to exist, as regards their powers of absorption. We followed various
molecules from a state of liquid to a state of gas, and found, in both
states of aggregation, the power of the individual molecules equally
asserted. The position of a vapour as an absorber of radiant heat was
shown to be determined by that of the liquid from which it is derived.
Reversing our conceptions, and regarding the molecules of gases and
vapours not as the recipients but as the originators of wave-motion;
not as absorbers but as radiators; it was proved that the powers of
absorption and radiation went hand in hand, the self-same chemical act
which rendered a body competent to intercept the waves of aether,
rendering it competent, in the same degree, to generate them. Perfumes
were next subjected to examination, and, notwithstanding their
extraordinary tenuity, they were found vastly superior, in point of
absorptive power, to the body of the air in which they were diffused.
We were led thus slowly up to the examination of the most widely
diffused and most important of all vapours--the aqueous vapour of our
atmosphere, and we found in it a potent absorber of the purely
calorific rays. The power of this substance to influence climate, and
its general influence on the temperature of the earth, were then
briefly dwelt upon. A cobweb spread above a blossom is sufficient to
protect it from nightly chill; and thus the aqueous vapour of our
air, attenuated as it is, checks the drain of terrestrial heat, and
saves the surface of our planet from the refrigeration which would
assuredly accrue, were no such substance interposed between it and the
voids of space. We considered the influence of vibrating period, and
molecular form, on absorption and radiation, and finally deduced, from
its action upon radiant heat, the exact amount of carbonic acid
expired by the human lungs.
Thus, in brief outline, were placed before you some of the results of
recent enquiries in the domain of Radiation, and my aim throughout has
been to raise in your minds distinct physical images of the various
processes involved in our researches. It is thought by some that
natural science has a deadening influence on the imagination, and a
doubt might fairly be raised as to the value of any study which would
necessarily have this effect. But the experience of the last hour
must, I think, have convinced you, that the study of natural science
goes hand in hand with the culture of the imagination. Throughout the
greater part of this discourse we have been sustained by this faculty.
We have been picturing atoms, and molecules, and vibrations, and
waves, which eye has never seen nor ear heard, and which can only be
discerned by the exercise of imagination. This, in fact, is the
faculty which enables us transcend the boundaries of sense, and
connect the phenomena of our visible world with those of an invisible
one. Without imagination we never could have risen to the conceptions
which have occupied us here today; and in proportion to your power of
exercising this faculty aright, and of associating definite mental
images with the terms employed, will be the pleasure and the profit
which you will derive from this lecture.
The outward facts of nature are insufficient to satisfy the mind. We
cannot be content with knowing that the light and heat of the sun
illuminate and warm the world. We are led irresistibly to enquire,
'What is light, and what is heat?' and this question leads us at once
out of the region of sense into that of imagination. [Footnote: This
line of thought was pursued further five years subsequently. See
'Scientific Use of the Imagination' in Vol. II.]
Thus pondering, and questioning, and striving to supplement that which
is felt and seen, but which is incomplete, by something unfelt and
unseen which is necessary to its completeness, men of genius have in
part discerned, not only the nature of light and heat, but also,
through them, the general relationship of natural phenomena. The
working power of Nature consists of actual or potential motion, of
which all its phenomena are but special forms. This motion manifests
itself in tangible and in intangible matter, being incessantly
transferred from the one to the other, and incessantly transformed by
the change. It is as real in the waves of the aether as in the waves
of the sea; the latter--derived as they are from winds, which in their
turn are derived from the sun--are, indeed, nothing more than the
heaped-up motion of the aether waves. It is the calorific waves
emitted by the sun which heat our air, produce our winds, and hence
agitate our ocean. And whether they break in foam upon the shore, or
rub silently against the ocean's bed, or subside by the mutual
friction of their own parts, the sea waves, which cannot subside
without producing heat, finally resolve themselves into waves of
aether, thus regenerating the motion from which their temporary
existence was derived. This connection is typical. Nature is not an
aggregate of independent parts, but an organic whole. If you open a
piano and sing into it, a certain string will respond. Change the
pitch of our voice; the first string ceases to vibrate, but another
replies. Change again the pitch; the first two strings are silent,
while another resounds. Thus is sentient man acted on by Nature, the
optic, the auditory, and other nerves of the human body being so many
strings differently tuned, and responsive to different forms of the
universal power.
********************
III ON RADIANT HEAT IN RELATION TO THE COLOUR AND CHEMICAL
CONSTITUTION OF BODIES.
[Footnote: A discourse delivered in the Royal Institution of Great
Britain, Jan. 19, 1866.]
ONE of the most important functions of physical science, considered as
a discipline of the mind, is to enable us by means of the sensible
processes of Nature to apprehend the insensible. The sensible
processes give direction to the line of thought; but this once given,
the length of the line is not limited by the boundaries of the senses.
Indeed, the domain of the senses, in Nature, is almost infinitely
small in comparison with the vast region accessible to thought which
lies beyond them. From a few observations of a comet, when it comes
within the range of his telescope, an astronomer can calculate its
path in regions which no telescope can reach: and in like manner, by
means of data furnished in the narrow world of the senses, we make
ourselves at home in other and wider worlds, which are traversed by
the intellect alone.
From the earliest ages the questions, 'What is light?' and 'What is
heat?' have occurred to the minds of men; but these questions never
would have been answered had they not been preceded by the question,
'What is sound?' Amid the grosser phenomena of acoustics the mind was
first disciplined, conceptions being thus obtained from direct
observation, which were afterwards applied to phenomena of a character
far too subtle to be observed directly. Sound we know to be due to
vibratory motion. A vibrating tuning-fork, for example, moulds the
air around it into undulations or waves, which speed away on all sides
with a certain measured velocity, impinge upon the drum of the ear,
shake the auditory nerve, and awake in the brain the sensation of
sound. When sufficiently near a sounding body we can feel the
vibrations of the air. A deaf man, for example, plunging his hand
into a bell when it is sounded, feels through the common nerves of his
body those tremors which, when imparted to the nerves of healthy ears,
are translated into sound. There are various ways of rendering those
sonorous vibrations not only tangible but visible; and it was not
until numberless experiments of this kind had been executed, that the
scientific investigator abandoned himself wholly, and without a shadow
of misgiving, to the conviction that what is sound within us is,
outside of us, a motion of the air.
But once having established this fact--once having proved beyond all
doubt that the sensation of sound is produced by an agitation of the
auditory nerve--the thought soon suggested itself that light might be
due to an agitation of the optic nerve. This was a great step in
advance of that ancient notion which regarded light as something
emitted by the eye, and not as anything imparted to it. But if light
be produced by an agitation of the retina, what is it that produces
the agitation? Newton, you know, supposed minute particles to be shot
through the humours of the eye against the retina, which he supposed
to hang like a target at the back of the eye. The impact of these
particles against the target, Newton believed to be the cause of
light. But Newton's notion has not held its ground, being entirely
driven from the field by the more wonderful and far more philosophical
notion that light, like sound, is a product of wave-motion.
The domain in which this motion of light is carried on lies entirely
beyond the reach of our senses. The waves of light require a medium
for their formation and propagation; but we cannot see, or feel, or
taste, or smell this medium. How, then, has its existence been
established? By showing, that by the assumption of this wonderful
intangible aether, all the phenomena of optics are accounted for, with
a fulness, and clearness, and conclusiveness, which leave no desire of
the intellect unsatisfied. When the law of gravitation first
suggested itself to the mind of Newton, what did he do? He set
himself to examine whether it accounted for all the facts. He
determined the courses of the planets; he calculated the rapidity of
the moon's fall towards the earth; he considered the precession of the
equinoxes, the ebb and flow of the tides, and found all explained by
the law of gravitation. He therefore regarded this law as
established, and the verdict of science subsequently confirmed his
conclusion. On similar, and, if possible, on stronger grounds, we
found our belief in the existence of the universal aether. It
explains facts far more various and complicated than those on which
Newton based his law. If a single phenomenon could be pointed out
which the aether is proved incompetent to explain, we should have to
give it up; but no such phenomenon has ever been pointed out. It is,
therefore, at least as certain that space is filled with a medium, by
means of which suns and stars diffuse their radiant power, as that it
is traversed by that force which holds in its grasp, not only our
planetary system, but the immeasurable heavens themselves.
There is no more wonderful instance than this of the production of a
line of thought, from the world of the senses into the region of pure
imagination. I mean by imagination here, not that play of fancy which
can give to airy nothings a local habitation and a name, but that
power which enables the mind to conceive realities which lie beyond
the range of the senses--to present to itself distinct images of
processes which, though mighty in the aggregate beyond all conception,
are so minute individually as to elude all observation. It is the
waves of air excited by a tuning-fork which render its vibrations
audible. It is the waves of aether sent forth from those lamps
overhead which render them luminous to us; but so minute are these
waves, that it would take from 30,000 to 60,000 of them placed end to
end to cover a single inch. Their number, however, compensates for
their minuteness. Trillions of them have entered your eyes, and hit
the retina at the backs of your eyes, in the time consumed in the
utterance of the shortest sentence of this discourse. This is the
steadfast result of modern research; but we never could have reached
it without previous discipline. We never could have measured the
waves of light, nor even imagined them to exist, had we not previously
exercised ourselves among the waves of sound. Sound and light are now
mutually helpful, the conceptions of each being expanded,
strengthened, and defined by the conceptions of the other.
The aether which conveys the pulses of light and heat not only fills
celestial space, swathing suns, and planets, and moons, but it also
encircles the atoms of which these bodies are composed. It is the
motion of these atoms, and not that of any sensible parts of bodies,
that the aether conveys. This motion is the objective cause of what,
in our sensations, are light and heat. An atom, then, sending its
pulses through the aether, resembles a tuning-fork sending its pulses
through the air. Let us look for a moment at this thrilling medium,
and briefly consider its relation to the bodies whose vibrations it
conveys. Different bodies, when heated to the same temperature,
possess very different powers of agitating the aether: some are good
radiators, others are bad radiators; which means that some are so
constituted as to communicate their atomic motion freely to the
aether, producing therein powerful undulations; while the atoms of
others are unable thus to communicate their motions, but glide through
the medium without materially disturbing its repose. Recent
experiments have proved that elementary bodies, except under certain
anomalous conditions, belong to the class of bad radiators. An atom,
vibrating in the aether, resembles a naked tuning-fork vibrating in
the air. The amount of motion communicated to the air by the thin
prongs is too small to evoke at any distance the sensation of sound.
But if we permit the atoms to combine chemically and form molecules,
the result, in many cases, is an enormous change in the power of
radiation. The amount of aethereal disturbance, produced by the
combined atoms of a body, may be many thousand times that produced by
the same atoms when uncombined.
The pitch of a musical note depends upon the rapidity of its
vibrations, or, in other words, on the length of its waves. Now, the
pitch of a note answers to the colour of light. Taking a slice of
white light from the sun, or from an electric lamp, and causing the
light to pass through an arrangement of prisms, it is decomposed. We
have the effect obtained by Newton, who first unrolled the solar beam
into the splendours of the solar spectrum. At one end of this
spectrum we have red light, at the other, violet; and between those
extremes lie the other prismatic colours. As we advance along the
spectrum from the red to the violet, the pitch of the light--if I may
use the expression--heightens, the sensation of violet being produced
by a more rapid succession of impulses than that which produces the
impression of red. The vibrations of the violet are about twice as
rapid as those of the red; in other words, the range of the visible
spectrum is about an octave.
There is no solution of continuity in this spectrum one colour changes
into another by insensible gradations. It is as if an infinite number
of tuning-forks, of gradually augmenting pitch, were vibrating at the
same time. But turning to another spectrum--that, namely, obtained
from the incandescent vapour of silver--you observe that it consists
of two narrow and intensely luminous green bands. Here it is as if
two forks only, of slightly different pitch, were vibrating. The
length of the waves which produce this first band is such that 47,460
of them, placed end to end, would fill an inch. The waves which
produce the second band are a little shorter; it would take of these
47,920 to fill an inch. In the case of the first band, the number of
impulses imparted, in one second, to every eye which sees it, is 677
millions of millions; while the number of impulses imparted, in the
same time, by the second band is 600 millions of millions. We may
project upon a white screen the beautiful stream of green light from
which these bands were derived. This luminous stream is the
incandescent vapour of silver. The rates of vibration of the atoms of
that vapour are as rigidly fixed as those of two tuning-forks; and to
whatever height the temperature of the vapour may be raised, the
rapidity of its vibrations, and consequently its colour, which wholly
depends upon that rapidity, remain unchanged.
The vapour of water, as well as the vapour of silver, has its definite
periods of vibration, and these are such as to disqualify the vapour,
when acting freely as such, from being raised to a white heat. The
oxyhydrogen flame, for example, consists of hot aqueous vapour. It is
scarcely visible in the air of this room, and it would be still less
visible if we could burn the gas in a clean atmosphere. But the
atmosphere, even at the summit of Mont Blanc, is dirty; in London it
is more than dirty; and the burning dirt gives to this flame the
greater portion of its present light. But the heat of the flame is
enormous. Cast iron fuses at a temperature of 2,000° Fahr; while the
temperature of the oxyhydrogen flame is 6,000° Fahr. A piece of
platinum is heated to vivid redness, at a distance of two inches
beyond the visible termination of the flame. The vapour which
produces incandescence is here absolutely dark. In the flame itself
the platinum is raised to dazzling whiteness, and is even pierced by
the flame. When this flame impinges on a piece of lime, we have the
dazzling Drummond light. But the light is here due to the fact that
when it impinges upon the solid body, the vibrations excited in that
body by the flame are of periods different from its own.
Thus far we have fixed our attention on atoms and molecules in a state
of vibration, and surrounded by a medium which accepts their
vibrations, and transmits them through space. But suppose the waves
generated by one system of molecules to impinge upon another system,
how will the waves be affected? Will they be stopped, or will they be
permitted to pass? Will they transfer their motion to the molecules
on which they impinge, or will they glide round the molecules, through
the intermolecular spaces, and thus escape?
The answer to this question depends upon a condition which may be
beautifully exemplified by an experiment on sound. These two
tuning-forks are tuned absolutely alike. They vibrate with the same
rapidity, and, mounted thus upon their resonant cases, you hear them
loudly sounding the same musical note. Stopping one of the forks, I
throw the other into strong vibration, and bring that other near the
silent fork, but not into contact with it. Allowing them to continue
in this position for four or five seconds, and then stopping the
vibrating fork, the sound does not cease. The second fork has taken
up the vibrations of its neighbour, and is now sounding in its turn.
Dismounting one of the forks, and permitting the other to remain upon
its stand, I throw the dismounted fork into strong vibration. You
cannot hear it sound. Detached from its case, the amount of motion
which it can communicate to the air is too small to be sensible at any
distance. When the dismounted fork is brought close to the mounted
one, but not into actual contact with it, out of the silence rises a
mellow sound. Whence comes it? From the vibrations which have been
transferred from the dismounted fork to the mounted one.
That the motion should thus transfer itself through the air it is
necessary that the two forks should be in perfect unison. If a morsel
of wax not larger than a pea be placed on one of the forks, it is
rendered thereby powerless to affect, or to be affected by, the other.
It is easy to understand this experiment. The pulses of the one fork
can affect the other, because they are _perfectly timed_. A single
pulse causes the prong of the silent fork to vibrate through an
infinitesimal space. But just as it has completed this small
vibration another pulse is ready to strike it. Thus, the impulses add
themselves together. In the five seconds during which the forks were
held near each other, the vibrating fork sent 1,280 waves against its
neighbour and those 1,280 shocks, all delivered at the proper moment,
all, as I have said, perfectly timed, have given such strength to the
vibrations of the mounted fork as to render them audible to all.
Another curious illustration of the influence of synchronism on
musical vibrations, is this: Three small gas-flames are inserted into
three glass tubes of different lengths. Each of these flames can be
caused to emit a musical note, the pitch of which is determined by the
length of the tube surrounding the flame. The shorter the tube the
higher is the pitch. The flames are now silent within their
respective tubes, but each of them can be caused to respond to a
proper note sounded anywhere in this room. With an instrument called
a syren, a powerful musical note, of gradually increasing pitch, can
be produced. Beginning with a low note, and ascending gradually to a
higher one, we finally attain the pitch of the flame in the longest
tube. The moment it is reached, the flame bursts into song. The
other flames are still silent within their tubes. But by urging the
instrument on to higher notes, the second flame is started, and the
third alone remains. A still higher note starts it also. Thus, as
the sound of the syren rises gradually in pitch, it awakens every
flame in passing, by striking it with a series of waves whose periods
of recurrence are similar to its own.
Now the wave-motion from the syren is in part taken up by the flame
which synchronises with the waves; and were these waves to impinge
upon a multitude of flames, instead of upon one flame only, the
transference might be so great as to absorb the whole of the original
wave motion. Let us apply these facts to radiant heat. This blue
flame is the flame of carbonic oxide; this transparent gas is carbonic
acid gas. In the blue flame we have carbonic acid intensely heated,
or, in other words, in a state of intense vibration. It thus
resembles the sounding fork, while this cold carbonic acid resembles
the silent one. What is the consequence? Through the synchronism of
the hot and cold gas, the waves emitted by the former are intercepted
by the latter, the transmission of the radiant heat being thus
prevented. The cold gas is intensely opaque to the radiation from
this particular flame, though highly transparent to heat of every
other kind. We are here manifestly dealing with that great principle
which lies at the basis of spectrum analysis, and which has enabled
scientific men to determine the substances of which the sun, the
stars, and even the nebulae are composed; the principle, namely, that
a body which is competent to emit any ray, whether of heat or light,
is competent in the same degree to absorb that ray. The absorption
depends on the synchronism existing between the vibrations of the atoms
from which the rays, or more correctly the waves, issue, and those of
the atoms on which they impinge.
To its almost total incompetence to emit white light, aqueous vapour
adds a similar incompetence to absorb white light. It cannot, for
example, absorb the luminous rays of the sun, though it can absorb the
non-luminous rays of the earth. This incompetence of the vapour to
absorb luminous rays is shared by water and ice--in fact, by all
really transparent substances. Their transparency is due to their
inability to absorb luminous rays. The molecules of such substances
are in dissonance with luminous waves; and hence such waves pass
through transparent bodies without disturbing the molecular rest. A
purely luminous beam, however intense may be its heat, is sensibly
incompetent to melt ice. We can, for example, converge a powerful
luminous beam upon a surface covered with hoar frost, without melting
a single spicula of the crystals. How then, it may be asked, are the
snows of the Alps swept away by the sunshine of summer? I answer,
they are not swept away by sunshine at all, but by rays which have no
sunshine whatever in them. The luminous rays of the sun fall upon the
snow-fields and are flashed in echoes from crystal to crystal, but
they find next to no lodgment within the crystals. They are hardly at
all absorbed, and hence they cannot produce fusion. But a body of
powerful dark rays is emitted by the sun; and it is these that cause
the glaciers to shrink and the snows to disappear; it is they that
fill the banks of the Arve and Arveyron, and liberate from their
frozen captivity the Rhone and the Rhine.