“Every secret which is disclosed, every discovery which is
made, every new effort which is brought to view, serves
to convince us of numberless more which remain concealed,
and which we had before no suspicion of.... Knowledge
is not our proper happiness. Whoever will in the least
attend to the thing will see that it is the gaining,
not the having of it, which is the entertainment of the
mind.”--BISHOP BUTLER.
Next, probably, to magnetism and electricity, the scientific
investigation of the laws of heat has yielded the most fruitful and the
most curious results. The science of heat made the greatest progress
about the middle of the present century, and Professor Tyndall was
one of its most successful investigators. Being a force co-related
to electricity, it is scarcely remarkable that the same natural
philosopher should reveal to us not a few of these silent operations of
magnetism and heat that previously were unobserved or were regarded as
mysteries.
When, in 1859, he turned his attention to the absorption of radiant
heat by gases and vapours, there was considerable diversity of opinion
as to the effect of the atmosphere on radiant heat; and great skill and
patience were required in devising experiments, and in detecting and
eliminating the various sources of error. Till then it was thought that
the subject was outside the realm of experiment, but Professor Tyndall
soon demonstrated that heat in gases and vapours was subject to various
laws which had most important effects in every part of the world. In
his first memoir he established not only the existence of absorption
and radiation in gases, but that the differences of absorption and
radiation were as great among gases as among liquids and solids. He
showed that the elementary gases, hydrogen, oxygen, nitrogen, as well
as air freed from moisture and carbonic acid, examined in a length of
four feet, absorb about 3½ per cent. of heat radiated from lamp-black
at 212°, the slightest impurity in the gas, however, altering the
rate of absorption. With compound gases and vapours very different
results were obtained. About twenty gases and vapours were examined,
and it was found that while the elementary gases already named gave
the feeblest action, olephiant gas showed the most energetic action,
absorbing 81 per cent. He also made the important discovery that by
arranging the various gases in order according to their power, first of
radiating heat and then of absorbing radiant heat, the order was the
same in both cases; in short, the order of radiation was exactly that
of absorption. In his second memoir he introduced a new and remarkable
method of determining absorption and radiation. This method he called
“dynamic radiation.” Dispensing with the use of any extraneous source
of heat, he obtained his results by the heat or cold produced by the
condensation or rarefication of the gases. Just as a ball striking a
target is heated by collision, so he heated gas contained in one part
of a tube by the collision of its particles against the surface of
another part into which they rushed to fill a vacuum. He found, he
said, by strict experiments that the dynamic radiation of an amount of
boracic ether vapour, possessing a tension of only one 1,012,500,000th
of an atmosphere, was easily measurable.
His researches on the relation of radiant heat to aqueous vapour,
published in 1863, were the most interesting and useful. Such were
the difficulties connected with the investigation of this part of the
subject that Professor Tyndall and his old friend Professor Magnus,
of Berlin, arrived at and long maintained opposite conclusions as
to the absorption of radiant heat by the air and the influence of
aqueous vapour. Early in his researches Professor Tyndall regarded the
action of the atmosphere as a particular part of his inquiry, and,
accordingly, his third memoir was specially devoted to the radiation of
aqueous vapour. The conclusion he came to was that the aqueous vapour
in our atmosphere intercepted or absorbed eighty times more heat than
the air, and as there was only one atom of aqueous vapour for every 200
of oxygen and nitrogen composing the air, it appeared that one atom
of the former absorbed 16,000 times more than one atom of oxygen or
nitrogen. This startling conclusion he verified by a system of checks
and counter-checks which were considered as decisive. The applications
of this discovery were manifold and important. The aqueous vapour which
absorbed so much heat he likened to a blanket which is more necessary
to the vegetable life of England than clothing is to man. “Remove for a
single summer night,” he said, “the aqueous vapour from the air which
overspreads this country, and you would assuredly destroy every plant
capable of being destroyed by a freezing temperature. The warmth of our
fields and gardens would pour itself unrequited into space, and the sun
would rise upon an island held fast in the iron grip of frost.” The
aqueous vapour constitutes a local dam, which deepens the temperature
at the earth’s surface, but which finally overflows and gives to
space all that we receive from the sun. This discovery presented an
explanation of some phenomena, which hitherto had been imperfectly
understood. It was evidently the absence of this aqueous screen which
made the winters in Central Asia almost unendurable; and it showed how
the burning heat of the Sahara during the day was followed by intense
cold at night.
Before Professor Tyndall had published all his observations on the
relations between radiant heat and aqueous vapour, his friend,
Professor Frankland, regarded them as sufficient to account for the
glacial era, and the action of glaciers over the entire globe. During
a visit to Norway in 1863 Frankland considered the subject afresh, and
came to the conclusion that the chief cause of the phenomena of the
glacial epoch was a higher temperature of the ocean than prevails at
present. The critics of the day pointed out that such a view depended
upon the accuracy of the assumption that our earth had gradually cooled
down from an originally incandescent state; and it is now generally
admitted by natural philosophers that the earth has cooled down from
a state of liquid heat. In that case the waters of the ocean, when
cooling down from the boiling point, would be at a higher temperature
than the present; and Professor Frankland maintained that it was in the
later stages of the cooling process that the glacial epoch occurred.
The great natural glacial apparatus he divided into three parts--the
evaporator, the condenser, and the receiver. The cooling ocean was the
evaporator; the mountains were the icebearers or receivers; while the
dry air which permitted the heat from the vapour to radiate into space,
acted as the condenser. He made numerous experiments to show that under
these conditions the land would cool more rapidly than the sea; and
he maintained that in the glacial epoch the “rays of heat streamed
into space from the ice-bearing surfaces with comparatively little
interruption, whilst the radiation from the sea was as effectually
retarded as if the latter had been protected with a thick envelope of
non-conducting material. Thus, whilst the ocean retained a temperature
considerably higher than at present, the icebearers had undergone a
considerably greater refrigeration.” He calculated that an increase
of 20° in the temperature of the coast of Norway would double the
evaporation from a given surface, and such an increased evaporation,
accompanied of course by a corresponding precipitation, “would suffice
to supply the higher portions of the land with that gigantic ice-burden
which ground down the mountain slopes during the glacial epoch.” Such
a view did not require the assumption of any natural convulsion or
catastrophe; on the contrary it accounted for the glacial epoch by
the evolution of thermal conditions, the existence of which is now
generally admitted.[3]
In his fourth memoir, published in 1864, “On the Radiation and
Absorption of Heat by Gaseous and Liquid Matter,” Professor Tyndall
showed that generally the absorption of non-luminous radiant heat by
vapours was the same as that of the liquids from which the vapours were
produced.
His fifth memoir, entitled “Contributions to Molecular Physics,” was
made the Bakerian lecture for that year. In it he deduced from numerous
experiments the remarkable law that the opacity of a substance with
respect to radiant heat from a source of comparatively low temperature
increases with the chemical complexity of its molecule. He examined
the effects of temperature on the transmission of radiant heat, the
radiation from flames of various kinds, and the influence of vibrating
periods on the absorption of radiant heat.
In November, 1864, the Royal Society presented him with the Rumford
medal for his researches on the absorption and radiation of heat by
gases and vapours; and General Sabine, in making the presentation, said
such had been the fate of Professor Tyndall that each last achievement
might almost be said to have dimmed the lustre of those which
preceded it. Curiously enough his very next achievements thereafter
did dim the lustre of those published prior to the presentation of the
Rumford Medal. It was the discovery of a means of separating light
from heat. Melloni had previously discovered a combination of screens
by which radiant heat could be arrested or separated from light,
an operation which is effected on a vast scale by the moon when it
reflects the light of the sun. Professor Tyndall effected the converse
operation. He discovered that a solution of iodine in bisulphide of
carbon entirely intercepted the light of the most brilliant flames.
A hollow prism filled with that opaque liquid and placed in the path
of the beam from an electric lamp, completely intercepted the light,
but transmitted the heat unimpaired. In this way he succeeded in
separating with marvellous sharpness the invisible from the visible
radiations of the lime light, the electric light, and the sun. He
not only produced combustion, fusion, and incandescence by invisible
radiation, but he proved that in the case of the electric light the
invisible rays are no less than eight times as powerful as the visible
radiations. He obtained all the colours of the solar spectrum from
a platinum foil raised to incandescence at the invisible focus; and
this rendering of a refractory body incandescent by invisible rays
he called _calorescence_. In connection with these investigations he
performed a daring experiment. Knowing that a layer of iodine placed
before the eye intercepted the light, he determined to place his own
eye in the focus of strong invisible rays. He knew that if in doing so
the dark rays were absorbed in a high degree by the humours of the eye,
the albumen of the humours might coagulate; and on the other hand, if
there was no high absorption, the rays might strike upon the retina
with a force sufficient to destroy it. When he first brought his eye,
undefended, near the dark focus, the heat on the parts surrounding the
pupil was too intense to be endured. He therefore made an aperture in a
plate of metal, and placing his eye behind this aperture, he gradually
approached the point of convergence of the invisible rays. First the
pupil and next the retina were placed in the focus without any sensible
damage. Immediately afterwards a sheet of platinum foil placed in the
position which the retina had occupied became red-hot.
In a subsequent memoir he dealt with the influence of colour and
mechanical condition upon radiant heat, demonstrating that white bodies
are far more potent absorbers of radiant heat than black ones.
During the first thirteen years of his researches in the laboratory of
the Royal Institution he produced thirteen papers, which were published
in the _Philosophical Transactions_. Conspicuous among these were his
papers on the radiation and absorption of heat, and his researches on
that subject have generally been admitted to be of the most thorough
and original character. A lucid epitome of the chief results he
obtained was given in the Rede lecture which he delivered before the
University of Cambridge in 1865, when the University conferred on him
the honorary degree of LL.D.
In 1863 he published the first edition of one of his most popular
books, _Heat Considered as a Mode of Motion_--a book which an eminent
electrician has recommended students of electricity to master; in
1867 he published a volume of lectures on “Sound”; and in 1869-74
he published his lectures on “Light.” These works have gone through
several editions. As an illustration of the interest with which he
can invest such impalpable subjects, it is worth remarking that a
Chinese official, named Hsii-chung-hu, was so pleased with the book on
Sound that he had it translated into the Chinese language and printed
at Shanghai, in order that his countrymen might participate in the
pleasure and instruction which he had derived from it. It was published
at the expense of the Chinese Government, and sold at 1_s._ 6_d._ a
copy.
During the ten years from 1859 to 1869, says Professor Tyndall,
“researches on radiant heat in its relations to the gaseous form of
matter occupied my continual attention.” But towards the close of that
period his main inquiry, as it extended into space, began to spread out
into various branches. In 1866 he entered upon an examination of the
chemical action of light upon vapours, and the action of heat of high
refrangibility as an explorer of the molecular condition of matter.
“In this investigation one obstacle to be overcome was the presence
of the floating matter in the air. The processes for the removal
of these particles became the occasion of an independent research,
branching out into various channels: on the one hand, it dealt with the
practical problem of the preservation of life among firemen exposed
to heated smoke; and, on the other, it approached the recondite
question of spontaneous generation. He subjected the compound vapours
of various substances to the action of a concentrated beam of light.
The vapours were decomposed, and non-volatile products were formed.
The decompositions always began with a blue cloud, which discharged
perfectly polarised light at right angles to the beam. This suggested
to him the origin of the blue colour of the sky; and as it showed the
extraordinary amount of light that may be scattered by cloudy matter
of extreme tenuity, he considered that it might be regarded as a
suggestion towards explaining the nature of a comet’s tail.”
Regions of cloud and smoke are proverbial as symbols of the negation
of human interest; but Professor Tyndall imparted new beauties to the
one and deprived the other of its terrors. He said to the chaotic
vapours “Light,” and that which was without form and void instantly
assumed the loveliest forms that Nature knows. Incredible as this
language may appear to some, it is no mere Oriental hyperbole. He
made the light from an electric lamp to pass through a great glass
tube containing transparent, invisible vapours, and the action of the
light at once commencing chemical decomposition, various cloud forms
resembling organic structures were seen in the tube. The following is
the beautiful description he gave to the Royal Society of the phenomena
presented by hydriodic acid:--
“The cloud extended for about eighteen inches along the tube, and
gradually shifted its position from the end nearest the lamp to the
most distant end. The portion quitted by the cloud proper was filled by
an amorphous haze, the decomposition, which was progressing lower down,
being here apparently complete. A spectral cone turned its apex towards
the distant end of the tube, and from its circular base filmy drapery
seemed to fall. Placed on the base of the cone was an exquisite vase,
from the interior of which sprang another vase of similar shape; over
the edges of these vases fell the faintest clouds, resembling spectral
sheets of liquid. From the centre of the upper vase a straight cord
of cloud passed for some distance along the axis of the experimental
tube, and at each end of this cord two involved and highly iridescent
vortices were generated. The frontal portion of the cloud which the
cord penetrated assumed in succession the form of roses, tulips,
and sunflowers. It also passed through the appearance of a series
of beautifully-shaped bottles placed one within the other. Once it
presented the shape of a fish, with eyes, gills, and feelers.”
In 1869 it was stated before the British Association that M. Morren,
while living in the South of France, had succeeded in producing similar
results by the use of sunlight instead of the electric light.
For a long time during his researches on the decomposition of vapours
he was troubled by the presence of floating matter revealed by a
powerful condensed beam of light, and he tried numerous expedients
for the purpose of intercepting this matter. At last he succeeded.
By causing the air intended for experimental purposes to pass over
the tip of a spirit-lamp flame, the floating matter disappeared. He
therefore concluded that it was organic matter, which had been burned
out by the flame. This discovery took place on October 5th, 1868. Till
then he regarded the dust of our air as for the most part inorganic
and noncombustible. This led him on to the investigation of the germ
theory. On the one hand he added proof to proof, and experiment to
experiment, to show that when a consuming heat was applied to air its
organic matter disappeared; and on the other hand he maintained that
as surely as a fig comes from a fig, a grape from a grape, and a thorn
from a thorn, so surely does the typhoid virus or seed, when planted or
scattered about among people, increase and multiply into typhoid fever,
scarlatina virus into scarlatina, and small-pox virus into small-pox.
These conclusions formed the subject of a famous lecture on “Dust and
Disease,” delivered at the Royal Institution on January 21st, 1870.
Among his audience were some of the foremost men of the day, such as
Mr. W. E. Gladstone, then Prime Minister, Earl Granville, Dean Stanley,
Sir Edwin Landseer, Sir Henry Holland, and Professor Huxley. The views
which Professor Tyndall then put forth were received with marked
disfavour among the medical profession. Even scientific men did not
hesitate to pour ridicule upon the germ theory. For example, Professor
Bloxam, Lecturer on Chemistry to the Department of Artillery Studies,
suggested in one of his lectures that the Committee on Explosives
should abandon gun cotton, and collecting the germs of small-pox
and similar malignant diseases in cotton or other dust-collecting
substances, should load shells with them, and we should then hear of
the enemy being dislodged from his position by a volley of typhus or
a few rounds of Asiatic cholera. Like most truths, the germ theory
survived the ridicule of its opponents.
The labours of Pasteur in relation to the germ theory always appeared
to command Professor Tyndall’s admiration. A large part of his lecture
on “Dust and Disease” consisted of an account of the successful way
in which Pasteur dealt with the epidemic among silkworms in France.
Writing in April, 1870, the Professor said: “There is more solid
science in one paper of Pasteur than in all the volumes and essays that
have been written against him. Schroeder and Pasteur have demonstrated
that air filtered through cotton-wool is deprived wholly, or in part,
of its power to produce animalcular life. Why? An experiment with a
beam of light answers the question; for while it proves our ordinary
air to be charged with floating matter, the beam pronounces air, which
has been carefully filtered through cotton-wool, to be visibly pure;
there are no germs afloat in it; hence it is impossible as a generator
of life. Again, Pasteur prepared twenty-one flasks, each containing
a decoction of yeast, which he boiled in order to destroy whatever
germs it might contain. While the space above the liquid was filled
with pure steam he sealed the necks of his flasks with a blow-pipe. He
opened ten of them in the damp, still caves of the Paris Observatory,
and eleven of them in the courtyard of the same establishment. Of the
former only one showed signs of life subsequently. In nine out of the
ten flasks no organisms of any kind were developed. In all the others
organisms speedily appeared. Pasteur ascribed this unexpected result to
the subsidence of the germs in the motionless air of the caves. Is this
surmise correct? The beam of light enables us to answer this question.
I have had a chamber constructed, the lower half of which is of wood,
and the upper half of glass. On the 6th February this chamber was
closed, and every crevice that could admit dust or cause a disturbance
of the air was carefully stopped. The electric beam when sent through
the glass showed the air at the outside to be loaded with floating
matter. The chamber was examined almost daily, and a gradual diminution
of the floating matter was observed. At the end of the week the chamber
was optically empty. The floating matters, germs included, had wholly
subsided, and the air held nothing in suspension. Here again the ocular
demonstration furnished by the luminous beam goes hand in hand with the
experimental result of Pasteur.”
Professor Tyndall did not, however, adopt the germ theory on the
authority of Pasteur. He not only discovered it for himself, but
demonstrated its accuracy by innumerable experiments, in the course
of which he made use of 10,000 vessels. To him, too, science owes
the use of the electric beam as an explorer of germ particles which
could not otherwise be made visible by the best optical aids. The most
exquisitely minute particles, which could not be detected by the most
powerful glasses, have been revealed in the air by the electric beam.
For some time he carried on a controversy with some doughty champions
of the old theory of spontaneous generation; but as the evidences in
favour of the germ theory increased, the antagonism to it diminished.
One practical evidence, not only of the reality, but of the utility
of the germ theory, was Pasteur’s discovery of the nature of the
organisms in yeast that produced “beer disease;” and when Pasteur
visited England, after that discovery, and explained the cause of beer
turning sour, Professor Tyndall afterwards visited some of the most
prominent breweries in London to make inquiries on the subject. He
was extremely surprised at the paucity of knowledge possessed by the
brewers, although they had over and over again incurred disastrous
losses in consequence of their lack of knowledge. He said that when the
brewers found their beer becoming bad they used to exchange their yeast
among themselves, and thus get on with their losses, when five minutes’
examination with the microscope would have prevented this waste and
loss; for it would have shown them the minute organisms which spoiled
the beer.
In connection with his researches on the germ theory, he produced a
useful invention which had a philanthropic rather than a commercial
object. To the title of inventor he never made any claim; on the
contrary, he repeatedly expressed his view of the difference between
a scientific discoverer and a mechanical inventor; contending that
while the practical man is not usually the man to make the necessary
antecedent discoveries, the cases are rare in which the discoverer in
science knows how to turn his labours to practical account.
Nevertheless scientific reflection enabled him to devise a form of
respirator which protects firemen from the stifling effects of dense
smoke. His attention had repeatedly been directed to the risks that
firemen encountered when in conflict with smoke and flame, and he
had been told that smoke was a greater enemy to them than flame.
He therefore endeavoured to find a means of protecting them from
suffocation. First he tried a respirator made of cotton-wool, but
that was insufficient; so to the cotton-wool he added glycerine; and
though this was an improvement, still it only enabled them to remain in
dense smoke for three or four minutes. He next added charcoal and this
greatly increased the utility of the respirator, which when complete
was composed of a layer of cotton-wool moistened with glycerine, next
a thin layer of dry wool, then a layer of charcoal fragments, succeeded
by another thin layer of dry cotton-wool and a layer of fragments of
caustic lime. These were inclosed in a wire gauze cover. The first
experiments with this respirator were made in a small cellar-like
chamber with stone flooring and stone walls in the basement of the
Royal Institution. A fire of resinous pine-wood was lighted, and
was so covered over as to generate dense smoke instead of flames.
Professor Tyndall and his assistant, having each put on one of the new
respirators, and suitable glasses to protect their eyes, were able to
remain for half an hour or longer in that apartment full of smoke so
dense and pungent that he believed a single inhalation through the
undefended mouth would have been perfectly unendurable. Captain Shaw,
the chief officer of the Metropolitan Fire Brigade, on being asked
whether such a respirator would be of use to him, replied that it
would be most valuable; but he had made himself acquainted with every
contrivance of the kind in this and other countries, and had found none
of them of any practical use. However, at the request of Professor
Tyndall, the Captain and some of his men went to the Royal Institution
to test the new invention. The small room was again filled with dense
smoke, three men went successively into it, and remained there as long
as their Captain desired. On coming out they declared that with the
respirators they had not felt the least discomfort, and that they could
have remained all day in the smoke. Captain Shaw himself then tested it
with the same result, and he afterwards stated that Professor Tyndall,
in the kindest possible manner, at once placed his invention at the
service of the Fire Brigade.
In 1870 he accompanied the eclipse expedition to Oran, and having been
disappointed in the special object of his journey, he determined in
returning to investigate the causes of the varying tints presented by
sea-water. On board H.M.S. _Urgent_, between Gibraltar and Spithead, he
filled nineteen bottles with sea-water, and afterwards examined them by
the electric light. This examination showed that the yellowish water
of the coast and harbours contained a large quantity of particles,
that in the green water the particles were finer and less abundant,
and that the blue water of the deep was comparatively clear of them.
The explanation he gave of the colours of the ocean, in a lecture at
the Royal Institution, was that when a beam of light entered the sea
the heat-rays were absorbed at the surface, the red rays by a very
superficial layer of water, the green rays next, and ultimately the
blue rays; but when the light encountered particles in the water the
green rays would be reflected by them. If there were no particles, the
green rays would continue their course till they were wholly quenched,
and thus water of more than ordinary depth and purity would appear as
black as ink.
In later years he made some practical additions to our knowledge
of sound. His advice had repeatedly been asked as to the laws
which affected the distribution of sound variously in different
buildings--a subject upon which volumes had been written, but which
was still imperfectly understood. As an illustration of the unexpected
circumstances that affected the transmission of sound, he sometimes
related what occurred to himself in the Senate House of Cambridge
University when he delivered the Rede lecture in 1865. On going to the
Senate House to test its acoustic qualities, he was astonished to find
that from the usual place of speaking his words could not be heard at
all by a friend whom he had placed at the extreme end of the hall as
his auditory. He found that the reverberation from the floor and walls
followed the direct sound of his voice in such a way as to destroy
the clearness of the words as they were uttered. Dismayed at this
effect, he made up his mind that in respect of audibleness his lecture
was doomed to be a failure. But the reverse was the case. The lecture
was in every respect a great success. An overflowing audience filled
the hall, and listened to him with rapt attention. During the hour
and a half that he spoke every syllable was heard by the most distant
hearer; and he attributed this unexpected result to the presence of
the audience, which, he said, quenched the prejudicial effect of the
reverberation of his voice produced by the sides and bottom of the
room. After that experience, he advocated the making of different
experiments with the view of extending the practical knowledge of
acoustics.
To that knowledge he himself became a valuable contributor. In 1873
he conducted a series of experiments with a view to determine the
properties of the atmosphere as a vehicle of sound. Navigators had
often been at a loss to understand how it was that the most powerful
fog-signals--such as gongs, whistles, and guns--were sometimes
easily heard at a great distance on rainy days, and were inaudible
at comparatively short distances on fine days. Even within a few
minutes the acoustic properties of the atmosphere sometimes underwent
remarkable variations. Professor Tyndall’s experiments led him to the
conclusion that the aqueous vapour raised by the sun, though often
invisible, produced a cloud which formed as impervious a barrier to
the waves of sound as a dense black cloud does to the waves of light.
The presence of water in a vaporous form being the real enemy to the
transmission of sound through the atmosphere, it was easy to understand
its frequent occurrence on days apparently clear and bright. This was
previously unknown.
He also furnished an interesting illustration of the corelation of heat
and sound.
Notwithstanding the elaborate data upon which he had founded his
conclusions as to the interaction of radiant heat on vapours, some
Continental physicists questioned their accuracy, and accordingly
Professor Tyndall in later years resumed the inquiry and obtained some
remarkable results. He had previously shown that heat will pass without
any loss through a long glass tube filled with nitrogen or air, and
closed up at the ends by lenses of crystal; but if the same tube is
filled with carbonic acid or the vapour of ether the heat, instead of
being transmitted through it, is almost entirely intercepted. In 1880
Mr. Graham Bell showed him that musical sounds were produced by a beam
of light striking upon thin discs of matter; and Professor Tyndall at
once discovered the secret of this surprising effect. He said that
before making an experiment he pictured in his mind a highly-absorbent
vapour exposed to the shocks of an intermittent beam suddenly expanding
at the moment of exposure, and as suddenly contracting when the beam
was intercepted; and thus pulses of an amplitude probably far greater
than those obtainable with solids would be produced, and would be
sufficient to give forth musical sounds. He soon proved this surmise
to be correct. He filled a glass tube or bulb with absorbent gas or
vapour, and between it and the limelight he placed a round piece of
cardboard with equi-distant holes in it; then by placing the bulb
in such a position that when the light passed through the holes it
impinged upon the glass bulb, and by causing the cardboard to revolve,
the action of the beam became intermittent, as it only reached the
vapour when one of the holes in the revolving cardboard came in front
of the bulb. By this contrivance a series of calorific shocks were
produced that gave sound vibrations of surprising intensity. When,
however, the bulbs were filled with gases or vapours, such as nitrogen
or air, that transmitted the heat, no sounds were produced. He tried
the sounding power of ten gases and eighty vapours, and found that
the sounds produced by chloride of methyl were the loudest; and that,
conveyed to the ear by a tube of indiarubber, they seemed as loud
as the peal of an organ. He also found that in respect of intensity
the order of the sound in gases was the same as the order of their
absorption of radiant heat. These marvellous results he described in
his Bakerian lecture for 1881, “On the Action of Free Molecules on
Radiant Heat and its Conversion thereby into Sound.”
FOOTNOTES:
[3] This glacier theory is all the more deserving of
prominence since the publication in 1886 of Lieutenant
Greely’s discovery of lakes, rivers, and valleys
rich in vegetation and animal life in the interior
of Grinnell Land at points the farthest north ever
reached by explorers.