power is stronger in proportion as the particles of bodies are more
closely united, the cohesive attraction of solid bodies is much greater
than that of fluids.
The thinner and lighter a fluid is, the less is the cohesive attraction
of its particles, because they are further apart; and in elastic fluids,
such as air, there is no cohesive attraction among the particles.
_Emily._ That is very fortunate; for it would be impossible to breathe
the air in a solid mass; or even in a liquid state.
But is the air a body of the same nature as other bodies?
_Mrs. B._ Undoubtedly, in all essential properties.
_Emily._ Yet you say that it does not possess one of the general
properties of bodies--attraction.
_Mrs. B._ The particles of air are not destitute of the power of
attraction, but they are too far distant from each other to be
influenced by it so as to produce cohesion: and the utmost efforts of
human art have proved ineffectual in the attempt to compress them, so as
to bring them within the sphere of each other's attraction, and make
them cohere.
_Emily._ If so, how is it possible to prove that they are endowed with
this power?
_Mrs. B._ The air is formed of particles precisely of the same nature as
those which enter into the composition of liquid and solid bodies, in
each of which we have a proof of their attraction.
_Emily._ It is then, I suppose, owing to the different degrees of
cohesive attraction in different substances, that they are hard or soft,
and that liquids are thick or thin.
_Mrs. B._ Yes; but you would express your meaning better by the term
_density_, which denotes the degree of closeness and compactness of the
particles of a body. In philosophical language, density is said to be
that property of bodies by which they contain a certain quantity of
matter, under a certain bulk or magnitude. _Rarity_ is the contrary of
density; it denotes the thinness and subtilty of bodies: thus you would
say that mercury or quicksilver was a very dense fluid; ether, a very
rare one. Those bodies which are the most dense, do not always cohere
the most strongly; lead is more dense than iron, yet its particles are
more easily separated.
_Caroline._ But how are we to judge of the quantity of matter contained
in a certain bulk?
_Mrs. B._ By the weight: under the same bulk bodies are said to be dense
in proportion as they are heavy.
_Emily._ Then we may say that metals are dense bodies, wood
comparatively a rare one, &c. But, Mrs. B., when the particles of a body
are so near as to attract each other, the effect of this power must
increase as they are brought by it closer together; so that one would
suppose that the body would gradually augment in density, till it was
impossible for its particles to be more closely united. Now, we know
that this is not the case; for soft bodies, such as cork, sponge, or
butter, never become, in consequence of the increasing attraction of
their particles, as hard as iron?
_Mrs. B._ In such bodies as cork and sponge, the particles which come in
contact are so few as to produce but a slight degree of cohesion: they
are porous bodies, which, owing to the peculiar arrangement of their
particles, abound with interstices, or pores, which separate the
particles. But there is also a fluid much more subtile than air, which
pervades all bodies, this is _heat_. Heat insinuates itself more or less
between the particles of all bodies, and forces them asunder; you may
therefore consider heat, and the attraction of cohesion, as constantly
acting in opposition to each other.
_Emily._ The one endeavouring to rend a body to pieces, the other to
keep its parts firmly united.
_Mrs. B._ And it is this struggle between the contending forces of heat
and attraction, which prevents the extreme degree of density which would
result from the sole influence of the attraction of cohesion.
_Emily._ The more a body is heated then, the more its particles will be
separated.
_Mrs. B._ Certainly: we find that bodies not only swell or dilate, but
lose their cohesion, by heat: this effect is very sensible in butter,
for instance, which expands by the application of heat, till at length
the attraction of cohesion is so far diminished that the particles
separate, and the butter becomes liquid. A similar effect is produced by
heat on metals, and all bodies susceptible of being melted. Liquids, you
know, are made to boil by the application of heat; the attraction of
cohesion then yields entirely to the repulsive power; the particles are
totally separated and converted into steam or vapour. But the agency of
heat is in no body more sensible than in air, which dilates and
contracts by its increase or diminution in a very remarkable degree.
_Emily._ The effects of heat appear to be one of the most interesting
parts of natural philosophy.
_Mrs. B._ That is true; but heat is so intimately connected with
chemistry, that you must allow me to defer the investigation of its
properties till you become acquainted with that science.
To return to its antagonist, the attraction of cohesion; it is this
power which restores to vapour its liquid form, which unites it into
drops when it falls to earth in a shower of rain, which gathers the dew
into brilliant gems on the blades of grass.
_Emily._ And I have often observed that after a shower, the water
collects into large drops on the leaves of plants; but I cannot say that
I perfectly understand how the attraction of cohesion produces this
effect.
_Mrs. B._ Rain, when it first leaves the clouds, is not in the form of
drops, but in that of mist or vapour, which is composed of very small
watery particles; these in their descent mutually attract each other,
and those that are sufficiently near in consequence unite and form a
drop, and thus the mist is transformed into a shower. The dew also was
originally in a state of vapour, but is, by the mutual attraction of the
particles, formed into small globules on the blades of grass: in a
similar manner the rain upon the leaf collects into large drops, which
when they become too heavy for the leaf to support, fall to the ground.
_Emily._ All this is wonderfully curious! I am almost bewildered with
surprise and admiration at the number of new ideas I have already
acquired.
_Mrs. B._ Every step that you advance in the pursuit of natural science,
will fill your mind with admiration and gratitude towards its Divine
Author. In the study of natural philosophy, we must consider ourselves
as reading the book of nature, in which the bountiful goodness and
wisdom of God are revealed to all mankind; no study can tend more to
purify the heart, and raise it to a religious contemplation of the
Divine perfections.
There is another curious effect of the attraction of cohesion which I
must point out to you; this is called capillary attraction. It enables
liquids to rise above their ordinary level in capillary tubes: these are
tubes, the bores of which are so extremely small that liquids ascend
within them, from the cohesive attraction between the particles of the
liquid and the interior surface of the tube. Do you perceive the water
rising in this small glass tube, above its level in the goblet of water,
into which I have put one end of it?
_Emily._ Oh yes; I see it slowly creeping up the tube, but now it is
stationary: will it rise no higher?
_Mrs. B._ No; because the cohesive attraction between the water and the
internal surface of the tube is now balanced by the weight of the water
within it; if the bore of the tube were narrower the water would rise
higher; and if you immerse several tubes of bores of different sizes,
you will see it rise to different heights in each of them. In making
this experiment, you should colour the water with a little red wine, in
order to render the effect more obvious.
All porous substances, such as sponge, bread, linen, &c. may be
considered as collections of capillary tubes: if you dip one end of a
lump of sugar into water, the fluid will rise in it, and wet it
considerably above the surface of the water into which you dip it.
_Emily._ In making tea I have often observed that effect, without being
able to account for it.
_Mrs. B._ Now that you are acquainted with the attraction of cohesion, I
must endeavour to explain to you that of _Gravitation_, which is
probably a modification of the same power; the first is perceptible only
in very minute particles, and at very small distances; the other acts on
the largest bodies, and extends to immense distances.
_Emily._ You astonish me: surely you do not mean to say that large
bodies attract each other?
_Mrs. B._ Indeed I do: let us take, for example, one of the largest
bodies in nature, and observe whether it does not attract other bodies.
What is it that occasions the fall of this book, when I no longer
support it?
_Emily._ Can it be the attraction of the earth? I thought that all
bodies had a natural tendency to fall.
_Mrs. B._ They have a natural tendency to fall, it is true; but that
tendency is produced entirely by the attraction of the earth: the earth
being so much larger than any body on its surface, forces every body,
which is not supported, to fall upon it.
_Emily._ If the tendency which bodies have to fall results from the
earth's attractive power, the earth itself can have no such tendency,
since it cannot attract itself, and therefore it requires no support to
prevent it from falling. Yet the idea that bodies do not fall of their
own accord, but that they are drawn towards the earth by its attraction,
is so new and strange to me, that I know not how to reconcile myself to
it.
_Mrs. B._ When you are accustomed to consider the fall of bodies as
depending on this cause, it will appear to you as natural, and surely
much more satisfactory, than if the cause of their tendency to fall were
totally unknown. Thus you understand that all matter is attractive, from
the smallest particle to the largest mass; and that bodies attract each
other with a force proportional to the quantity of matter they contain.
_Emily._ I do not perceive any difference between the attraction of
cohesion and that of gravitation; is it not because every particle of
matter is endowed with an attractive power, that large bodies consisting
of a great number of particles, are so strongly attractive?
_Mrs. B._ True. There is, however, this difference between the
attraction of particles and that of masses, that the former takes place
only when the particles are contiguous, whilst the latter is exerted
when the masses are far from each other. The attraction of particles
frequently counteracts the attraction of gravitation. Of this you have
an instance in the attraction of capillary tubes, in which liquids
ascend by the attraction of cohesion, in opposition to that of gravity.
It is on this account that it is necessary that the bore of the tube
should be extremely small; for if the column of water within the tube is
not very minute, the attraction of cohesion would not be able either to
raise or support it in opposition to its gravity; because the increase
of weight, in a column of water of a given height, is much greater than
the increase in the attracting surface of the tube, when its size is
increased.
You may observe also, that all solid bodies are enabled by the force of
the cohesive attraction of their particles to resist that of gravity,
which would otherwise disunite them, and bring them to a level with the
ground, as it does in the case of a liquid, the cohesive attraction of
which is not sufficient to enable it to resist the power of gravity.
_Emily._ And some solid bodies appear to be of this nature, as sand, and
powder for instance: there is no attraction of cohesion between their
particles?
_Mrs. B._ Every grain of powder, or sand, is composed of a great number
of other more minute particles, firmly united by the attraction of
cohesion; but amongst the separate grains there is no sensible
attraction, because they are not in sufficiently close contact.
_Emily._ Yet they actually touch each other?
_Mrs. B._ The surface of bodies is in general so rough and uneven, that
when in apparent contact, they touch each other only by a few points.
Thus, when I lay this book upon the table, the binding of which appears
perfectly smooth, so few of the particles of its under surface come in
contact with the table, that no sensible degree of cohesive attraction
takes place; for you see that it does not stick or cohere to the table,
and I find no difficulty in lifting it off.
It is only when surfaces, perfectly flat and well polished, are placed
in contact, that the particles approach in sufficient number, and
closely enough, to produce a sensible degree of cohesive attraction.
Here are two plates of polished metal, I press their flat surfaces
together, having previously interposed a few drops of oil, to fill up
every little porous vacancy. Now try to separate them.
_Emily._ It requires an effort beyond my strength, though there are
handles for the purpose of pulling them asunder. Is the firm adhesion of
the two plates merely owing to the attraction of cohesion?
_Mrs. B._ There is no force more powerful, since it is by this that the
particles of the hardest bodies are held together. It would require a
weight of several pounds to separate these plates. In the present
example, however, much of the cohesive force is due to the attraction
subsisting between the metal and the oil which is interposed; as without
this, or some other fluid, the points of contact would still be
comparatively few, although we may have employed our utmost art, in
giving flat surfaces to the plates.
_Emily._ In making a kaleidoscope, I recollect that the two plates of
glass, which were to serve as mirrors, stuck so fast together, that I
imagined some of the gum I had been using had by chance been interposed
between them; but I am now convinced that it was their own natural
cohesive attraction which produced this effect.
_Mrs. B._ Very probably it was so; for plate-glass has an extremely
smooth, flat surface, admitting of the contact of a great number of
particles, when two plates are laid upon each other.
_Emily._ But, Mrs. B., the cohesive attraction of some substances is
much greater than that of others; thus glue, gum and paste, cohere with
singular tenacity.
_Mrs. B._ Bodies which differ in their natures in other respects, differ
also in their cohesive attraction; it is probable that there are no two
bodies, the particles of which attract each other with precisely the
same force.
There are some other modifications of attraction peculiar to certain
bodies; namely, that of magnetism, of electricity, and of affinity, or
chemical attraction; but we shall confine our attention merely to the
attraction of cohesion and of gravity; the examination of the latter we
shall resume at our next meeting.
Questions
1. (Pg. 10) What is intended by the term _bodies_?
2. (Pg. 10) Is the term _matter_, restricted to substances of a
particular kind?
3. (Pg. 10) Name those properties of bodies, which are called inherent.
4. (Pg. 10) What is meant by impenetrability?
5. (Pg. 10) Can a liquid be said to be impenetrable?
6. (Pg. 11) How can you prove that air is impenetrable?
7. (Pg. 11) If air is impenetrable, what causes the water to rise some
way into a goblet, if I plunge it into water with its mouth downward?
8. (Pg. 11) When I drive a nail into wood, do not both the iron and the
wood occupy the same space?
9. (Pg. 11) In how many directions, is a body said to have extension?
10. (Pg. 11) How do we distinguish the terms height and depth?
11. (Pg. 12) What constitutes the _figure_, or _form_ of a body?
12. (Pg. 12) What is said respecting the form of minerals?
13. (Pg. 12) What of the vegetable and animal creation?
14. (Pg. 12) What of artificial, and accidental forms?
15. (Pg. 12) What is meant by divisibility?
16. (Pg. 12) What examples can you give, to prove that the particles of
a body are minute in the extreme?
17. (Pg. 13) What produces the odour of bodies?
18. (Pg. 13) How do odours exemplify the minuteness of the particles of
matter?
19. (Pg. 13) Can matter be in any way annihilated?
20. (Pg. 13) What becomes of the fuel, which disappears in our fires?
21. (Pg. 14) How can that part which evaporates, be still said to
possess a substantial form?
22. (Pg. 14) What do we mean by _inertia_?
23. (Pg. 14) Give an example to prove that force is necessary, either to
give or to stop motion.
24. (Pg. 14) What general power do the particles of matter exert upon
other particles?
25. (Pg. 15) What is that species of attraction called, which keeps
bodies in a solid state?
26. (Pg. 15) Does the attraction of cohesion exist in liquids, and how
is its existence proved?
27. (Pg. 15) If the particles of air attract each other, why do they not
cohere?
28. (Pg. 15) From what then do you infer that they possess attraction?
29. (Pg. 15) How do you account for some bodies being hard and others
soft?
30. (Pg. 16) What is meant by the term _density_?
31. (Pg. 16) Do the most dense bodies always cohere the most strongly?
32. (Pg. 16) How do we know that one body is more dense than another?
33. (Pg. 16) What is there which acts in opposition to cohesive
attraction, tending to separate the particles of bodies?
34. (Pg. 17) What would be the consequence if the repulsive power of
heat were not exerted?
35. (Pg. 17) If we continue to increase the heat, what effects will it
produce on bodies?
36. (Pg. 17) What body has its dimensions most sensibly affected by
change of temperature?
37. (Pg. 17) What power restores vapours to the liquid form?
38. (Pg. 17) What examples can you give?
39. (Pg. 17) How are drops of rain and of dew said to be formed?
40. (Pg. 18) What is meant by a capillary tube?
41. (Pg. 18) What effect does attraction produce when these are immersed
in water?
42. (Pg. 18) What is the reason that the water rises to a certain height
only?
43. (Pg. 18) Give some familiar examples of capillary attraction.
44. (Pg. 18) In what does _gravitation_ differ from cohesive attraction?
45. (Pg. 18) What causes bodies near the earth's surface, to have a
tendency to fall towards it?
46. (Pg. 19) What remarkable difference is there between the attraction
of gravitation, and that of cohesion?
47. (Pg. 19) In what instances does the power of cohesion counteract
that of gravitation?
48. (Pg. 19) Why will water rise to a less height, if the size of the
tube is increased?
49. (Pg. 20) Why do not two bodies cohere, when laid upon each other?
50. (Pg. 20) Can two bodies be made sufficiently flat to cohere with
considerable force?
51. (Pg. 20) What is the reason that the adhesion is greater when oil is
interposed?
52. (Pg. 21) What other modifications of attraction are there, besides
those of cohesion and of gravitation?
CONVERSATION II.
ON THE ATTRACTION OF GRAVITY.
ATTRACTION OF GRAVITATION, CONTINUED. OF WEIGHT. OF THE FALL OF BODIES.
OF THE RESISTANCE OF THE AIR. OF THE ASCENT OF LIGHT BODIES.
EMILY.
I have related to my sister Caroline all that you have taught me of
natural philosophy, and she has been so much delighted by it, that she
hopes you will have the goodness to admit her to your lessons.
_Mrs. B._ Very willingly; but I did not think you had any taste for
studies of this nature, Caroline.
_Caroline._ I confess, Mrs. B., that hitherto I had formed no very
agreeable idea either of philosophy, or philosophers; but what Emily has
told me has excited my curiosity so much, that I shall be highly pleased
if you will allow me to become one of your pupils.
_Mrs. B._ I fear that I shall not find you so tractable a scholar as
Emily; I know that you are much biased in favour of your own opinions.
_Caroline._ Then you will have the greater merit in reforming them, Mrs.
B.; and after all the wonders that Emily has related to me, I think I
stand but little chance against you and your attractions.
_Mrs. B._ You will, I doubt not, advance a number of objections; but
these I shall willingly admit, as they will afford an opportunity of
elucidating the subject. Emily, do you recollect the names of the
general properties of bodies?
_Emily._ Impenetrability, extension, figure, divisibility, inertia and
attraction.
_Mrs. B._ Very well. You must remember that these are properties common
to all bodies, and of which they cannot be deprived; all other
properties of bodies are called accidental, because they depend on the
relation or connexion of one body to another.
_Caroline._ Yet surely, Mrs. B., there are other properties which are
essential to bodies, besides those you have enumerated. Colour and
weight, for instance, are common to all bodies, and do not arise from
their connexion with each other, but exist in the bodies themselves;
these, therefore, cannot be accidental qualities?
_Mrs. B._ I beg your pardon; these properties do not exist in bodies
independently of their connexion with other bodies.
_Caroline._ What! have bodies no weight? Does not this table weigh
heavier than this book; and, if one thing weighs heavier than another,
must there not be such a thing as weight?
_Mrs. B._ No doubt: but this property does not appear to be essential to
bodies; it depends upon their connexion with each other. Weight is an
effect of the power of attraction, without which the table and the book
would have no weight whatever.
_Emily._ I think I understand you; it is the attraction of gravity which
makes bodies heavy.
_Mrs. B._ You are right. I told you that the attraction of gravity was
proportioned to the quantity of matter which bodies contain: now the
earth consisting of a much greater quantity of matter than any body upon
its surface, the force of its attraction must necessarily be greatest,
and must draw every thing so situated towards it; in consequence of
which, bodies that are unsupported fall to the ground, whilst those that
are supported, press upon the object which prevents their fall, with a
weight equal to the force with which they gravitate towards the earth.
_Caroline._ The same cause then which occasions the fall of bodies,
produces their weight also. It was very dull in me not to understand
this before, as it is the natural and necessary consequence of
attraction; but the idea that bodies were not really heavy of
themselves, appeared to me quite incomprehensible. But, Mrs. B., if
attraction is a property essential to matter, weight must be so
likewise; for how can one exist without the other?
_Mrs. B._ Suppose there were but one body existing in universal space,
what would its weight be?
_Caroline._ That would depend upon its size; or more accurately
speaking, upon the quantity of matter it contained.