mystery also. The cloud takes no note of size on the part of the waves
of ether, but reflects them all alike. It exercises no selective
action. Now the cause of this may be that the cloud particles are so
large in comparison with the size of the waves of ether as to reflect
them all indifferently. A broad cliff reflects an Atlantic roller as
easily as a ripple produced by a sea-bird’s wing; and in the presence
of large reflecting surfaces, the existing differences of magnitude
among the waves of ether may disappear. But supposing the reflecting
particles, instead of being very large, to be very small, in comparison
with the size of the waves. In this case, instead of the whole wave
being fronted and in great part thrown back, a small portion only is
shivered off. The great mass of the wave passes over such a particle
without reflection. Scatter then a handful of such minute foreign
particles in our atmosphere, and set imagination to watch their action
upon the solar waves. Waves of all sizes impinge upon the particles,
and you see at every collision a portion of the impinging wave struck
off by reflection. All the waves of the spectrum, from the extreme red
to the extreme violet, are thus acted upon. But in what proportions
will the waves be scattered? A clear picture will enable us to
anticipate the experimental answer. Remembering that the red waves are
to the blue much in the relation of billows to ripples, let us consider
whether those extremely small particles are competent to scatter
all the waves in the same proportion. If they be not--and a little
reflection will make it clear to you that they are not--the production
of colour must be an incident of the scattering. Largeness is a thing
of relation; and the smaller the wave, the greater is the relative
size of any particle on which the wave impinges, and the greater also
the ratio of the reflected portion to the total wave. A pebble placed
in the way of the ring-ripples produced by our heavy rain-drops on a
tranquil pond will throw back a large fraction of the ripple incident
upon it, while the fractional part of a larger wave thrown back by the
same pebble might be infinitesimal. Now we have already made it clear
to our minds that to preserve the solar light white, its constituent
proportions must not be altered; but in the act of division performed
by these very small particles we see that the proportions are altered;
an undue fraction of the smaller waves is scattered by the particles,
and, as a consequence, in the scattered light, blue will be the
predominant colour. The other colours of the spectrum must, to some
extent, be associated with the blue. They are not absent but deficient.
We ought, in fact, to have them all, but in diminishing proportions,
from the violet to the red.
We have here presented a case to the imagination, and, assuming the
undulatory theory to be a reality, we have, I think, fairly reasoned
our way to the conclusion, that were particles, small in comparison
to the size of the ether waves, sown in our atmosphere, the light
scattered by those particles would be exactly such as we observe in
our azure skies. When this light is analysed, all the colours of the
spectrum are found; but they are found in the proportions indicated by
our conclusion.
Let us now turn our attention to the light which passes unscattered
among the particles. How must it be finally affected? By its successive
collisions with the particles the white light is more and more robbed
of its shorter waves; it therefore loses more and more of its due
proportion of blue. The result may be anticipated. The transmitted
light, where short distances are involved, will appear yellowish.
But as the sun sinks towards the horizon the atmospheric distances
increase, and consequently the number of the scattering particles.
They abstract in succession the violet, the indigo, the blue, and even
disturb the proportions of green. The transmitted light under such
circumstances must pass from yellow through orange to red. This also is
exactly what we find in nature. Thus, while the reflected light gives
us at noon the deep azure of the Alpine skies, the transmitted light
gives us at sunset the warm crimson of the Alpine snows. The phenomena
certainly occur _as if_ our atmosphere were a medium rendered
slightly turbid by the mechanical suspension of exceedingly small
foreign particles.
Here, as before, we encounter our sceptical ‘_as if_.’ It is
one of the parasites of science, ever at hand, and ready to plant
itself and sprout, if it can, on the weak points of our philosophy.
But a strong constitution defies the parasite, and in our case, as we
question the phenomena, probability grows like growing health, until
in the end the malady of doubt is completely extirpated. The first
question that naturally arises is--Can small particles be really
proved to act in the manner indicated? No doubt of it. Each one of
you can submit the question to an experimental test. Water will not
dissolve resin, but spirit will; and when spirit which holds resin in
solution is dropped into water, the resin immediately separates in
solid particles, which render the water milky. The coarseness of this
precipitate depends on the quantity of the dissolved resin. You can
cause it to separate in thick clots or in exceedingly fine particles.
Professor Brücke has given us the proportions which produce particles
particularly suited to our present purpose. One gramme of clean mastic
is dissolved in eighty-seven grammes of absolute alcohol, and the
transparent solution is allowed to drop into a beaker containing clear
water kept briskly stirred. An exceedingly fine precipitate is thus
formed, which declares its presence by its action upon light. Placing a
dark surface behind the beaker, and permitting the light to fall into
it from the top or front, the medium is seen to be distinctly blue. It
is not perhaps so perfect a blue as I have seen on exceptional days,
this year, among the Alps, but it is a very fair sky-blue. A trace of
soap in water gives a tint of blue. London, and I fear Liverpool milk,
makes an approximation to the same colour through the operation of the
same cause; and Helmholtz has irreverently disclosed the fact that a
blue eye is simply a turbid medium.
The action of turbid media upon light was illustrated by Goethe,
who, though unacquainted with the undulatory theory, was led by his
experiments to regard the firmament as an illuminated turbid medium
with the darkness of space behind it. He describes glasses showing a
bright yellow by transmitted, and a beautiful blue by reflected light.
Professor Stokes, who was probably the first to discern the real nature
of the action of small particles on the waves of ether, describes a
glass of a similar kind.[3] Capital specimens of such glass are to be
found at Salviati’s in St. James’s Street. What artists call ‘chill’ is
no doubt an effect of this description. Through the action of minute
particles, the browns of a picture often present the appearance of
the bloom of a plum. By rubbing the varnish with a silk handkerchief
optical continuity is established, and the chill disappears. Some
years ago I witnessed Mr. Hirst experimenting at Zermatt on the turbid
water of the Visp, which was charged with the finely divided matter
ground down by the glaciers. When kept still for a day or so, the
grosser matter sank, but the finer matter remained suspended, and gave
a distinctly blue tinge to the water. The blueness of certain Alpine
lakes has been shown to be in part due to this cause. Prof. Roscoe has
noticed several striking cases of a similar kind. In a very remarkable
paper the late Principal Forbes showed that steam issuing from the
safety-valve of a locomotive, when favourably observed, exhibits at a
certain stage of its condensation the colours of the sky. It is blue by
reflected light, and orange or red by transmitted light. The effect,
as pointed out by Goethe, is to some extent exhibited by peat smoke.
More than ten years ago I amused myself at Killarney by observing on
a calm day the straight smoke-columns rising from the chimneys of the
cabins. It was easy to project the lower portion of a column against a
dark pine, and its upper portion against a bright cloud. The smoke in
the former case was blue, being seen mainly by reflected light; in the
latter case it was reddish, being seen mainly by transmitted light.
Such smoke was not in exactly the condition to give us the glow of the
Alps, but it was a step in this direction. Brücke’s fine precipitate
above referred to looks yellowish by transmitted light, but by duly
strengthening the precipitate you may render the white light of noon
as ruby-coloured as the sun when seen through Liverpool smoke, or upon
Alpine horizons. I do not, however, point to the gross smoke arising
from coal as an illustration of the action of small particles, because
such smoke soon absorbs and destroys the waves of blue instead of
sending them to the eyes of the observer.
These multifarious facts, and numberless others which cannot now be
referred to, are explained by reference to the single principle, that
where the scattering particles are small in comparison to the size
of the waves we have in the reflected light a greater proportion of
the smaller waves, and in the transmitted light a greater proportion
of the larger waves, than existed in the original white light. The
physiological consequence is that in the one light blue is predominant,
and in the other light orange or red. And now let us push our enquiries
forward. Our best microscopes can readily reveal objects not more than
¹⁄₅₀₀₀₀th of an inch in diameter. This is less than the length of a
wave of red light. Indeed a first-rate microscope would enable us to
discern objects not exceeding in diameter the length of the smallest
waves of the visible spectrum. By the microscope therefore we can
submit our particles to an experimental test. If they are as large as
the light-waves they will infallibly be seen; and if they are not seen
it is because they are smaller. I placed in the hands of our President
a bottle containing Brücke’s particles in greater number and coarseness
than those examined by Brücke himself. The liquid was a milky blue, and
Mr. Huxley applied to it his highest microscopic power. He satisfied
me at the time that had particles of even ¹⁄₁₀₀₀₀₀th of an inch in
diameter existed in the liquid they could not have escaped detection.
But no particles were seen. Under the microscope the turbid liquid was
not to be distinguished from distilled water. Brücke, I may say, also
found the particles to be of ultra-microscopic magnitude.
But we have it in our power to imitate far more closely than we have
hitherto done the natural conditions of this problem. We can generate
in air, as many of you know, artificial skies, and prove their perfect
identity with the natural one, as regards the exhibition of a number
of wholly unexpected phenomena. By a continuous process of growth
moreover we are able to connect sky-matter, if I may use the term,
with molecular matter on the one side, and with molar matter, or
matter in sensible masses, on the other. In illustration of this, I
will take an experiment described by M. Morren of Marseilles at the
last meeting of the British Association. Sulphur and oxygen combine to
form sulphurous acid gas. It is this choking gas that is smelt when a
sulphur match is burnt in air. Two atoms of oxygen and one of sulphur
constitute the molecule of sulphurous acid. Now it has been recently
shown in a great number of instances that waves of ether issuing from
a strong source, such as the sun or the electric light, are competent
to shake asunder the atoms of gaseous molecules. A chemist would call
this ‘decomposition’ by light; but it behoves us, who are examining
the power and function of the imagination, to keep constantly before
us the physical images which we hold to underlie our terms. Therefore
I say, sharply and definitely, that the components of the molecules
of sulphurous acid are shaken asunder by the ether waves. Enclosing
the substance in a suitable vessel, placing it in a dark room, and
sending through it a powerful beam of light, we at first see nothing:
the vessel containing the gas is as empty as a vacuum. Soon, however,
along the track of the beam a beautiful sky-blue colour is observed,
which is due to the liberated particles of sulphur. For a time the blue
grows more intense: it then becomes whitish; and from a whitish blue
it passes to a more or less perfect white. If the action be continued
long enough, we end by filling the tube with a dense cloud of sulphur
particles, which by the application of proper means may be rendered
visible.
Here then our ether waves untie the bond of chemical affinity, and
liberate a body--sulphur--which at ordinary temperatures is a solid,
and which therefore soon becomes an object of the senses. We have
first of all the free atoms of sulphur, which are both invisible and
incompetent to stir the retina sensibly with scattered light. But
these atoms gradually coalesce and form particles, which grow larger
by continual accretion until after a minute or two they appear as
sky-matter. In this condition they are invisible themselves, but
competent to send an amount of wave-motion to the retina sufficient to
produce the firmamental blue. The particles continue, or may be caused
to continue, in this condition for a considerable time, during which no
microscope can cope with them. But they continually grow larger, and
pass by insensible gradations into the state of _cloud_, when they
can no longer elude the armed eye. Thus without solution of continuity
we start with matter in the molecule, and end with matter in the mass,
sky-matter being the middle term of the series of transformations.
Instead of sulphurous acid, we might choose from a dozen other
substances, and produce the same effect with any of them. In the case
of some--probably in the case of all--it is possible to preserve
matter in the skyey condition for fifteen or twenty minutes under
the continual operation of the light. During these fifteen or twenty
minutes the particles are constantly growing larger, without ever
exceeding the size requisite to the production of the celestial blue.
Now when two vessels are placed before you, each containing sky-matter,
it is possible to state with great distinctness which vessel contains
the largest particles. The retina is very sensitive to differences
of light, when, as here, the eye is in comparative darkness, and when
the quantities of wave-motion thrown against the retina are small.
The larger particles declare themselves by the greater whiteness of
their scattered light. Call now to mind the observation, or effort
at observation, made by our President, when he failed to distinguish
the particles of mastic in Brücke’s medium, and when you have done so
follow me. I permitted a beam of light to act upon a certain vapour.
In two minutes the azure appeared, but at the end of fifteen minutes
it had not ceased to be azure. After fifteen minutes, for example,
its colour, and some other phenomena, pronounced it to be a blue of
distinctly smaller particles than those sought for in vain by Mr.
Huxley. These particles, as already stated, must have been less than
¹⁄₁₀₀₀₀₀th of an inch in diameter. And now I want you to submit to
your imagination the following question: Here are particles which have
been growing continually for fifteen minutes, and at the end of that
time are demonstrably smaller than those which defied the microscope
of Mr. Huxley:--_what must have been the size of these particles at
the beginning of their growth?_ What notion can you form of the
magnitude of such particles? The distances of stellar space give us
simply a bewildering sense of vastness without leaving any distinct
impression on the mind, and the magnitudes with which we have here to
do bewilder us equally in the opposite direction. We are dealing with
infinitesimals compared with which the test objects of the microscope
are literally immense.
From their perviousness to stellar light, and other considerations,
Sir John Herschel drew some startling conclusions regarding the
density and weight of comets. You know that these extraordinary and
mysterious bodies sometimes throw out tails 100,000,000 of miles in
length, and 50,000 miles in diameter. The diameter of our earth is
8,000 miles. Both it and the sky, and a good portion of space beyond
the sky, would certainly be included in a sphere 10,000 miles across.
Let us fill this sphere with cometary matter, and make it our unit
of measure. An easy calculation informs us that to produce a comet’s
tail of the size just mentioned about 300,000 such measures would
have to be emptied into space. Now suppose the whole of this stuff
to be swept together, and suitably compressed, what do you suppose
its volume would be? Sir John Herschel would probably tell you that
the whole mass might be carted away at a single effort by one of your
dray-horses. In fact, I do not know that he would require more than
a small fraction of a horse-power to remove the cometary dust. After
this you will hardly regard as monstrous a notion I have sometimes
entertained concerning the quantity of matter in our sky. Suppose a
shell to surround the earth at a height above the surface which would
place it beyond the grosser matter that hangs in the lower regions of
the air--say at the height of the Matterhorn or Mont Blanc. Outside
this shell we have the deep blue firmament. Let the atmospheric space
beyond the shell be swept clean, and let the sky-matter be properly
gathered up. What is its probable amount? I have sometimes thought that
a lady’s portmanteau would contain it all. I have thought that even a
gentleman’s portmanteau--possibly his snuffbox--might take it in. And
whether the actual sky be capable of this amount of condensation or
not, I entertain no doubt that a sky quite as vast as ours, and as good
in appearance, could be formed from a quantity of matter which might be
held in the hollow of the hand.
Small in mass, the vastness in point of number of the particles of
our sky may be inferred from the continuity of its light. It is not
in broken patches, nor at scattered points that the heavenly azure
is revealed. To the observer on the summit of Mont Blanc the blue
is as uniform and coherent as if it formed the surface of the most
close-grained solid. A marble dome would not exhibit a stricter
continuity. And Mr. Glaisher will inform you that if our hypothetical
shell were lifted to twice the height of Mont Blanc above the earth’s
surface, we should still have the azure overhead. Everywhere through
the atmosphere those sky-particles are strewn. They fill the Alpine
valleys, spreading like a delicate gauze in front of the slopes of
pine. They sometimes so swathe the peaks with light as to abolish
their definition. This year I have seen the Weisshorn thus dissolved
in opalescent air. By proper instruments the glare thrown from the
sky-particles against the retina may be quenched, and then the mountain
which it obliterated starts into sudden definition. Its extinction in
front of a dark mountain resembles exactly the withdrawal of a veil. It
is the light then taking possession of the eye, and not the particles
acting as opaque bodies, that interfere with the definition. By day
this light quenches the stars; even by moonlight it is able to exclude
from vision all stars between the fifth and the eleventh magnitude.
It may be likened to a noise, and the stellar radiance to a whisper
drowned by the noise. What is the nature of the particles which shed
light? The celebrated De la Rive ascribes the haze of the Alps in
fine weather to floating organic germs. Now the possible existence
of germs in such profusion has been held up as an absurdity. It has
been affirmed that they would darken the air, and on the assumed
impossibility of their existence in the requisite numbers, without
invasion of the solar light, a powerful argument has been based by
believers in spontaneous generation. Similar arguments have been used
by the opponents of the germ theory of epidemic disease, and both
parties have triumphantly challenged an appeal to the microscope and
the chemist’s balance to decide the question. Such arguments are
absolutely valueless. Without committing myself in the least to De la
Rive’s notion, without offering any objection here to the doctrine of
spontaneous generation, without expressing any adherence to the germ
theory of disease, I would simply draw attention to the fact that in
the atmosphere we have particles which defy both the microscope and the
balance, which do not darken the air, and which exist, nevertheless,
in multitudes sufficient to reduce to insignificance the Israelitish
hyperbole regarding the sands upon the seashore.
The varying judgments of men on these and other questions may perhaps
be, to some extent, accounted for by that doctrine of Relativity which
plays so important a part in philosophy. This doctrine affirms that
the impressions made upon us by any circumstance, or combination of
circumstances, depends upon our previous state. Two travellers upon
the same peak, the one having ascended to it from the plain, the other
having descended to it from a higher elevation, will be differently
affected by the scene around them. To the one nature is expanding, to
the other it is contracting, and feelings are sure to differ which
have two such different antecedent states. In our scientific judgments
the law of relativity may also play an important part. To two men, one
educated in the school of the senses, who has mainly occupied himself
with observation, and the other educated in the school of imagination
as well, and exercised in the conceptions of atoms and molecules to
which we have so frequently referred, a bit of matter, say ¹⁄₅₀₀₀₀th of
an inch in diameter, will present itself differently. The one descends
to it from his molar heights, the other climbs to it from his molecular
lowlands. To the one it appears small, to the other large. So also as
regards the appreciation of the most minute forms of life revealed by
the microscope. To one of these men they naturally appear conterminous
with the ultimate particles of matter, and he readily figures the
molecules from which they directly spring; with him there is but a
step from the atom to the organism. The other discerns numberless
organic gradations between both. Compared with his atoms, the smallest
vibrios and bacteria of the microscopic field are as behemoth and
leviathan. The law of relativity may to some extent explain the
different attitudes of these two men with regard to the question of
spontaneous generation. An amount of evidence which satisfies the
one entirely fails to satisfy the other; and while to the one the
last bold defence and startling expansion of the doctrine will appear
perfectly conclusive, to the other it will present itself as imposing a
profitless labour of demolition on subsequent investigators.[4]