ATTRACTION.
53. =Nature of Attraction.=--If you attempt to break a very
tenacious solid substance, why do you not succeed? It is because
the particles are so strongly fastened together. But how? By some
kind of cement or glue, or by some mechanical contrivances as nails
or hooks? No. They are fastened together by some unseen force. We
know nothing of the nature of this force. We know only that it
exists, and we call it attraction. The name is a proper one, for
it simply expresses the fact that one particle attracts or draws
another particle toward itself.
54. =Newton's Idea of Attraction.=--It was stated in § 20 that the
particles of matter, even in the densest substances, are not in
actual contact, but have spaces around them. Now it was supposed by
Newton that there is some kind of ethereal substance pervading all
these spaces, which causes this attraction between the particles.
He supposed also that this ether was every where in space,
causing attraction between masses of matter. But all this is mere
supposition, and we know not whether there is this sort of ethereal
glue keeping the universe together, or whether it is some property
in the particles themselves that makes them thus attract each
other. But the fact of the attraction we know, and we can observe
the phenomena which it produces, and discover the laws or rules by
which this force is regulated in its action.
55. =Attraction in Solids.=--Attraction is stronger in some solids
than in others. The mason with his trowel easily divides a brick;
but he can not do this with a piece of granite, for its particles
have a greater attraction for each other than those of the brick.
So a rap which would break a glass dish would not injure a copper
one of the same thickness. A weight that would hang securely from
an iron wire would break a lead wire of the same size; that is,
it would tear the particles apart, because they are not strongly
attracted to each other. Attraction has different modes of action
in different solids. It therefore fastens their particles together
in different ways, and thus produces all the various qualities,
already noticed, which are so useful to us--tenacity, hardness,
softness, ductility, flexibility, etc.
56. =Attraction in Liquids.=--In a liquid the attraction between
the particles is very feeble compared with that in solids. The
attraction of the particles of steel is in strength about three
million times that of the particles of water. We make the estimate
in this way: We find that a steel wire will sustain a weight
equal to 39,000 feet of the wire. But a drop of water hanging to
the end of a stick can not be more than one-sixth of an inch in
length; that is, water will hold together by the attraction of its
particles only to this extent, which is a little less than the
three millionth part of the length of steel wire which could hang
without breaking.
57. =Freeness of Movement of the Particles of Liquids.=--There
is one prominent characteristic of liquids which is probably not
entirely owing to the feeble attraction of their particles--I mean
the freeness with which these particles are moved among each other.
This is owing probably in part to some peculiar arrangement of the
atoms in making the particles of a liquid. I will illustrate this
in a coarse way. If the atoms of lead in shot were so arranged as
to make irregular jagged forms, they could not readily be moved
among each other. We suppose the ultimate atoms of a liquid to be
so arranged in the formation of particles as to make them not only
round but very smooth. Hence comes the great ease with which they
circulate among each other.
[Illustration: Fig. 9.]
[Illustration: Fig. 10.]
58. =Globular Shape of Drops of Liquids.=--As the particles of a
liquid move thus freely among each other, their attraction disposes
them to assume a globular or round shape. The reason of this can be
made plain by Figs. 9 and 10. The outside of a perfect sphere is
all at the same distance from the centre. So all the circumference
of a circle is at the same distance from the centre, as represented
in Fig. 9. But this is not true of all parts of the surface of
a cube or of a square: _a_, for example, is farther from the
centre than _b_ is. Now in a drop of liquid all the particles are
attracted toward the centre, for in that line from each particle
lies the largest number of particles to attract it. This can be
made obvious by taking some point in the drop, as represented in
Fig. 10, and drawing lines from it through the centre and in other
directions. If _a_ be the point in the drop, it is plain that the
line from it through the centre is longer than _a b_ or _a c_.
Therefore a particle, _a_, will be attracted toward the centre
rather than in the direction _a b_ or _a c_, because there are more
particles in the direction of the centre, and the more particles
there are the stronger is the attraction. But this is not all. The
particles in the line _a c_, tending to make _a_ go toward _c_, are
balanced by the particles in the line _a e_, tending to make it go
toward _e_. The two lines of particles therefore together tend
to make it go in a middle line between them, that is, toward the
centre, just as two strings pulling equally, the one to _c_ and the
other to _e_, would make a body, _a_, move in a middle line between
these two directions. The same can be shown of the two lines
of particles _a b_ and _a d_, and so of any other two alike in
situation on each side of the line through the centre. The tendency
of every particle is, then, to go toward the centre, and it would
go there if there were not particles between to prevent it. You see
how this would operate in the case of the particles on the surface
of the drop. As these are all striving, as we may say, in obedience
to attraction, to get to the centre, none of them will be raised up
into an angle or a point, as would be the case if the drop were in
the shape of a cube. If this should be done it would show that some
of the particles were not as strongly attracted toward the centre
as others are, which is an impossibility.
59. =The Globular Form in Different Liquids.=--The disposition
to form a sphere is seen more distinctly in mercury than in any
other liquid. If you drop a little of it upon a plate it separates
into globules, which roll about like shot. Why can not the same
thing be done with water? Why do the drops of water hang upon the
window-pane, showing only in an imperfect way their disposition
to the globular arrangement? It is because the particles of water
have a greater attraction for other substances, and less attraction
for each other, than the particles of the quicksilver have. Water
sometimes exhibits its disposition to the globular form in full on
the leaves of some plants, and rolls about in balls like mercury.
This is because there is something on the surface of the leaf which
repels rather than attracts the water. If you put your finger,
however, on one of these drops, it will spoil it, and your finger
will be moistened, because there is an attraction between the
particles of your skin and of the water. Take another illustration
of this difference in attraction. If you drop a little oil upon
the surface of water it will float about in round drops. This is
because the water repels the oil, as the surface of some kinds of
leaves does water. But when oil is spilled upon wood or cloth its
particles have so strong an attraction for their particles that
they unite with them, instead of gathering up into little round
companies as they do on the surface of water.
60. =Manufacture of Shot.=--We have a beautiful example of the
tendency of fluids to the globular arrangement in the manufacture
of shot. The melted lead is poured into a large vessel in the top
of the shot-tower. This vessel has holes in its bottom, from which
the metal falls in drops. Each drop, as it whirls round and round
in its fall, takes the globular form. By the time that it reaches
the end of its journey, about two hundred feet, it becomes so far
cooled as to be solid, and as it is received in a reservoir of
water, its globular form is retained. Bullets can not be made in
this way, because a quantity of melted lead sufficient to make a
bullet will not hold together in a globular form.
61. =Globular Form of the Earth and the Heavenly Bodies.=--It is
supposed that the sun, moon, earth, and all the heavenly bodies
were once in a liquid state, and that they owe their globular
shape to this fact. As they whirled on in this condition in their
course, the different solids were gradually formed, and at length
they acquired their present state. How all the mighty changes could
be effected in our earth, converging it from a liquid into a body
with a solid crust, having such various substances in it, and so
variously arranged, with its depressions containing water, and the
whole covered with its robe of air fifty miles in thickness, we
can not understand. And yet there are some portions of the process
which chemistry and geology have revealed to us, giving us some
glimpses of the wonders which, during the lapse of ages, God
wrought in our earth in preparing it for the habitation of man.
[Illustration: Fig. 11. Fig. 12.]
62. =Crystallization.=--The arrangement of the particles of solid
substances is different from that of liquids. The tendency here
is to straight lines and angles; that is, to crystalline forms.
Alum or common salt, when it becomes solid from a solution, forms
crystals. So also does sugar. The crystals of different substances
are different. In Fig. 11 you have the crystal of common salt, and
in Fig. 12 that of alum. We see this crystalline tendency every
where, even in the rude rocks and common stones. The rocks are
disposed to exhibit regular layers, or columns, or battlements, and
always do so except when interfering circumstances prevent. And
when you examine their composition, or that of the stone under your
feet, you see the same crystalline disposition in detail that you
see in the mass.
63. =Crystallization of Water.=--Water, when it changes into a
solid, shows the same disposition, of which the crystals of the
snow and the frost-work on our windows are familiar examples.
When snow forms, the water of the clouds is suddenly crystallized
by the cold air, the particles taking their regular places more
readily and certainly than if they were guided by intelligence,
because in obedience to an unerring law established by the Creator.
We sometimes have an example of this sudden crystallization of
water under our eye. The water in a pitcher may remain fluid,
although it is cooled down to the freezing point, and even below
it, if it be kept perfectly still. But on taking up the pitcher
the water at once becomes filled with a net-work of ice-crystals.
The explanation is this: The stillness of the water has prevented
its particles from taking on the new arrangement needed for the
formation of ice; but the jostling of them in taking up the pitcher
has served to make them do it thus suddenly.
[Illustration: Fig. 13.]
64. =Frost and Snow.=--The frost-work on our windows is a wonderful
exhibition of the variety of forms that crystallization can
produce. It sometimes presents figures like leaves and flowers,
such as we see chased on vessels of silver, but much more delicate
and beautiful. So varied and fantastic are the forms in which these
water-crystals are arranged, that it is very natural to ascribe
them, as is done universally in the dialect of the nursery, to the
ingenuity of a strange and tricksy spirit. Every snow-flake is a
bundle of little crystals as regular and beautiful as the crystals
which you so much admire in a mineralogical cabinet. And there is
great variety in the grouping of these crystals. You have some
specimens of these groups in Fig. 13 as they appear on examining
them with the microscope. Over six hundred different forms have
been enumerated, and a hundred have been delineated. It is a very
quick operation by which the particles of water in the clouds thus
marshal themselves, as if by magic, in these regular forms. But a
quicker operation is that by which hail is formed--so quick that
the particles have not time to set themselves in the crystalline
arrangement, but are huddled together without order. The brilliant
and glistening whiteness of the snow is owing to the reflection of
light from its minute crystals. In the arctic regions the beauty
of the snow is often much greater than with us. "The snow crystals
of last night," says Captain M'Clintock in his "Discovery of the
Fate of Sir John Franklin," "were extremely beautiful. The largest
kind is an inch in length; its form exactly resembles the end
of a pointed feather. Stellar crystals two-tenths of an inch in
diameter have also fallen; these have six points, and are the most
exquisite things when seen under a microscope. In the sun, or even
in moonlight, all these crystals glisten most brilliantly; and as
our masts and rigging are abundantly covered with them, the _Fox_
never was so gorgeously arrayed as she now appears."
65. =Order in Nature.=--We see in this general tendency to
crystallization a striking illustration of the fact that God
is a God of order. Disorderly arrangement is never seen except
where there is an obvious necessity for it. And even when there
is apparent disorder, a little examination generally shows that
essentially there is order. The rocks that give so much variety
to scenery are not piled up in confusion, and order has evidently
reigned in their construction. Pick up a common stone, and on
breaking it you will see the crystalline arrangement in its
interior. Nay, more, much of the very soil is made up of separated
and broken crystals.
[Illustration: Fig. 14.]
66. =Particles Must be very Near to Each Other to Adhere.=--Why
is it that when you have broken any thing made of glass, however
accurately you may bring the two parts together, you can not make
them unite in one again? It is simply because the particles of
substances will not attract each other enough to be united unless
they are brought very near together. Now it is impossible to bring
the particles on the two surfaces of a broken piece of glass as
near together as they were before it was broken. If you could do
so, no crack would be visible. You can join them by some kind of
cement. This is because the particles of the cement, while it is
soft, can be insinuated among the particles of the glass; and thus,
when it dries, it becomes a bond of union between the particles
on each side of the breach. For the same reason you can make the
cut surfaces of some yielding substances adhere. If you divide
a piece of India-rubber with a clean cut, you can make the two
surfaces adhere by pressing them together firmly. The particles in
this case are not unyielding, as those of the glass are, and some
of them are therefore brought into such near neighborhood as to
attract each other sufficiently to unite together. So, too, if you
cut two bullets so as to have a very smooth flat surface on each,
you can make them adhere quite strongly by pressing them together,
especially if you give a little turning motion at the same time
that you press, for this will cause the particles on the two
surfaces to be somewhat mingled together. If you have quite large
balls of lead with handles, as represented in Fig. 14, it will
require considerable force to separate them when they have been
thus pressed together.
67. =Other Illustrations.=--Silver and gold may be made to adhere
to iron by a very great and sudden pressure. The iron must be made
very smooth, and the silver or gold plate very thin. A powerful
blow brings the particles of the thin plate into such nearness to
those of the iron that union is effected, or, in other words, that
they attract each other sufficiently to be united. So, also, a
sheet of tin and one of lead can be made to adhere so as to form
one sheet by the pressure of the rollers of a flatting-mill. Two
very smooth panes of glass laid one upon another may have so many
particles brought into great nearness as to occasion some adhesion.
It will be slight, however, few comparatively of all the particles
coming near enough to adhere, for the smoothest glass is full of
inequalities, as may be seen by the microscope.
68. =Strength of Adhesion.=--In no case do particles come in actual
contact (§ 20), and their adhesion depends on the nearness of their
neighborhood to each other. The strength of union, then, between
two surfaces depends on the number of particles brought near enough
to adhere together. In the case of the two bullets or the lead
balls, if all the particles of the two surfaces were near enough
to adhere, the lead would be just as strong at the junction as any
where else. The reason that so strong adhesion takes place between
portions of some substances when we soften them by heat is that
the particles of the two softened ends are all brought near enough
together for adhesion. Thus the two ends of a broken stick of
sealing-wax may be firmly united by heating them and then pressing
them together. The same thing can be done with glass. When iron is
welded, as it is termed, some hammering is required to make the
particles of the two softened ends of the iron unite.
[Illustration: Fig. 15.]
69. =Attraction Between Solids and Liquids.=--The attraction which
solids and liquids have for each other furnishes us with many
interesting phenomena. The adhesion of drops of water to glass
and other solids is a familiar example of this attraction. If you
dip your hand into water, it is wet on taking it out, because
your skin has sufficient attraction for the water to retain some
of it. A towel will retain more of it for two reasons--with the
interstices between its fibres it presents much more surface to
the water, and it has none of the oily substance which on your
skin, though being in small quantity, serves somewhat to repel the
water. The attraction of solids and fluids for each other is shown
very prettily in the experiment represented in Fig. 15 (p. 47).
A piece of wood is attached by the string, _a_, to one end of a
balance, and weights just sufficient to balance it are placed in
the opposite scale. If now the wood is brought in contact with the
water in the vessel, _b_, it will require additional weight in the
scale to separate the wood from the water.
[Illustration: Fig. 16. Fig. 17.]
[Illustration: Fig. 18.]
[Illustration: Fig. 19.]
70. =Farther Illustrations.=--When you see stems of plants rising
above the surface of stagnant water you will observe that the water
is considerably raised about them. This is from the attraction
between them and the water. For the same reason water is not as
high in the middle of a tumbler as it is at the sides. If you
immerse a piece of glass in water, the water will rise at its sides
as represented in Fig. 16. If you immerse two together, as in
Fig. 17, the water will rise higher between than outside of them,
because the particles between are attracted by two surfaces, while
those outside are attracted only by one. It is for the same reason
that two men can raise a weight higher than one of them can alone.
And if the pieces of glass be brought quite near together, as in
Fig. 18, the water will be raised higher still, because there is
less to be raised by the two surfaces. It is just as two men can
raise a small weight higher than they can a large one. The same
thing may be beautifully illustrated in this way: Let two pieces
of glass, as represented in Fig. 19, be immersed in colored water,
with two of their edges joined together at A B, the opposite edges
at E D C being separated. The height to which the fluid rises will
make a curved line, A F C, it being lowest at the edges which are
separated, and highest at the edges which are joined together.
[Illustration: Fig. 20.]
[Illustration: Fig. 21.]
71. =Rise of Liquids in Tubes.=--For the same reason that water
rises higher between plates of glass than outside, so it will rise
higher in a tube than it will outside of it. The diagram in Fig. 20
will make this clear. I represent in this a transverse section of
a tube, enlarged so that the demonstration may be plain. We will
take a particle on the inside and the outside at equal distances
from the glass. It is clear that the particle _a_ is not as near to
as many particles of the glass as the particle _b_ is. The lines
drawn show this. The longest lines extending from the particles _a_
and _b_ to the glass are equal in length; that is, _a e_ and _a f_
are equal to _b g_ and _b h_. It is clear, therefore, that all the
glass between the lines at _c_ and _d_ is as near to the particle
_b_ as the glass between the lines at e and _f_ is to the particle
_a_. But this is not all. The particle _b_ is near enough to all
the inside of the tube to be attracted by it, while very little
attraction is exerted upon _a_ by any part of the glass beyond
that which is included between _e_ and _f_. The same difference
can be shown with regard to all the particles on the inside of the
tube compared with those outside. The former are nearer to more
particles of the glass than the latter, and therefore are more
strongly attracted. Again, as the nearer the plates of glass are
(§ 70) the higher the water rises between them, so the smaller
the tube is the higher will the water rise in it. You can try
the experiment as represented in Fig. 21. It is obvious that the
particle _b_ (Fig. 20) would not be very strongly attracted by the
part of the tube opposite if the tube were a large one; but it
would be if the tube were very small, for then it would be quite
near to that part.
72. =Capillary Attraction.=--The term capillary (derived from
the Latin word _capilla_, hair) has been commonly applied to the
attraction exhibited under the circumstances just noticed, because
it is most obvious and was first observed in tubes of very fine
bore. The same term is used when the attraction is seen in the
rising or spreading of a liquid in interstices as well as in tubes.
Thus capillary attraction causes the rising of oil or burning fluid
in the wicks of lamps. The liquid goes up in the interstices or
spaces between the fibres, as it does in the spaces of tubes. I
will give some other examples. If you let one end of a towel be
in a bowl of water, the other end lying over upon the table, the
whole towel will become wet from the spreading of the water among
the fibres in obedience to capillary attraction. If you suspend a
piece of sponge so that it merely touch the surface of some water,
or if you lay it in a plate with water in it, the whole sponge will
become wet. So, too, if you dip the end of a lump of sugar in your
tea, and hold it there a little time, the whole will be moistened.
In very damp weather the wood-work in our houses swells from
the spreading of water in the pores of the wood in obedience to
capillary attraction. Especially will this be so in basement rooms,
where the water can go up from the ground in the pores of the
walls, as well as from the damp air. In watering plants in pots,
if the water be poured into the saucers, it will pass up through
the earth by capillary attraction. For the same reason plants and
trees near streams grow luxuriantly, being abundantly supplied
with water, which rises to their roots through the pores of the
soil. The disposition of wood to imbibe moisture in its pores
has sometimes been made use of very effectually in getting out
millstones. First a large block of stone is hewn into a cylindrical
shape. Then grooves are cut into it all around where a separation
is desired, and wooden wedges are driven tightly into them. These
absorb moisture from the dews and rain, and therefore swell so much
as to split the stone in the direction of the grooves. The blotter
which you use furnishes an illustration of capillary attraction,
the ink being taken up among the fibres of the paper. Ordinary
writing paper will not answer as a blotter, because the sizing
fills up the interstices between the fibres.