John Tyndall.

Fragaments of science; a series of detached essays, addresses, and reviews (Volume 1) online

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on the temperature of the earth, were then briefly
dwelt upon. A cobweb spread above a blossom ia



RADIATION. 71

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
Eadiation, and my aim throughout has been to raise in
your minds distinct physical images of the various pro-
cesses 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 N the ima-
gination. 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 ima-
gination. This, in fact, is the faculty which enables us
to transcend the boundaries of sense, and connect the
phenomena of our visible world with those of an in-
visible one. Without imagination we never could have
risen to the conceptions which have occupied us here
to-day ; 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.
6



72 FBAGMENTS OF SCIENCE.

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. 1
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 ether 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 ether
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 ether,
thus regenerating the motion from which their tempo-
rary 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

This line of thought was pursued further five years subse-
quently. See ' Scientific Use of the Imagination ' in Vol. IL



RADIATION. 73

it, a certain string will respond. Change the pitch of
your 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.*

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 com-
parison 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 ques-
tion, ' What is sound ? ' Amid the grosser phenomena
of acoustics the mind was first disciplined, conceptiona

1 A discourse delivered in the Royal Institution of
Britain, Jan. 19,



RADIANT HEAT AND ITS RELATIONS. 75

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 he due to vibratory motion. A vibrating tuning-
fork, for example, moulds the air around it into un-
dulations 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 band 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 any-
thing 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



76 FEAGMENTS OF SCIENCE.

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 assump-
tion of this wonderful intangible ether, all the pheno-
mena of optics are accounted for, with a fulness, and
clearness, and collusiveness, 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 ether. 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 ether 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 theii radiant power, as that it is traversed by
that force which holds in its grasp, not only our



RADIANT HEAT AND ITS RELATIONS. 77

planetary system, but the immeasurable heavens them-
selves.

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 He 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 ether
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, how-
ever, 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 ether 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



78 FKA.GMENTS OF SCIENCE.

bodies, that the ether conveys. This motion is the
objective cause of what, in our sensations, are light and
heat. An atom,' then, sending its pulses through the
ether, 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 dif-
ferent powers of agitating the ether : some are good
radiators, others are bad radiators ; which means that
some are so constituted as to communicate their atomic
motion freely to the ether, 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 ether, re-
sembles 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 sensa-
tion of sound. But if we permit the atoms to com-
bine chemically and form molecules, the result, in
many cases, is an enormous change in the power of
radiation. The amount of ethereal disturbance, pro-
duced 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,



RADIANT HEAT AND ITS RELATIONS 79

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 gradu-
ally augmenting pitch, were vibrating at the same time.
But turning to another spectrum that, namely, ob-
tained 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
577 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 tern-



80 FRAGMENTS OF SCIENCE.

perature of the vapour may be raised, the rapidity oi
its vibrations, and consequently its colour, which wholly
depends upon that rapidity, remain unchanged.

The vapoui 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 tempera-
ture 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



RADIANT HEAT AND ITS RELATIONS. 81

impinge, or will they glide round the molecules, through
the intermolecular spaces, and thus escape ?

The answer to this question depends upon a condi-
tion which may be beautifully exemplified by an experi-
ment 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. Slopping
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 vibra-
tion. 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 actualcontact 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 per-
fectly 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.



82 FKAGMENTS OF SCIENCE.

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



RADIANT HEAT AM) ITS BELATIONS. 83

motion. Let us apply these facts to radiant heat. This
blue flame is the flame of carbonic oxide ; this trans-
parent 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 lumi-
nous rays of the sun, though it can absorb the non-lumi-
nous 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 trans-
parency is due to their inability to absorb luminous rays.
The molecules of such substances are in dissonance with
the luminous waves ; and hence such waves pass through
transparent bodies without disturbing the molecular
rest. A purely luminous beam, however intense may



84 FKAGMENTS OF SCIENCE.

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 sun-
shine 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 power-
ful dark rays is emitted \ y 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 Ehone and
the Rhine.

Placing a concave silvered mirror behind the electric
light its rays are converged to a focus of dazzling bril-
liancy. Placing in the path of the rays, between the
light and the focus, a vessel of water, and introducing
at the focus a piece of ice, the ice is not melted by the
concentrated beam. Matches, at the same place, are
ignited, and wood is set on fire. The powerful heat,
then, of this luminous beam is incompetent to melt the
ice. On withdrawing the cell of water, the ice imme-
diately liquefies, and the water trickles from it in drops.
Reintroducing the cell of water, the fusion is arrested,
and the drops cease to fall. The transparent water of
the cell exerts no sensible absorption on the luminous
rays, still it withdraws something from the beam, which,
when permitted to act, is competent to melt the ice.
This something is the dark radiation of the electric
light. Again, I place a slab of pure ice in front of the
electric lamp ; send a luminous beam first through our



EAD1AKT HEAT AND ITS EELAT10NS. 85

cell of water and then through the ice. By means of



Online LibraryJohn TyndallFragaments of science; a series of detached essays, addresses, and reviews (Volume 1) → online text (page 6 of 33)