protuberance was strong or weak according to the position of the
prism. The summit also underwent striking changes. In one position
of the prism it exhibited a pale white against a dark background; in
the rectangular position it was a dark mauve against a light
background. The red of the Matterhorn changed in a similar manner;
but the whole mountain also passed through wonderful changes of
definition. The air at the time was filled with a silvery haze, in
which the Matterhorn almost disappeared. This could be wholly
quenched by the Nicol, and then the mountain sprang forth with
astonishing solidity and detachment from the surrounding air. The
changes of the Dom were still more wonderful. A vast amounts of light
could be removed from the sky behind it, for it occupied the position
of maximum polarisation. By a little practice with the Nicol it was
easy to render the extinction of the light, or its restoration, almost
instantaneous. When the sky was quenched, the four minor peaks and
buttresses, and the summit of the Dom, together with the shoulder of
the Alphubel, glowed as if set suddenly on fire. This was immediately
dimmed by turning the Nicol through an angle of 90°. It was not the
stoppage of the light of the sky behind the mountains alone which
produced this startling effect; the air between them and me was highly
opalescent, and the quenching of this intermediate glare augmented
remarkably the distinctness of the mountains.
On the morning of August 24 similar effects were finely shown. At 10
A.M. all three mountains, the Dom, the Matterhorn, and the Weisshorn,
were powerfully affected by the Nicol. But in this instance also, the
line drawn to the Dom being very nearly perpendicular to the solar
beams, the effects on this mountain were most striking. The grey
summit of the Matterhorn, at the same time, could scarcely be
distinguished from the opalescent haze around it; but when the Nicol
quenched the haze, the summit became instantly isolated, and stood out
in bold definition. It is to be remembered that in the production of
these effects the only things changed are the sky behind, and the
luminous haze in front of the mountains; that these are changed
because the light emitted from the sky and from the haze is plane
polarised light, and that the light from the snows and from the
mountains, being sensibly unpolarised, is not directly affected by the
Nicol. It will also be understood that it is not the interposition of
the haze _as an opaque body_ that renders the mountains indistinct, but
that it is the _light_ of the haze which dims and bewilders the eye, and
thus weakens the definition of objects seen through it.
These results have a direct bearing upon what artists call 'aerial
perspective.' As we look from the summit of Mont Blanc, or from a
lower elevation, at the serried crowd of peaks, especially if the
mountains be darkly coloured--covered with pines, for example--every
peak and ridge is separated from the mountains behind it by a thin
blue haze which renders the relations of the mountains as to distance
unmistakable. When this haze is regarded through the Nicol
perpendicular to the sun's rays, it is in many cases wholly quenched,
because the light which it emits in this direction is wholly
polarised. When this happens, aerial perspective is abolished, and
mountains very differently distant appear to rise in the same vertical
plane. Close to the Bel Alp for instance, is the gorge of the Massa,
and beyond the gorge is a high ridge darkened by pines. This ridge
may be projected upon the dark slopes at the opposite side of the
Rhone valley, and between both we have the blue haze referred to,
throwing the distant mountains far away. But at certain hours of the
day the haze may be quenched, and then the Massa ridge and the
mountains beyond the Rhone seem almost equally distant from the eye.
The one appears, as it were, a vertical continuation of the other. The
haze varies with the temperature and humidity of the atmosphere. At
certain times and places it is almost as blue as the sky itself; but
to see its colour, the attention must be withdrawn from the mountains
and from the trees which cover them. In point of fact, the haze is a
piece of more or less perfect sky; it is produced in the same manner,
and is subject to the same laws, as the firmament itself. We live _in_
the sky, not _under_ it.
These points were further elucidated by the deportment of the
selenite, plate, with which the readers of the foregoing pages
are so well acquainted. On some of the sunny days of August the
haze in the valley of the Rhone, as looked at from the Bel Alp,
was very remarkable. Towards evening the sky above the mountains
opposite to my place of observation yielded a series of the most
splendidly-coloured iris-rings; but on lowering the selenite until it
had the darkness of the pines at the opposite side of the Rhone
'valley, instead of the darkness of space, as a background, the
colours were not much diminished in brilliancy. I should estimate the
distance across the valley, as the crow flies, to the opposite
mountain, at nine miles; so that a body of air of this thickness can,
under favourable circumstances, produce chromatic effects of
polarisation almost as vivid as those produced by the sky itself.
Again: the light of a landscape, as of most other things, consists of
two parts; the one, coming purely from superficial reflection, is
always of the same colour as the light which falls upon the landscape;
the other part reaches us from a certain depth within the objects
which compose the landscape, and it is this portion of the total light
which gives these objects their distinctive colours. The white light
of the sun enters all substances to a certain depth, and is partly
ejected by internal reflection; each distinct substance absorbing and
reflecting the light, in accordance with the laws of its own molecular
constitution. Thus the solar light is _sifted_ by the landscape, which
appears in such colours and variations of colour as, after the sifting
process, reach the observer's eye. Thus the bright green of grass, or
the darker colour of the pine, never comes to us alone, but is always
mingled with an amounts of light derived from superficial reflection.
A certain hard brilliancy is conferred upon the woods and meadows by
this superficially-reflected light. Under certain circumstances, it
may be quenched by a Nicol's prism, and we then obtain the true colour
of the grass and foliage. Trees and meadows, thus regarded, exhibit a
richness and softness of tint which they never show as long as the
superficial light is permitted to mingle with the true interior
emission. The needles of the pines show this effect very well,
large-leaved trees still better; while a glimmering field of maize
exhibits the most extraordinary variations when looked at through the
rotating Nicol.
Thoughts and questions like those here referred to took me, in August
1869, to the top of the Aletschhorn. The effects described in the
foregoing paragraphs were for the most part reproduced on the summit
of the mountain. I scanned the whole of the sky with my Nicol. Both
alone, and in conjunction with the selenite, it pronounced the
perpendicular to the solar beams to be the direction of maximum
polarisation.
But at no portion of the firmament was the polarisation complete. The
artificial sky produced in the experiments recorded in the preceding
pages could, in this respect, be rendered far more perfect than the
natural one; while the gorgeous 'residual blue' which makes its
appearance when the polarisation of the artificial sky ceases to be
perfect, was strongly contrasted with the lack-lustre hue which, in
the case of the firmament, outlived the extinction of the brilliancy.
With certain substances, however, artificially treated, this dull
residue may also be obtained.
All along the arc from the Matterhorn to Mont Blanc the light of the
sky immediately above the mountains was powerfully acted upon by the
Nicol. In some cases the variations of intensity were astonishing. I
have already said that a little practice enables the observer to shift
the Nicol from one position to another so rapidly as to render the
alternative extinction and restoration of the light immediate. When
this was done along the arc to which I have referred, the alternations
of light and darkness resembled the play of sheet lightning behind the
mountains. There was an element of awe connected with the suddenness
with which the mighty masses, ranged along the line referred to,
changed their aspect and definition under the operation of the prism.
*****
The physical reason of the blueness of both natural and artificial
skies is, I trust, correctly given in the essay on the Scientific use
of the Imagination published in the second volume of these Fragments.
********************
V. ON DUST AND DISEASE.
[Footnote: A discourse delivered before the Royal Institution of Great
Britain, January 21, 1870.]
Experiments on Dusty Air.
SOLAR light, in passing through a dark room, reveals its track by
illuminating the dust floating in the air. 'The sun,' says Daniel
Culverwell, 'discovers atomes, though they be invisible by
candle-light, and makes them dance naked in his beams.'
In my researches on the decomposition of vapours by light, I was
compelled to remove these 'atoms' and this dust. It was essential that
the space containing the vapours should embrace no visible thing--that
no substance capable of scattering light in the slightest sensible
degree should, at the outset of an experiments, be found in the wide
'experimental tube' in which the vapour was enclosed.
For a long time I was troubled by the appearance there of floating
matter, which, though invisible in diffuse daylight, was at once
revealed by a powerfully condensed beam. Two U-tubes were placed in
succession in the path of the air, before it entered the liquid whose
vapour was to be carried into the experimental tube. One of the
U-tubes contained fragments of marble wetted with a strong solution of
caustic potash; the other, fragments of glass wetted with concentrated
sulphuric acid which, while yielding no vapour of its own, powerfully
absorbs the aqueous vapour of the air. [Footnote: The apparatus is
figured in Fig. 3.] To my astonishment, the air of the Royal
Institution, sent through these tubes at a rate sufficiently slow to
dry it, and to remove its carbonic acid, carried into the experimental
tube a considerable amounts of mechanically suspended matter, which
was illuminated when the beam passed through the tube. The effect was
substantially the same when the air was permitted to bubble through
the liquid acid, and through the solution of potash.
I tried to intercept this floating matter in various ways; and on
October 5, 1868, prior to sending the air through the drying
apparatus, it was carefully permitted to pass over the tip of a
spirit-lamp flame. The floating matter no longer appeared, having
been burnt up by the flame. It was therefore _organic matter_. I was
by no means prepared for this result; having previously thought that
the dust of our air was, in great part, inorganic and non-combustible.
[Footnote: According to an analysis kindly furnished to me by Dr.
Percy, the dust collected _from the walls_ of the British Museum
contains fully 50 per cent. of inorganic matter. I have every
confidence in the results of this distinguished chemist; they show
that the _floating_ dust of our rooms is, as it were, winnowed from the
heavier matter. As bearing directly upon this point I may quote the
following passage from Pasteur: 'Mais ici se présente une remarque: la
poussière que Pon trouve à la surface de tous les corps est soumise
constamment à des courants d'air, qui doivent soulever des particules
les plus légères, au nombre desquelles se trouvent, sans doute, de
préférence les corpuscules organisés, oeufs ou spores, moins lourds
généralement que les particules minérales.']
I had constructed a small gas-furnace, now much employed by chemists,
containing a platinum tube, which could be heated to vivid redness.
[Footnote: Pasteur was, I believe, the first to employ such a tube.]
The tube contained a roll of platinum gauze, which, while it permitted
the air to pass through it, ensured the practical contact of the dust
with the incandescent metal. The air of the laboratory was permitted
to enter the experimental tube, sometimes through the cold, and
sometimes through the heated, tube of platinum. In the first column
of the following fragment of a long table the quantity of air operated
on is expressed by the depression of the mercury gauge of the
air-pump. In the second column the condition of the platinum tube is
mentioned, and in the third the state of the air in the experimental
tube.
Quantity of air State of platinum tube State of experimental tube
15 inches Cold Full of particles.
30 inches Red-hot Optically empty.
The phrase 'optically empty' shows that when the conditions of perfect
combustion were present, the floating matter totally disappeared.
*****
In a cylindrical beam, which strongly illuminated the dust of the
laboratory, I placed an ignited spirit-lamp. Mingling with the flame,
and round its rim, were seen curious wreaths of darkness resembling an
intensely black smoke. On placing the flame at some distance below
the beam, the same dark masses stormed upwards. They were blacker
than the blackest smoke ever seen issuing from the funnel of a
steamer; and their resemblance to smoke was so perfect as to lead the
most practised observer to conclude that the apparently pure flame of
the alcohol lamp required but a beam of sufficient intensity to reveal
its clouds of liberated carbon. But is the blackness smoke? This
question presented itself in a moment and was thus answered: A red-hot
poker was placed underneath the beam: from it the black wreaths also
ascended. A large hydrogen flame was next employed, and it produced
those whirling masses of darkness, far more copiously than either the
spirit-flame or poker. Smoke was therefore out of the question.
[Footnote: In none of the public rooms of the United States where I
had the honour to lecture was this experiment made. The organic dust
was too scanty. Certain rooms in England--the Brighton Pavilion, for
example--also lack the necessary conditions.]
What, then, was the blackness? It was simply that of stellar space;
that is to say, blackness resulting from the absence from the track of
the beam of all matter competent to scatter its light. When the flame
was placed below the beam the floating matter was destroyed _in situ_;
and the air, freed from this matter, rose into the beam, jostled aside
the illuminated particles, and substituted for their light the
darkness due to its own perfect transparency. Nothing could more
forcibly illustrate the invisibility of the agent which renders all
things visible. The beam crossed, unseen, the black chasm formed by
the transparent air, while, at both sides of the gap, the thick-strewn
particles shone out like a luminous solid under the powerful
illumination.
It is not, however, necessary to burn the particles to produce a
stream of darkness. Without actual combustion, currents may be
generated which shall displace the floating matter, and appear dark
amid the surrounding brightness. I noticed this effect first on
placing a red-hot copper ball below the beam, and permitting it to
remain there until its temperature had fallen below that of boiling
water. The dark currents, though much enfeebled, were still produced.
They may also be produced by a flask filled with hot water.
To study this effect a platinum wire was stretched across the beam,
the two ends of the wire being connected with the two poles of a
voltaic battery. To regulate the strength of the current a rheostat
was placed in the circuit. Beginning with a feeble current the
temperature of the wire was gradually augmented; but long before it
reached the heat of ignition, a flat stream of air rose from it, which
when looked at edgeways appeared darker and sharper than one of the
blackest lines of Fraunhofer in the purified spectrum. Right and left
of this dark vertical band the floating matter rose upwards, bounding
definitely the non-luminous stream of air. What is the explanation?
Simply this: The hot wire rarefied the air in contact with it, but it
did not equally lighten the floating matter. The convection current
of pure air therefore passed upwards among the inert particles,
dragging them after it right and left, but forming between them an
impassable black partition. This elementary experiments enables us to
render an account of the dark currents produced by bodies at a
temperature below that of combustion.
But when the platinum wire is intensely heated, the floating matter is
not only displaced, but destroyed. I stretched a wire about 4 inches
long through the air of an ordinary glass shade resting on
cotton-wool, which also surrounded the rim. The wire being raised to
a white heat by an electric current, the air expanded, and some of it
was forced through the cotton-wool. When the current was interrupted,
and the air within the shade cooled, the returning air did not carry
motes along with it, being filtered by the wool. At the beginning of
this experiments the shade was charged with floating matter; at the
end of half an hour it was optically empty.
On the wooden base of a cubical glass shade, a cubic foot in volume,
upright supports were fixed, and from one support to the other 38
inches of platinum wire were stretched in four parallel lines. The
ends of the platinum wire were soldered to two stout copper wires
which passed through the base of the shade and could be connected with
a battery. As in the last experiments the shade rested upon
cotton-wool. A beam sent through the shade revealed the suspended
matter. The platinum wire was then raised to whiteness. In five
minutes there was a sensible diminution of the matter, and in ten
minutes it was totally consumed.
Oxygen, hydrogen, nitrogen, carbonic acid, so prepared as to exclude
all floating particles, produce, when poured or blown into the beam,
the darkness of stellar space. Coal-gas does the same. An ordinary
glass shade, placed in the air with its mouth downwards, permits the
track of the beam to be seen crossing it. When coal-gas or hydrogen
is allowed to enter the shade by a tube reaching to its top, the gas
gradually fills the shade from above downwards. As soon as it
occupies the space crossed by the beam, the luminous track is
abolished. Lifting the shade so as to bring the common boundary of
gas and air above the beam, the track flashes forth. After the shade
is full, if it be inverted, the pure gas passes upwards like a black
smoke among the illuminated particles.
The Germ Theory of Contagious Disease.
There is no respite to our contact with the floating matter of the
air; and the wonder is, not that we should suffer occasionally from
its presence, but that so small a portion of it, and even that but
rarely diffused over large areas, should appear to be deadly to man.
And what is this portion? It was some time ago the current belief
that epidemic diseases generally were propagated by a kind of malaria,
which consisted of organic matter in a state of motor-decay; that when
such matter was taken into the body through the lungs, skin, or
stomach, it had the power of spreading there the destroying process by
which itself had been assailed. Such a power was visibly exerted in
the case of yeast. A little leaven was seen to leaven the whole
lump--a mere speck of matter, in this supposed state of decomposition,
being apparently competent to propagate indefinitely its own decay.
Why should not a bit of rotten malaria act in a similar manner within
the human frame? In 1836 a very wonderful reply was given to this
question. In that year Cagniard de la Tour discovered the
yeast-plant--a living organism, which when placed in a proper medium
feeds, grows, and reproduces itself, and in this way carries on the
process which we name fermentation. By this striking discovery
fermentation was connected with organic growth.
Schwann, of Berlin, discovered the yeast-plant independently about the
same time; and in February, 1837, he also announced the important
result, that when a decoction of meat is effectually screened from
ordinary air, and supplied solely with calcined air, putrefaction
never sets in. Putrefaction, therefore, he affirmed to be caused, not
by the air, but by something which could be destroyed by a
sufficiently high temperature. The results of Schwann were confirmed
by the independent experiments of Helmholtz, Ure, and Pasteur, while
other methods, pursued by Schultze, and by Schroeder and Dusch, led to
the same result.
But as regards fermentation, the minds of chemists, influenced
probably by the great authority of Gay-Lussac, fell back upon the old
notion of matter in a state of decay. It was not the living
yeast-plant, but the dead or dying parts of it, which, assailed by
oxygen, produced the fermentation. Pasteur, however, proved the real
'ferments,' mediate or immediate, to be organised beings which find in
the reputed ferments their necessary food.
Side by side with these researches and discoveries, and fortified by
them and others, has run the germ theory of epidemic disease. The
notion was expressed by Kircher, and favoured by Linnaeus, that
epidemic diseases may be due to germs which float in the atmosphere,
enter the body, and produce disturbance by the development within the
body of parasitic life. The strength of this theory consists in the
perfect parallelism of the phenomena of contagious disease with those
of life. As a planted acorn gives birth to an oak, competent to
produce a whole crop of acorns, each gifted with the power of
reproducing its parent tree; and as thus from a single seedling a
whole forest may spring; so, it is contended, these epidemic diseases
literally plant their seeds, grow, and shake abroad new germs, which,
meeting in the human body their proper food and temperature, finally
take possession of whole populations. There is nothing to my
knowledge in pure chemistry which resembles the power of propagation
and self-multiplication possessed by the matter which produces
epidemic disease. If you sow wheat you do not get barley; if you sow
small-pox you do not get scarlet-fever, but small-pox indefinitely
multiplied, and nothing else. The matter of each contagious disease
reproduces itself as rigidly as if it were (as Miss Nightingale puts
it) dog or cat.
Parasitic Diseases of Silkworms. Pasteur's Researches.
It is admitted on all hands that some diseases are the product of
parasitic growth. Both in man and in lower creatures, the existence
of such diseases has been demonstrated. I am enabled to lay before
you an account of an epidemic of this kind, thoroughly investigated
and successfully combated by M. Pasteur. For fifteen years a plague
had raged among the silkworms of France. They had sickened and died
in multitudes, while those that succeeded in spinning their cocoons
furnished only a fraction of the normal quantity of silk. In 1853 the
silk culture of France produced a revenue of one hundred and thirty
millions of francs. During the twenty previous years the revenue had
doubled itself, and no doubt was entertained as to its further
augmentation. The weight of the cocoons produced in 1853 was
26,000,000 kilogrammes; in 1865 it had fallen to 4,000,000, the fall
entailing, in a single year, a loss of 100,000,000 francs.