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 color through the operation of the same cause; and Helmholtz
has irreverently disclosed the fact that a blue eye is simply a
turbid medium.
Numerous instances of the kind might be cited. The action of turbid
media upon light was fully and beautifully illustrated by Goethe,
who, though unacquainted with the undulatory theory, was led by his
experiments to regard the blue of the firmament as caused by 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. 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. No doubt the blueness
of certain Alpine lakes is in part due to this cause. Professor
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 favorably
observed, exhibits at a certain stage of its condensation the colors
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 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 colored
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 inquiries forward. Our best microscopes can readily reveal
objects not more than 1/50000 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 1/100000 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 behooves 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
color 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 eye 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
resin in Brücke’s medium, and when you have done so follow me. I
permitted a beam of light to act upon a certain vapor. 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
color 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
1/100000 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?
As the distances of stellar space give us simply a bewildering sense
of vastness without leaving any distinct impression on the mind, so
the magnitudes with which we have here to do impress us with a
bewildering sense of smallness. 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, then, to surround the earth at a
hight above the surface which would place it beyond the grosser
matter that hangs in the lower regions of the air—say at the hight
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 snuff-box—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 hight 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 this light? On points of controversy I will not here
enter, but I may say that 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. 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 travelers 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 conception of atoms and molecules to which we have so
frequently referred, a bit of matter, say 1/50000 of an inch in
diameter, will present itself differently. The one descends to it
from his molar hights, 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 defense and startling expansion of the doctrine will
appear perfectly conclusive, to the other it will present itself as
imposing a profitless labor of demolition on subsequent
investigators. The proper and possible attitude of these two men is
that each of them should work as if it were his aim and object to
establish the view entertained by the other.
I trust, Mr. President, that you—whom untoward circumstances have
made a biologist, but who still keep alive your sympathy with that
class of inquiries which nature intended you to pursue and
adorn—will excuse me to your brethren if I say that some of them
seem to form an inadequate estimate of the distance which separates
the microscopic from the molecular limit, and that, as a
consequence, they sometimes employ a phraseology which is calculated
to mislead.
When, for example, the contents of a cell are described as perfectly
homogeneous, as absolutely structureless, because the microscope
fails to distinguish any structure, then I think the microscope
begins to play a mischievous part. A little consideration will make
it plain to all of you that the microscope can have no voice in the
real question of germ structure. Distilled water is more perfectly
homogeneous than the contents of any possible organic germ. What
causes the liquid to cease contracting at 39° F., and to grow bigger
until it freezes? It is a structural process of which the microscope
can take no note, nor is it likely to do so by any conceivable
extension of its powers. Place this distilled water in the field of
an electro-magnet, and bring a microscope to bear upon it. Will any
change be observed when the magnet is excited? Absolutely none; and
still profound and complex changes have occurred.
First of all, the particles of water are rendered diamagnetically
polar; and secondly, in virtue of the structure impressed upon it by
the magnetic strain of its molecules, the liquid twists a ray of
light in a fashion perfectly determinate both as to quantity and
direction. It would be immensely interesting to both you and me if
one here present, who has brought his brilliant imagination to bear
upon this subject, could make us see as he sees the entangled
molecular processes involved in the rotation of the plane of
polarization by magnetic force. While dealing with this question he
lived in a world of matter and of motion to which the microscope has
no passport, and in which it can offer no aid. The cases in which
similar conditions hold are simply numberless. Have the diamond, the
amethyst, and the countless other crystals formed in the
laboratories of nature and of man, no structure? Assuredly they
have, but what can the microscope make of it? Nothing. It cannot be
too distinctly borne in mind that between the microscopic limit and
the true molecular limit there is room for infinite permutations and
combinations. It is in this region that the poles of the atoms are
arranged, that tendency is given to their powers, so that when these
poles and powers have free action and proper stimulus in a suitable
environment, they determine first the germ and afterwards the
complete organism. This first marshaling of the atoms on which all
subsequent action depends baffles a keener power than that of the
microscope. Through pure excess of complexity, and long before
observation can have any voice in the matter, the most highly
trained intellect, the most refined and disciplined imagination,
retires in bewilderment from the contemplation of the problem. We
are struck dumb by an astonishment which no microscope can relieve,
doubting not only the power of our instrument, but even whether we
ourselves possess the intellectual elements which will ever enable
us to grapple with the ultimate structural energies of nature.
But the speculative faculty, of which imagination forms so large a
part, will nevertheless wander into regions where the hope of
certainty would seem to be entirely shut out. We think that though
the detailed analysis may be, and may ever remain, beyond us,
general notions may be attainable. At all events, it is plain that
beyond the present outposts of microscopic inquiry lies an immense
field for the exercise of the imagination. It is only, however, the
privileged spirits who know how to use their liberty without abusing
it, who are able to surround imagination by the firm frontiers of
reason, that are likely to work with any profit here. But freedom to
them is of such paramount importance that, for the sake of securing
it, a good deal of wildness on the part of weaker brethren may be
overlooked. In more senses than one Mr. Darwin has drawn heavily
upon the scientific tolerance of his age. He has drawn heavily upon
_time_ in his development of species, and he has drawn adventurously
upon _matter_ in his theory of pan-genesis. According to this
theory, a germ already microscopic is a world of minor germs. Not
only is the organism as a whole wrapped up in the germ, but every
organ of the organism has there its special seed.
This, I say, is an adventurous draft on the power of matter to
divide itself and distribute its forces. But, unless we are
perfectly sure that he is overstepping the bounds of reason, that he
is unwittingly sinning against observed fact or demonstrated law—for
a mind like that of Darwin can never sin wittingly against either
fact or law—we ought, I think, to be cautious in limiting his
intellectual horizon. If there be the least doubt in the matter, it
ought to be given in favor of the freedom of such a mind. To it a
vast possibility is in itself a dynamic power, though the
possibility may never be drawn upon.
It gives me pleasure to think that the facts and reasonings of this
discourse tend rather towards the justification of Mr. Darwin than
towards his condemnation, that they tend rather to augment than to
diminish the cubic space demanded by this soaring speculator; for
they seem to show the perfect competence of matter and force, as
regards divisibility and distribution, to bear the heaviest strain
that he has hitherto imposed upon them.