endowment conferred once for all on man at his creation, we find him
here gifted with a mind curious to know the causes of things, and
surrounded by objects which excite its questionings, and raise the
desire for an explanation. It is related of a young Prince of one of
the Pacific Islands, that when he first saw himself in a
looking-glass, he ran round the glass to see who was standing at the
back. And thus it is with the general human intellect, as regards the
phenomena of the external world. It wishes to get behind and learn
the causes and connections of these phenomena. What is the sun, what
is the earth, what should we see if we came to the edge of the earth
and looked over? What is the meaning of thunder and lightning, of
hail, rain, storm, and snow? Such questions presented themselves to
early men, and by and by it was discovered that this desire for
knowledge was not implanted in vain. After many trials it became
evident that man's capacities were, so to speak, the complement of
nature's facts, and that, within certain limits, the secret of the
universe was open to the human understanding. It was found that the
mind of man had the power of penetrating far beyond the boundaries of
his five senses; that the things which are seen in the material world
depend for their action upon things unseen; in short, that besides the
phenomena which address the senses, there are laws and principles and
processes which do not address the senses at all, but which must be,
and can be, spiritually discerned.
To the subjects which require this discernment belong the phenomena of
molecular force. But to trace the genesis of the notions now
entertained upon this subject, we have to go a long way back. In the
drawing of a bow, the darting of a javelin, the throwing of a
stone--in the lifting of burdens, and in personal combats, even savage
man became acquainted with the operation of _force_. Ages of
discipline, moreover, taught him foresight. He laid by at the proper
season stores of food, thus obtaining time to look about him, and to
become an observer and enquirer. Two things which he noticed must
have profoundly stirred his curiosity. He found that a kind of resin
dropped from a certain tree possessed, when rubbed, the power of
drawing light bodies to itself, and of causing them to cling to it;
and he also found that a particular stone exerted a similar power over
a particular kind of metal. I allude, of course, to electrified
amber, and to the load-stone, or natural magnet, and its power to
attract particles of iron. Previous experience of his own muscles had
enabled our early enquirer to distinguish between a push and a pull.
Augmented experience showed him that in the case of the magnet and the
amber, pulls and pushes--attractions and repulsions--were also
exerted; and, by a kind of poetic transfer, be applied to things
external to himself, conceptions derived from himself. The magnet and
the rubbed amber were credited with pushing and pulling, or, in other
words, with exerting force.
In the time of the great Lord Bacon the margin of these pushes and
pulls was vastly extended by Dr. Gilbert, a man probably of firmer
scientific fibre, and of finer insight, than Bacon himself. Gilbert
proved that a multitude of other bodies, when rubbed, exerted the
power which, thousands of years previously, had been observed in
amber. In this way the notion of attraction and repulsion in external
nature was rendered familiar. It was a matter of experience that
bodies, between which no visible link or connection existed, possessed
the power of acting upon each other; and the action came to be
technically called 'action at a distance.'
But out of experience in science there grows something finer than mere
experience. Experience furnishes the soil for plants of higher
growth; and this observation of action at a distance provided material
for speculation upon the largest of problems. Bodies were observed to
fall to the earth. Why should they do so? The earth was proved to
revolve round the sun; and the moon to revolve round the earth. Why
should they do so? What prevents them from flying straight off into
space? Supposing it were ascertained that from a part of the earth's
rocky crust a firmly fixed and tightly stretched chain started towards
the sun, we might be inclined to conclude that the earth is held in
its orbit by the chain--that the sun twirls the earth around him, as a
boy twirls round his head a bullet at the end of a string. But why
should the chain be needed? It is a fact of experience that bodies
can attract each other at a distance, without the intervention of any
chain. Why should not the sun and earth so attract each other? and
why should not the fall of bodies from a height be the result of their
attraction by the earth? Here then we reach one of those higher
speculations which grow out of the fruitful soil of observation.
Having started with the savage, and his sensations of muscular force,
we pass on to the observation of force exerted between a magnet and
rubbed amber and the bodies which they attract, rising, by an unbroken
growth of ideas, to a conception of the force by which sun and planets
are held together.
This idea of attraction between sun and planets had become familiar in
the time of Newton. He set himself to examine the attraction; and
here, as elsewhere, we find the speculative mind falling back for its
materials upon experience. It had been observed, in the case of
magnetic and electric bodies, that the nearer they were brought
together the stronger was the force exerted between them; while, by
increasing the distance, the force diminished until it became
insensible. Hence the inference that the assumed pull between the
earth and the sun would be influenced by their distance asunder.
Guesses had been made as to the exact manner in which the force varied
with the distance; but Newton supplemented the guess by the severe
test of experiment and calculation. Comparing the pull of the earth
upon a body close to its surface, with its pull upon the moon, 240,000
miles away, Newton rigidly established the law of variation with the
distance. But on his way to this result Newton found room for other
conceptions, some of which, indeed, constituted the necessary
stepping-stones to his result. The one which here concerns us is,
that not only does the sun attract the earth, and the earth attract
the sun, as wholes, but every particle of the sun attracts every
particle of the earth, and the reverse. His conclusion was, that the
attraction of the masses was simply the sum of the attractions of
their constituent particles.
This result seems so obvious that you will perhaps wonder at my
dwelling upon it; but it really marks a turning point in our notions
of force. You have probably heard of certain philosophers of the
ancient world named Democritus, Epicurus, and Lucretius. These men
adopted, developed, and diffused the doctrine of atoms and molecules,
which found its consummation at the hands of the illustrious John
Dalton. But the Greek and Roman philosophers I have named, and their
followers, up to the time of Newton, pictured their atoms as falling
and flying through space, hitting each other, and clinging together by
imaginary hooks and claws. They missed the central idea that atoms
and molecules could come together, not by being fortuitously knocked
Against each other, but by their own mutual attractions. This is one
of the great steps taken by Newton. He familiarised the world with
the conception of _molecular force_.
Newton, you know, was preceded by a grand fellow named John Kepler--a
true working man--who, by analysing the astronomical observations of
his master, Tycho Brahe, had actually found that the planets moved as
they are now known to move. Kepler knew as much about the motion of
the planets as Newton did; in fact, Kepler taught Newton and the world
generally the facts of planetary motion. But this was not enough. The
question arose--Why should the facts be so? This was the great
question for Newton, and it was the solution of it which renders his
name and fame immortal. Starting from the principle that every
particle of matter in the solar system attracts every other particle
by a force which varies as the inverse square of the distance between
the particles, he proved that the Planetary motions must be what
observation makes them to be. He showed that the moon fell towards
the earth, and that the planets fell towards the sun, through the
operation of the same force that pulls an apple from its tree. This
all-pervading force, which forms the solder of the material universe,
and the conception of which was necessary to Newton's intellectual
peace, is called the force of gravitation.
Gravitation is a purely attractive force, but in electricity and
magnetism, repulsion had been always seen to accompany attraction.
Electricity and magnetism are double or _polar forces_. In the case of
magnetism, experience soon pushed the mind beyond the bounds of
experience, compelling it to conclude that the polarity of the magnet
was resident in its molecules. I hold a magnetised strip of steel by
its centre, and find that one half of the strip attracts, and the
other half repels, the north end of a magnetic needle. I break the
strip in the middle, find that this half, which a moment ago attracted
throughout its entire length the north pole of a magnetic needle, is
now divided into two new halves, one of which wholly attracts, and the
other of which wholly repels, the north pole of the needle. The half
proves to be as perfect a magnet as the whole. You may break this
half and go on till further breaking becomes impossible through the
very smallness of the fragments; the smallest fragment is found
endowed with two poles, and is, therefore, a perfect magnet. But you
cannot stop here: you _imagine_ where you cannot _experiment_; and reach
the conclusion entertained by all scientific men, that the magnet
which you see and feel is an assemblage of molecular magnets which you
cannot see and feel, but which, as before stated, must be
intellectually discerned.
Magnetism then is a polar force; and experience hints that a force of
this kind may exert a certain structural power. It is known, for
example, that iron filings strewn round a magnet arrange themselves in
definite lines, called, by some, 'magnetic curves,' and, by others,
'lines of magnetic force.' Over two magnets now before me is spread a
sheet of paper. Scattering iron filings over the paper, polar force
comes into play, and every particle of the iron responds to that
force. We have a kind of architectural effort--if I may use the
term--exerted on the part of the iron filings. Here then is a fact of
experience which, as you will see immediately, furnishes further
material for the mind to operate upon, rendering it possible to attain
intellectual clearness and repose, while speculating upon apparently
remote phenomena.
The magnetic force has here acted upon particles visible to the eye.
But, as already stated, there are numerous processes in nature which
entirely elude the eye of the body, and must be figured by the eye of
the mind. The processes of chemistry are examples of these. Long
thinking and experimenting has led philosophers to conclude that
matter is composed of atoms from which, whether separate or in
combination, the whole material world is built up. The air we
breathe, for example, as mainly a mechanical mixture of the atoms of
oxygen and nitrogen. The water we drink is also composed of oxygen
and hydrogen. But it differs from the air in this particular, that in
water the oxygen and hydrogen are not mechanically mixed, but
chemically combined. The atoms of oxygen and those of hydrogen exert
enormous attractions on each other, so that when brought into
sufficient proximity they rush together with an almost incredible
force to form a chemical compound. But powerful as is the force with
which these atoms lock themselves together, we have the means of
tearing them asunder, and the agent by which we accomplish this may
here receive a few moments' attention.
Into a vessel containing acidulated water I dip two strips of metal,
the one being zinc and the other platinum, not permitting them to
touch each other in the liquid. I connect the two upper ends of the
strips by a piece of copper wire. The wire is now the channel of
what, for want of a better name, we call an 6 electric current.' What
the inner change of the wire is we do not know, but we do know that a
change has occurred, by the external effects produced by the wire. Let
me show you one or two of these effects. Before you is a series of
ten vessels, each with its pair of metals, and I wish to get the added
force of all ten. The arrangement is called a voltaic battery. I
plunge a piece of copper wire among these iron filings; they refuse to
cling to it. I employ the selfsame wire to connect the two ends of
the battery, and subject it to the same test. The iron filings now
crowd round the wire and cling to it. I interrupt the current, and
the filings immediately fall; the power of attraction continues only
so long as the wire connects the two ends of the battery.
Here is a piece of similar wire, overspun with cotton, to prevent the
contact of its various parts, and formed into a coil. I make the coil
part of the wire which connects the two ends of the voltaic battery.
By the attractive force with which it has become suddenly endowed, it
now empties this tool-box of its iron nails. I twist a covered copper
wire round this common poker; connecting the wire with the two ends of
the voltaic battery, the poker is instantly transformed into a strong
magnet. Two flat spirals are here suspended facing each other, about
six inches apart. Sending a current through both spirals, they clash
suddenly together; reversing what is called the direction of the
current in one of the spirals, they fly asunder. All these effects
are due to the power which we name an electric current, and which we
figure as flowing through the wire when the voltaic circuit is
complete.
By the same agent we tear asunder the locked atoms of a chemical
compound. Into this small cell, containing water, dip two thin wires.
A magnified image of the cell is thrown upon the screen before you,
and you see plainly the images of the wires. From a small battery I
send an electric current from wire to wire. Bubbles of gas rise
immediately from each of them, and these are the two gases of which
the water is composed. The oxygen is always liberated on the one
wire, the hydrogen on the other. The gases may be collected either
separately or mixed. I place upon my hand a soap bubble filled with
the mixture of both gases. Applying a taper to the bubble, a loud
explosion is heard. The atoms have rushed together with detonation,
and without injury to my hand, and the water from which they were
extracted is the result of their re-union.
*****
One consequence of the rushing together of the atoms is the
development of heat. What is this heat? Here are two ivory balls
suspended from the same point of support by two short strings. I draw
them thus apart and then liberate them. They clash together, but, by
virtue of their elasticity, they quickly recoil, and a sharp vibratory
rattle succeeds their collision. This experiment will enable you to
figure to your mind a pair of clashing atoms. We have in the first
place, a motion of the one atom towards the other--a motion of
translation, as it is usually called--then a recoil, and afterwards a
motion of vibration. To this vibratory motion we give the name of
heat. Thus, three things are to be kept before the mind--first, the
atoms themselves; secondly, the force with which they attract each
other; and thirdly, the motion consequent upon the exertion of that
force. This motion must be figured first as a motion of translation,
and then as a motion of vibration, to which latter we give the name of
heat. For some time after the act of combination this motion is so
violent as to prevent the molecules from coming together, the water
being maintained in a state of vapour. But as the vapour cools, or in
other words loses its motion, the molecules coalesce to form a liquid.
And now we approach a new and wonderful display of force. As long as
the substance remains in a liquid or vaporous condition, the play of
this force is altogether masked and bidden. But as the heat is
gradually withdrawn, the molecules prepare for new arrangements and
combinations. Solid crystals of water are at length formed, to which
we give the familiar name of ice. Looking at these beautiful edifices
and their internal structure, the pondering mind has forced upon it
the question, How are they built up? We have obtained clear
conceptions of polar force; and we infer from our broken magnet that
polar force may be resident in the molecules or smallest particles of
matter, and that by the play of this force structural arrangement is
possible. What, in relation to our present question, is the natural
action of a mind furnished with this knowledge? It is compelled to
transcend experience, and endow the atoms and molecules of which
crystals are built with definite poles whence issue attractions and
repulsions. In virtue of these forces some poles are drawn together,
while some retreat from each other; atom is added to atom, and
molecule to molecule, not boisterously or fortuitously, but silently
and symmetrically, and in accordance with laws more rigid than those
which guide a human builder when he places his materials together.
Imagine the bricks and stones of this town of Dundee endowed with
structural power.
Imagine them attracting and repelling, and arranging themselves into
streets and houses and Kinnaird Halls--would not that be wonderful?
Hardly less wonderful is the play of force by which the molecules of
water build themselves into the sheets of ice which every winter roof
your ponds and lakes.
If I could show you the actual progress of this molecular
architecture, its beauty would delight and astonish you. A reversal
of the process of crystallisation may be actually shown. The
molecules of a piece of ice may be taken asunder before your eyes; and
from the manner in which they separate, you may to some extent infer
the manner in which they go together. When a beam is sent from our
electric lamp through a plate of glass, a portion of the beam is
intercepted, and the glass is warmed by the portion thus retained
within it. When the beam is sent through a plate of ice, a portion of
the beam is also absorbed; but instead of warming the ice, the
intercepted heat melts it internally. It is to the delicate silent
action of this beam within the ice that I now wish to direct your
attention. Upon the screen is thrown a magnified image of the slab of
ice: the light of the beam passes freely through the ice without
melting it, and enables us to form the image; but the heat is in great
part intercepted, and that heat now applies itself to the work of
internal liquefaction. Selecting certain points for attack, round
about those points the beam works silently, undoing the crystalline
architecture, and reducing to the freedom of liquidity molecules which
had been previously locked in a solid embrace. The liquefied spaces
are rendered visible by strong illumination. Observe those
six-petaled flowers breaking out over the white surface, and expanding
in size as the action of the beam continues. These flowers are
liquefied ice. Under the action of the heat the molecules of the
crystals fall asunder, so as to leave behind them these exquisite
forms. We have here a process of demolition which clearly reveals the
reverse process of construction. In this fashion, and in strict
accordance with this hexangular type, every ice molecule takes its
place upon our ponds and lakes during the frosts of winter. To use
the language of an American poet, 'the atoms march in tune,' moving to
the music of law, which thus renders the commonest substance in nature
a miracle of beauty.
It is the function of science, not as some think to divest this
universe of its wonder and mystery, but, as in the case before us, to
point out the wonder and the mystery of common things. Those
fern-like forms, which on a frosty morning overspread your
windowpanes, illustrate the action of the same force. Breathe upon
such a pane before the fires are lighted, and reduce the solid
crystalline film to the liquid condition; then watch its subsequent
resolidification. You will see it all the better if you look at it
through a common magnifying glass. After you have ceased breathing,
the film, abandoned to the action of its own forces, appears for a
moment to be alive. Lines of motion run through it; molecule closes
with molecule, until finally the whole film passes from the state of
liquidity, through this state of motion, to its final crystalline
repose.
I can show you something similar. Over a piece of perfectly clean
glass I pour a little water in which certain crystals have been
dissolved. A film of the solution clings to the glass. By means of a
microscope and a lamp, an image of the plate of glass is thrown upon
the screen. The beam of the lamp, besides illuminating the glass,
also heats it; evaporation sets in, and at a certain moment, when the
solution has become supersaturated, splendid branches of crystal shoot
out over the screen. A dozen square feet of surface are now covered
by those beautiful forms. With another solution we obtain crystalline
spears, feathered right and left by other spears. From distant nuclei
in the middle of the field of view the spears shoot with magical
rapidity in all directions. The film of water on a window-pane on a
frosty morning exhibits effects quite as wonderful as these. Latent
in these formless solutions, latent in every drop of water, lies this
marvellous structural power, which only requires the withdrawal of
opposing forces to bring it into action.
The clear liquid now held up before you is a solution of nitrate of
silver--a compound of silver and nitric acid. When an electric
current is sent through this liquid the silver is severed from the
acid, as the hydrogen was separated from the oxygen in a former
experiment; and I would ask you to observe how the metal behaves when
its molecules are thus successively set free. The image of the cell,
and of the two wires which dip into the liquid of the cell, are now
clearly shown upon the screen. Let us close the circuit, and send the
current through the liquid. From one of the wires a beautiful silver
tree commences immediately to sprout. Branches of the metal are
thrown out, and umbrageous foliage loads the branches. You have here
a growth, apparently as wonderful as that of any vegetable, perfected
in a minute before your eyes. Substituting for the nitrate of silver
acetate of lead, which is a compound of lead and acetic acid, the
electric current severs the lead from the acid, and you see the metal
slowly branching into exquisite metallic ferns, the fronds of which,
as they become too heavy, break from their roots and fall to the
bottom of the cell.
These experiments show that the common matter of our earth--'brute
matter,' as Dr. Young, in his _Night Thoughts_, is pleased to call
it--when its atoms and molecules are permitted to bring their forces
into free play, arranges itself, under the operation of these forces,
into forms which rival in beauty those of the vegetable world. And
what is the vegetable world itself, but the result of the complex play
of these molecular forces? Here, as elsewhere throughout nature, if
matter moves it is force that moves it, and if a certain structure,
vegetable or mineral, is produced, it is through the operation of the
forces exerted between the atoms and molecules.