to enter it in the dark. The slightly convergent beam of the electric
light was then sent through the mixture. For a moment the tube was
_optically empty_, nothing whatever being seen within it; but before a
second had elapsed a shower of particles was precipitated on the beam.
The cloud thus generated became denser as the light continued to act,
slowing at some places vivid iridescence.
The lens of the electric lamp was now placed so as to form within the
tube a strongly convergent cone of rays. The tube was cleansed and
again filled in darkness. When the light was sent through it, the
precipitation upon the beam was so rapid and intense that the cone,
which a moment before was invisible, flashed suddenly forth like a
solid luminous spear. The effect was the same when the air and vapour
were allowed to enter the tube in diffuse daylight. The cloud,
however, which shone with such extraordinary radiance under the
electric beam, was invisible in the ordinary light of the laboratory.
The quantity of mixed air and vapour within the experimental tube
could of course be regulated at pleasure. The rapidity of the action
diminished with the attenuation of the vapour. When, for example, the
mercurial column associated with the experimental tube was depressed
only five inches, the action was not nearly so rapid as when the tube
was full. In such cases, however, it was exceedingly interesting to
observe, after some seconds of waiting, a thin streamer of delicate
bluish-white cloud slowly forming along the axis of the tube, and
finally swelling so as to fill it.
Fig. 2.
Fig. 3.
When dry oxygen was employed to carry in the vapour the effect was the
same as that obtained with air.
When dry hydrogen was used as a vehicle, the effect was also the same.
The effect, therefore, is not due to any interaction between the
vapour of the nitrite and its vehicle.
This was further demonstrated by the deportment of the vapour itself.
When it was permitted to enter the experimental tube unmixed with air
or any other gas, the effect was substantially the same. Hence the
seat of the observed action is the vapour.
This action is not to be ascribed to heat. As regards the glass of
the experimental tube, and the air within the tube, the beam employed
in these experiments was perfectly cold. It had been sifted by
passing it through a solution of alum, and through the thick
double-convex lens of the lamp. When the unsifted beam of the lamp
was employed, the effect was still the same; the obscure calorific
rays did not appear to interfere with the result.
My object here being simply to point out to chemists a method of
experiments which reveals a new and beautiful series of reactions, I
left to them the examination of the products of decomposition. The
group of atoms forming the molecule of nitrite of amyl is obviously
shaken asunder by certain specific waves of the electric beam, nitric
oxide and other products, of which the _nitrate_ of amyl is probably
one, being the result of the decomposition. The brown fumes of
nitrous acid were seen mingling with the cloud within the experimental
tube. The nitrate of amyl, being less volatile than the nitrite, and
not being able to maintain itself in the condition of vapour, would be
precipitated as a visible cloud along the track of the beam.
In the anterior portions of the tube a powerful sifting of the beam by
the vapour occurs, which diminishes the chemical action in the
posterior portions. In some experiments the precipitated cloud only
extended halfway down the tube. When, under these circumstances, the
lamp was shifted so as to send the beam through the other end of the
tube, copious precipitation occurred there also.
Solar light also effects the decomposition of the nitrite-of-amyl
vapour. On October 10, 1868, I partially darkened a small room in the
Royal Institution, into which the sun shone, permitting the light to
enter through an open portion of the window-shutter. In the track of
the beam was placed a large plano-convex lens, which formed a fine
convergent cone in the dust of the room behind it. The experimental
tube was filled in the laboratory, covered with a black cloth, and
carried into the partially darkened room. On thrusting one end of the
tube into the cone of rays behind the lens, precipitation within the
cone was copious and immediate. The vapour at the distant end of the
tube was in part shielded by that in front, and was also more feebly
acted on through the divergence of the rays. On reversing the tube, a
second and similar cone was precipitated.
Physical Considerations.
I sought to determine the particular portion of the light which
produced the foregoing effects. When, previous to entering the
experimental tube, the beam was caused to pass through a red glass,
the effect was greatly weakened, but not extinguished. This was also
the case with various samples of yellow glass. A blue glass being
introduced before the removal of the yellow or the red, on taking the
latter away prompt precipitation occurred along the track of the blue
beam. Hence, in this case, the more refrangible rays are the most
chemically active. The colour of the liquid nitrite of amyl indicates
that this must be the case; it is a feeble but distinct yellow: in
other words, the yellow portion of the beam is most freely
transmitted. It is not, however, the transmitted portion of any beam
which produces chemical action, but the absorbed portion. Blue, as
the complementary colour to yellow, is here absorbed, and hence the
more energetic action of the blue rays.
This reasoning, however, assumes that the same rays are absorbed by
the liquid and its vapour. The assumption is worth testing. A
solution of the yellow chromate of potash, the colour of which may be
made almost, if not altogether, identical with that of the liquid
nitrite of amyl, was found far more effective in stopping the chemical
rays than either the red or the yellow glass. But of all substances
the liquid nitrite itself is most potent in arresting the rays which
act upon its vapour. A layer one-eighth of an inch in thickness,
which scarcely perceptibly affected the luminous intensity, absorbed
the entire chemical energy of the concentrated beam of the electric
light.
The close relation subsisting between a liquid and its vapour, as
regards their action upon radiant heat, has been already amply
demonstrated. [Footnote: 'Phil. Trans.' 1864; 'Heat, a Mode of
Motion,' chap, xii; and P. 61 of this volume.] As regards the nitrite
of amyl, this relation is more specific than in the cases hitherto
adduced; for here the special constituent of the beam, which provokes
the decomposition of the vapour, is shown to be arrested by the
liquid.
A question of extreme importance in molecular physics here arises:
What is the real mechanism of this absorption, and where is its seat?
[Footnote: My attention was very forcibly directed to this subject
some years ago by a conversation with my excellent friend Professor
Clausius.]
I figure, as others do, a molecule as a group of atoms, held together
by their mutual forces, but still capable of motion among themselves.
The vapour of the nitrite of amyl is to be regarded as an assemblage
of such molecules. The question now before us is this: In the act of
absorption, is it the molecules that are effective, or is it their
constituent atoms? Is the _vis viva_ of the intercepted light-waves
transferred to the molecule as a whole, or to its constituent parts?
The molecule, as a whole, can only vibrate in virtue of the forces
exerted between it and its neighbour molecules. The intensity of
these forces, and consequently the rate of vibration, would, in this
case, be a function of the distance between the molecules. Now the
identical absorption of the liquid and of the vaporous nitrite of amyl
indicates an identical vibrating period on the part of liquid and
vapour, and this, to my mind, amounts to an experimental proof that
the absorption occurs in the main _within_ the molecule. For it can
hardly be supposed, if the absorption were the act of the molecule as
a whole, that it could continue to affect waves of the same period
after the substance had passed from the vaporous to the liquid state.
In point of fact, the decomposition of the nitrite of amyl is itself
to some extent an illustration of this internal molecular absorption;
for were the absorption the act of the molecule as a whole, the
relative motions of its constituent atoms would remain unchanged, and
there would be no mechanical cause for their separation. It is
probably the synchronism of the vibrations of one portion of the
molecule with the incident waves, that enables the amplitude of those
vibrations to augment, until the chain which binds the parts of the
molecule together is snapped asunder.
I anticipate wide, if not entire, generality for the fact that a
liquid and its vapour absorb the same rays. A cell of liquid chlorine
would, I imagine, deprive light more effectually of its power of
causing chlorine and hydrogen to combine than any other filter of the
luminous rays. The rays which give chlorine its colour have nothing
to do with this combination, those that are absorbed by the chlorine
being really effective rays. A highly sensitive bulb, containing
chlorine and hydrogen, in the exact proportions necessary for the
formation of hydrochloric acid, was placed at one end of an
experimental tube, the beam of the electric lamp being sent through it
from the other. The bulb did not explode when the tube was filled
with chlorine, while the explosion was violent and immediate when the
tube was filled with air. I anticipate for the liquid chlorine an
action similar to, but still more energetic than, that exhibited by
the gas. If this should prove to be the case, it will favour the view
that chlorine itself is _molecular_ and not _monatomic_.
Production of Sky-blue by the Decomposition of Nitrite of Amyl.
When the quantity of nitrite vapour is considerable, and the light
intense, the chemical action is exceedingly rapid, the particles
precipitated being so large as to whiten the luminous beam. Not so,
however, when a well-mixed and highly attenuated vapour fills the
experimental tube. The effect now to be described was first obtained
when the vapour of the nitrite was derived from a portion of its
liquid which had been accidentally introduced into the passage through
which the dry air flowed into the experimental tube.
In this case, the electric beam traversed the tube for several seconds
before any action was visible. Decomposition then visibly commenced,
and advanced slowly. When the light was very strong, the cloud
appeared of a milky blue. When, on the contrary, the intensity was
moderate, the blue was pure and deep. In Brücke's important
experiments on the blue of the sky and the morning and evening red,
pure mastic is dissolved in alcohol, and then dropped into water well
stirred. When the proportion of mastic to alcohol is correct, the
resin is precipitated so finely as to elude the highest microscopic
power. By reflected light, such a medium appears bluish, by
transmitted light yellowish, which latter colour, by augmenting the
quantity of the precipitate, can be caused to pass into orange or red.
But the development of colour in the attenuated nitrite-of-amyl vapour
is doubtless more similar to what takes place in our atmosphere. The
blue, moreover, is far purer and more sky-like than that obtained from
Bruecke's turbid medium. Never, even in the skies of the Alps, have I
seen a richer or a purer blue than that attainable by a suitable
disposition of the light falling upon the precipitated vapour.
Iodide of Allyl.--Among the liquids hitherto subjected to the
concentrated electric light, iodide of allyl, in point of rapidity and
intensity of action, comes next to the nitrite of amyl. With the
iodide I have employed both oxygen and hydrogen, as well as air, as a
vehicle, and found the effect in all cases substantially the same. The
cloud-column here was exquisitely beautiful. It revolved round the
axis of the decomposing beam; it was nipped at certain places like an
hour-glass, and round the two bells of the glass delicate
cloud-filaments twisted themselves in spirals. It also folded itself
into convolutions resembling those of shells. In certain conditions
of the atmosphere in the Alps I have often observed clouds of a
special pearly lustre; when hydrogen was made the vehicle of the
iodide-of allyl vapour a similar lustre was most exquisitely shown.
With a suitable disposition of the light, the purple hue of
iodine-vapour came out very strongly in the tube.
The remark already made, as to the bearing of the decomposition of
nitrite of amyl by light on the question of molecular absorption,
applies here also; for were the absorption the work of the molecule as
a whole, the iodine would not be dislodged from the allyl with which
it is combined. The non-synchronism of iodine with the waves of
obscure heat is illustrated by its marvellous transparency to such
heat. May not its synchronism with the waves of light in the present
instance be the cause of its divorce from the allyl?
Iodide of Isopropyl.--The action of light upon the vapour of this
liquid is, at first, more languid than upon iodide of allyl; indeed
many beautiful reactions may be overlooked, in consequence of this
languor at the commencement. After some minutes' exposure, however,
clouds begin to form, which grow in density and in beauty as the light
continues to act. In every experiment hitherto made with this
substance the column of cloud filling the experimental tube, was
divided into two distinct parts near the middle of the tube. In one
experiments a globe of cloud formed at the centre, from which, right
and left, issued an axis uniting the globe with two adjacent
cylinders. Both globe and cylinders were animated by a common motion
of rotation. As the action continued, paroxysms of motion were
manifested; the various parts of the cloud would rush through each
other with sudden violence. During these motions beautiful and
grotesque cloud-forms were developed. At some places the nebulous
mass would become ribbed so as to resemble the graining of wood; a
longitudinal motion would at times generate in it a series of curved,
transverse bands, the retarding influence of the sides the tube
causing an appearance resembling, on a small scale, the dirt-bands of
the Mer de Glace. In the anterior portion of the tube those sudden
commotion were most intense; here buds of cloud would sprout forth,
and grow in a few seconds into perfect flower-like forms. The cloud
of iodide of isopropyl had a character Of its own, and differed
materially from all others that I had seen. A gorgeous mauve colour
was observed in the last twelve inches of the tube; the vapour of
iodine was present, and it may have been the sky-blue scattered by the
precipitated particles which, mingling with the purple of the iodine,
produced the mauve. As in all other cases here adduced, the effects
were proved to be due to the light; they never occurred in darkness.
The forms assumed by some of those actinic clouds, as I propose to
call them, in consequence of rotations and other motions, due to
differences of temperature, are perfectly astounding. I content
myself here with a meagre description of one more of them.
The tube being filled with the sensitive mixture, the beam was sent
through it, the lens at the same time being so placed as to produce a
cone of very intense light. Two minutes elapsed before anything was
visible; but at the end of this time a faint bluish cloud appeared to
hang itself on the most concentrated portion of the beam.
Soon afterwards a second cloud was formed five inches farther down the
experimental tube. Both clouds were united by a slender cord of the
same bluish tint as themselves.
As the action of the light continued, the first cloud gradually
resolved itself into a series of parallel disks of exquisite delicacy,
which rotated round an axis perpendicular to their surfaces, and
finally blended to a screw surface with an inclined generatrix. This
gradually changed into a filmy funnel, from the narrow end of which
the 'cord' extended to the cloud in advance.
The latter also underwent slow but incessant modification. It first
resolved itself into a series of strata resembling those of the
electric discharge. After a little time, and through changes which it
was difficult to follow, both clouds presented the appearance of a
series of concentric funnels set one within the other, the interior
ones being seen through the outer ones. Those of the distant cloud
resembled claret-glasses in shape. As many as six funnels were thus
concentrically set together, the two series being united by the
delicate cord of cloud already referred to. Other cords and Blender
tubes were afterwards formed, which coiled themselves in delicate
spirals around the funnels.
Rendering the light along the connecting-cord more intense, it
diminished in thickness and became whiter; this was a consequence of
the enlargement of its particles. The cord finally disappeared, while
the funnels melted into two ghost-like films, shaped like parasols.
They were barely visible, being of an exceedingly delicate blue tint.
They seemed woven of blue air. To compare them with cobweb or with
gauze would be to liken them to something infinitely grosser than
themselves.
In all cases a distant candle-flame, when looked at through the cloud,
was sensibly undimmed.
2. ON THE BLUE COLOUR OF THE SKY, AND THE POLARISATION OF SKYLIGHT.
[Footnote: In my 'Lectures on Light' (Longman), the polarisation of
light will be found briefly, but, I trust, clearly explained.]
1869.
After the communication to the Royal Society of the foregoing brief
account of a new Series of Chemical Reactions produced by Light, the
experiments upon this subject were continued, the number of substances
thus acted on being considerably increased.
I now, however, beg to direct attention to two questions glanced at
incidentally in the preceding pages--the blue colour of the sky, and
the polarisation of skylight. Reserving the historic treatment of the
subject for a more fitting occasion, I would merely mention now that
these questions constitute, in the opinion of our most eminent
authorities, the two great standing enigmas of meteorology. Indeed it
was the interest manifested in them by Sir John Herschel, in a letter
of singular speculative power, addressed to myself, that caused me to
enter upon the consideration of these questions so soon.
The apparatus with which I work consists, as already stated, of a
glass tube about a yard in length, and from 2.5 to 3 inches internal
diameter. The vapour to be examined is introduced into this tube in
the manner already described, and upon it the condensed beam of the
electric lamp is permitted to act, until the neutrality or the
activity of the substance has been declared.
It has hitherto been my aim to render the chemical action of light
upon vapours visible. For this purpose substances have been chosen,
one at least of whose products of decomposition under light shall have
a boiling-point so high, that as soon as the substance is formed it
shall be precipitated. By graduating the quantity of the vapour, this
precipitation may be rendered of any degree of fineness, forming
particles distinguishable by the naked eye, or far beyond the reach of
our highest microscopic powers. I have no reason to doubt that
particles may be thus obtained, whose diameters constitute but a small
fraction of the length of a wave of violet light.
In all cases when the vapours of the liquids employed are sufficiently
attenuated, no matter what the liquid may be, the visible action
commences with the formation of a _blue cloud_. But here I must guard
myself against all misconception as to the use of this term. The
'cloud' here referred to is totally invisible in ordinary daylight. To
be seen, it requires to be surrounded by darkness, _it only_ being
illuminated by a powerful beam of light. This blue cloud differs in
many important particulars from the finest ordinary clouds, and might
justly have assigned to it an intermediate position between such
clouds and true vapour. With this explanation, the term 'cloud,' or
'incipient cloud,' or 'actinic cloud,' as I propose to employ it,
cannot, I think, be misunderstood.
I had been endeavouring to decompose carbonic acid gas by light. A
faint bluish cloud, due it may be, or it may not be, to the residue of
some vapour previously employed, was formed in the experimental tube.
On looking across this cloud through a Nicol's prism, the line of
vision being horizontal, it was found that when the short diagonal of
the prism was vertical, the quantity of light reaching the eye was
greater than when the long diagonal was vertical. When a plate of
tourmaline was held between the eye and the bluish cloud, the quantity
of light reaching the eye when the axis of the prism was perpendicular
to the axis of the illuminating beam, was greater than when the axes
of the crystal and of the beam were parallel to each other.
This was the result all round the experimental tube. Causing the
crystal of tourmaline to revolve round the tube, with its axis
perpendicular to the illuminating beam, the quantity of light that
reached the eye was in all its positions a maximum. When the
crystallographic axis was parallel to the axis of the beam, the
quantity of light transmitted by the crystal was a minimum.
From the illuminated bluish cloud, therefore, polarised light was
discharged, the direction of maximum polarisation being at right
angles to the illuminating beam; the plane of vibration of the
polarised light was perpendicular to the beam. [Footnote: This is
still an undecided point; but the probabilities are so much in its
favour, and it is in my opinion so much preferable to have a physical
image on which the mind can rest, that I do not hesitate to employ the
phraseology in the text.]
Thin plates of selenite or of quartz, placed between the Nicol and the
actinic cloud, displayed the colours of polarised light, these colours
being most vivid when the line of vision was at right angles to the
experimental tube. The plate of selenite usually employed was a
circle, thinnest at the centre, and augmenting uniformly in thickness
from the centre outwards. When placed in its proper position between
the Nicol and the cloud, it exhibited a system of splendidly-coloured
rings.
The cloud here referred to was the first operated upon in the manner
described. It may, however, be greatly improved upon by the choice of
proper substances, and by the application, in proper quantities, of
the substances chosen. Benzol, bisulphide of carbon, nitrite of amyl,
nitrite of butyl, iodide of allyl, iodide of isopropyl, and many other
substances may be employed. I will take the nitrite of butyl as
illustrative of the means adopted to secure the best result, with
reference to the present question.
And here it may be mentioned that a vapour, which when alone, or mixed
with air in the experimental tube, resists the action of light, or
shows but a feeble result of this action, may, when placed in
proximity with another gas or vapour, exhibit vigorous, if not violent
action. The case is similar to that of carbonic acid gas, which,
diffused in the atmosphere, resists the decomposing action of solar
light, but when placed in contiguity with chlorophyl in the leaves of
plants, has its molecules shaken asunder.