ACTINISM--CHEMICAL RADIATIONS.
The Sun-ray and its Powers--Darkening of Horn Silver--Niepce’s
Discovery--Prismatic Spectrum--Refrangibility of Light, Heat, and
Actinism--Daguerre’s Discovery--Photography--Chemical Effects
produced by Solar Radiations--Absorption of Actinism--Phenomena of
the Daguerreotype--Chemical Change produced upon all Bodies--Power
of Matter to restore its Condition--Light protects from Chemical
Change--Photographs taken in Darkness--Chemical Effects of Light
on organized Forms--Chemical Effects of Solar Heat--Influence
of Actinism on Electricity--Radiations in Darkness--Moser’s
Discoveries, &c.
Heat and light are derived from the sun, and we have attempted to show,
not only that the phenomena of these two principles are different, but
that they can scarcely, in the present condition of our knowledge, be
regarded as modified manifestations of _one_ superior power. Associated
with these two remarkable elements, others may exist in the solar rays.
Electrical phenomena are certainly developed by both heat and light,
and peculiar electric changes are produced by exposure to sunshine.
Electricity may be merely _excited_ by the solar rays, or it may
_flow_ like light from the sun. Chemical action may be only due to the
disturbance of some diffused principle; or it may be directly owing to
some agency which is radiated at once from the sun.
A sun ray is a magical thing: we connect it in our fancy with the
most ethereal of possible creations. Yet in its action on matter it
produces colour; it separates the particles of solid masses farther
from each other, and it breaks up some of the strongest forces of
chemical affinity. To modern science is entirely due the knowledge
we have gained of the marvellous powers of the sunbeam; and it has
rendered us familiar with phenomena, to which the incantation scenes
of the Cornelius Agrippas of the dark ages were but ill-contrived
delusions, and their magic mirrors poor instruments. The silver tablets
of the photographic artist receiving fixed impressions of the objects
represented in the dark chamber by a lens, are far superior as examples
of natural magic.
In the dark ages, or rather as the earliest gleams of the bright
morning of inductive research were dispelling the mists of that
phantom-peopled period, it was observed, for the first time, that the
sun’s rays turned a white compound black. Man must have witnessed, long
before, that change which is constantly taking place in all vegetable
colours: some darkening by exposure to sunlight, while others were
bleached by its influence. Yet those phenomena excited no attention,
and the world knew nothing of the mighty changes which were constantly
taking place around them. The alchemists--sublime pictures of credulous
humanity--toiling in the smoke of their secret laboratories, waiting
and watching for every change which could be produced by fire, or by
their “royal waters,” caught the first faint ray of an opening truth;
and their wild fancy, that light could change silver into gold, if they
but succeeded in getting its subtile beams to interpenetrate the metal,
was the clue afforded to the empirical philosopher to guide him through
a more than Cretan labyrinth.[117]
The first fact recorded upon this, point was, that horn silver
blackened when exposed to the light. Without doubt many anxious
thoughts were given by these alchemists to that fact. Here was, as it
appeared, a mixing up of light and matter, and behold the striking
change! It was a step towards the realization of their dreams. Alas!
poor visionaries! in pursuing an ideality they lost the reality which
was within their grasp.
Truths come slowly upon man, and long it is before these angel visits
are acknowledged by humanity. The world clings to its errors, and
avoids the truth, lest its light should betray their miserable follies.
At length a man of genius announced that “_No substance can be
exposed to the sun’s rays without undergoing a chemical change_;”
but his words fell idly upon the ear. His friends looked upon his
light-produced pictures as singular; they preserved them in their
cabinets of curiosities; but the truths which he enunciated were soon
forgotten. Howbeit his words were recorded, and it is due to the
solitary experimentalist of Châlons on the Saône, to couple the name
of Niepce with the discovery of a fact which is scarcely second to
the development of the great law of universal gravitation.[118] But
an examination awaits us, which, for its novelty, has more charms
than most branches of science, and which, for the extensive views it
opens to the inquirer, has an interest in nowise inferior to any other
physical investigation.
The prismatic spectrum affords us the means of examining the conditions
of the solar rays with great facility. In bending the ray of white
light out of its path, by means of a triangular piece of glass, we
divide it in a remarkable manner. We learn that heat is less refracted
by the glass than the other powers; we find the maximum point of the
calorific rays but slightly thrown out of the right line, which the
solar pencil would have taken, had it not been interrupted by the
prism; and the thermic action is found to diminish with much regularity
on either side of this line. We discover that the luminous power is
subject to greater refraction, and that its maximum lies considerably
above that of heat; and that, in like manner, on each side the light
diminishes, producing orange, red, and crimson colours below the
maximum point, and green, blue, and violet above it. Again, we find
that the radiations which produce chemical change are more refrangible
than either of the others, and the maximum of this power is found
at the point where light rapidly diminishes, and where scarcely any
heat can be detected: it extends in full activity, above its maximum,
to a considerable distance, where no trace of light under ordinary
conditions exists, and below that point, until light, appearing to
act as an interfering agent, quenches its peculiar properties. These
are strong evidences that _light_ and _actinism_--as this principle
has been named--are not identical: and we may separate them most
easily and effectually from each other. Certain glasses, stained dark
blue, with oxide of cobalt, admit scarcely any light; but they offer
no interruption to the passage of actinism or the chemical rays; on
the contrary, a pure yellow glass, or a yellow fluid, which does not
sensibly reduce the intensity of any one colour of the chromatic band
of luminous rays, completely cuts off this chemical principle, whatever
it may be. In addition to these, there are other results which we shall
have to describe, which prove that, although associated in the solar
beam, light and actinism are in constant antagonism.
When Daguerre first published his great discovery, the European public
regarded his metal tablets with feelings of wonder: we have grown
accustomed to the beautiful phenomena of this art, and we have become
acquainted with a number of no less beautiful processes on paper,
all of which, if studied aright, must convince the most superficial
thinker, that a world of wonder lies a little beyond our knowledge,
but within the reach of industrious and patient research. Photography
is the name by which the art of sun-painting will be for ever known.
We regard this as unfortunate, conveying as it does a false idea,--the
pictures not being _light-drawn_. Could we adopt the name given
by Niepce to the process, the difficulty would be avoided, since
Heliography involves no hypothesis, and strictly tells the undeniable
truth, that our pictures are _sun-drawn_. That pictures can be produced
by the rays from artificial sources, presents no objection to this;
these rays were still originally derived from the sun.
By whatever name we determine to convey our ideas of these phenomena,
it is certain that they involve a series of effects which are of the
highest interest to every lover of nature, and of the utmost importance
to the artist and the amateur. By easy manipulation we are now enabled
to give permanence to the charming pictures which are produced by means
of that pleasing invention of Baptista Porta, the _Camera Obscura_. Any
image, which being refracted by the lens of this instrument falls upon
the table in its dark chamber, may be secured with its most delicate
gradations of shadows, upon either a metallic or a paper tablet.
But let us proceed to the examination of a few of the more striking
phenomena of these chemical changes. To commence with some of the more
simple but no less important results.
Chlorine and hydrogen will not unite in darkness, nor will chlorine and
carbonic oxide; but, if either of those gaseous mixtures is exposed to
sunshine, they combine rapidly, and often with explosion. A solution of
the sulphate of iron in ordinary water may be preserved for a long time
in the dark without undergoing any change; expose it to the sunshine,
and a precipitation of oxide of iron is very rapidly produced. The
mineral chameleon, the _manganesiate of potash_ in solution, is
almost instantly decomposed in daylight; but it is a long time before
it undergoes any change in darkness. The same thing occurs with a
combination of platinum and lime: indeed, it appears that precipitation
is at all times, and under all circumstances, accelerated by the solar
rays. As these precipitations are in exact agreement with the quantity
of actinic radiation to which the solutions have been exposed, we may
actually weigh off the relative quantities, representing in grains the
equivalent numbers to the amount of actinism which has influenced the
chemical compound.[119]
We have evidence which appears to prove that this chemical agent may
be absorbed by simple bodies, and that by this absorption an actual
change of condition, is produced, in many respects analogous to those
allotropic changes which we have previously considered. Chlorine, in
its ordinary state, does not combine with hydrogen in the dark. If we
employ the yellow medium of chlorine gas, for the purpose of analyzing
the sun’s rays previously to their falling upon some chemical compound
which is sensitive to actinic power, we shall find that the chlorine
obstructs all this actinism, and, however unstable the compound,
it remains unchanged. But the chlorine gas which has interrupted
this wonderful agent, appears to have absorbed it, and it is so far
altered in its constitution that _it will unite with hydrogen in the
dark_.[120] In like manner, if, of two portions of the same solution
of sulphate of iron, one is kept in the dark and the other exposed
to the sunshine, it will be found that the solution which has been
exposed will precipitate gold and silver from their combinations much
more speedily than that which has been preserved in darkness--the
temperature and every other condition being the same.
The phenomena of the Daguerreotype involve many strange conditions.
A plate of silver, on which a slight chemical action has been
established by the use of iodine, is exposed to the lenticular image
in the camera obscura. If allowed to remain under the influence of
these radiations for a sufficient length of time, a faithful picture
of the illuminated objects is delineated on the plate, as shown by
the visible decomposition and darkening of the iodized surface. The
plate is not, however, in practice allowed to assume this condition;
after an exposure of a few seconds the radiant influence is cut off,
and the eye cannot detect any evidence of change upon the yellow
plate. It is now exposed to the vapour of mercury, and that metal in
a state of exceedingly fine division is condensed upon the plate;
but the condensation is not uniformly spread upon its face. The
deposit of mercurial vapour is in exact proportion to the amount of
chemical action produced. Is the change, by which this peculiar power
of condensation is effected, a chemical, calorific, electrical, or
merely a molecular one? The evidences, at present, are not sufficient
to determine the question. It has lately been suggested, that the
mercury acts chemically only, and effects the full decomposition of
the iodide of silver; and that the picture is due to this, and not to
the deposition actually of the mercury vapour. In all probability we
have the involved action of several forces. We have some experiments
which show, clearly enough, that mercury is deposited in proportions
which correspond with the intensity of solar action. A chemically
prepared surface is not necessary to exhibit this result. A polished
plate of metal, of glass, of marble, or a piece of painted wood, being
partially exposed, will, when breathed upon, or presented to the action
of mercurial vapour, show that a disturbance has been produced upon
the portions which were illuminated, whereas no change can be detected
upon the parts which were kept in the dark. It was thought, until
lately, that a few chemical compounds, such as the iodide of silver,
the material employed in the Daguerreotype and Calotype,--chloride of
silver, the ordinary photographic agent,--a few salts of gold, and one
or two of lead and iron, were the only materials upon which these very
remarkable changes were produced. We now know that it is impossible
to expose any body, simple or compound, to the sun’s rays, without
its being influenced by this chemical and molecular disturbing power.
To take our examples from inorganic nature, the granite rock which
presents its uplifted head in firmness to the driving storm, the stones
which genius has framed into forms of architectural beauty, or the
metal which is intended to commemorate the great acts of man, and which
in the human form proclaims the hero’s deeds and the artist’s talent,
are all alike destructively acted upon during the hours of sunshine,
and, but for provisions of nature no less wonderful, would soon perish
under the delicate touch of the most subtile of the agencies of the
universe.
Niepce was the first to show that all bodies which underwent this
change during daylight possessed the power of restoring themselves
to their original conditions during the hours of night, when this
excitement was no longer influencing them. Resins, the Daguerreotype
plate, the unprepared metal tablet, and numerous photographic
preparations, prove this in a remarkable manner.[121]
The picture which we receive to-day, unless we adopt some method of
securing its permanency, fades away before the morrow, and we try to
restore it in vain. With some of our chemical preparations this is
very remarkably shown, but by none in so striking a manner as by paper
prepared with the iodide of platinum, which, being impressed with an
image by heliographic power, which is represented by dark brown tints,
restores itself in the dark, in a few minutes, to its former state of
a yellow colour, and recovers its sensibility to sunshine.[122] The
inference we alone can draw from all the evidences which the study
of actino-chemistry affords, is, that the hours of darkness are as
necessary to the inorganic creation as we know night and sleep to be
to the organic kingdom. But we must not forget that there does exist
in the solar rays a balance of forces which materially modifies the
amount of disturbing influence exerted by them on matter. Not only do
we find that the chemical action is not extended over the whole length
of the prismatic spectrum, but we discover that over spaces, which
correspond with the maximum points of light and heat, a protective
action is exerted. That is, that highly sensitive photographic agents,
which blacken rapidly under exposure to diffused daylight, are entirely
protected from change in full sunshine, if at the same time as a strong
light is thrown upon them by reflection, the yellow and extra red rays
are brought to bear upon their surface. Not only so, but by employing
media which will cut off all the chemical rays of the spectrum,
admitting freely at the same time the luminous and calorific rays, we
find that a protected band, the length of the spectrum, remains white,
whilst every other portion has blackened.[123]
Among the many curious instances of natural magic, none are more
remarkable than an experiment not long since proposed, by which
Daguerreotype pictures may be taken in absolute darkness to the human
eye. This is effected in the following manner:--A large prismatic
spectrum is thrown upon a lens fitted into one side of a dark chamber;
and as we know that the actinic power resides in great activity beyond
the violet ray, where there is no light, the only rays which we allow
to pass the lens into the chamber are those which are extra-spectral
and non-luminous. These are directed upon, any white object, and from
that object radiated upon a highly sensitive plate in a camera obscura.
Thus a copy of the subject will be obtained by the agency of radiations
which produce no sensible effect upon the optic nerve. This experiment
is the converse of those which show us that we may illuminate any
object with the strongest sunlight which has passed through yellow
glass, the yellow solution of sulphuret of calcium, or of the
bichromate of potash--these being non-transparent to the chemical
rays--and yet fail to secure any Daguerreotype copy of it, even upon
the most exquisitely sensitive plate. Indeed, the image of the sun
itself, when setting through an atmosphere which reduces its light to
a red or rich yellow colour, not only produces no chemical change,
but protects an iodized plate from it; and whilst every other part
of the tablet gives a picture of surrounding objects in the ordinary
character, the bright sun itself is represented by a spot upon which
no change has taken place.[124] In tropical climes, where a brilliant
sun is giving the utmost degree of illumination to all surrounding
objects, all photographic preparations are acted upon relatively more
slowly than in the climate of England, where the light is less intense.
As a remarkable instance of this fact, a circumstance may be mentioned,
which is curiously illustrative of the power of light to interfere with
actinism:--
A gentleman, well acquainted with the Daguerreotype process, obtained
in the city of Mexico all the necessary apparatus and chemicals,
expecting, under the bright light and cloudless skies of that climate,
to produce pictures of superior excellence. Failure upon failure was
the result; and although every care was used, and every precaution
adopted, it was not until the rainy season set in that he could secure
a good Daguerreotype of any of the buildings of that southern city.
The first attempts, which were made at the instigation of M. Arago,
by order of the French Government, to copy the Egyptian tombs and
temples, and the remains of the Aztecs in Central America, were
failures. Although the photographers employed succeeded to admiration
in Paris, in producing pictures in a few minutes, they found often that
an exposure of an hour was insufficient under the bright and glowing
illumination of a southern sky.
Experiments with the spectrum have been made in different latitudes,
and it is found, that, as we proceed towards the equator, a band which
is always left unchanged, corresponding exactly with the rays of
greatest illuminating power, regularly enlarges in size, thus proving
the increase of light over actinism--and the interfering power of the
former.
By increasing the sensibility of the photographic preparation, this
difficulty is overcome, and particularly when any organic compound
enters into the preparation. So that we are now enabled to copy nature
in all her varying moods, whether we employ our photographic tablets in
temperate Europe, or in tropical Africa.
The degree of sensibility which has been attained is remarkable. Mr.
Fox Talbot, by uniting a process devised by Dr. Woods, of Parsonstown,
and another which was first introduced by the author of this volume,
and combining them with an ether, obtains a most unstable compound,
which he thus employs. A glass plate is covered with albumen united
with the above solution, and then with nitrate of silver: this forms
the sensitive surface. The plate being placed in the dark, in a camera,
it is so adjusted that the image of a printed bill fixed upon a wheel
may fall upon it when uncovered, and the wheel illuminated. The wheel
is made to revolve with the utmost rapidity, in a perfectly dark
room, and the sensitive plate uncovered. Then the whirling bill is
illuminated for an inappreciably short space of time by the discharge
of a Leyden jar. Notwithstanding the rapid rate at which the pointed
paper is moving, and the instantaneous nature of the illumination--a
miniature flash of lightning--the bill is found to be copied with
unfailing fidelity upon the photographic plate. It unfortunately
happens, that the preparation by which this extraordinary degree of
sensibility is obtained, is very uncertain in its action--and hence it
is not generally useful; but here we have the evidence to show that
at a speed as rapid as that of a rifle-ball an impression may be made
upon a photographic plate. There are, however, some new processes which
promise eventually to rival the above for sensibility, and to be by no
means of difficult manipulation. Of this character is the collodion
process. The gun cotton dissolved in ether possesses some very great
accelerating properties, and in combination with the silver salts, and
one of the vegetable acids, it forms a sensitive surface upon which
pictures may be obtained in less than a second of time.
Colour, natural colour too, has been very decidedly secured. The sun
has been solicited to display his palette, and the answer has been a
picture in which colour for colour in all their fidelity have been
impressed. The plate upon which this result has been obtained is of a
dark brown colour, and the chromatic variety is, as it were, eaten out
by the solar rays. These colours have not yet been permanently fixed
upon the plate employed, but from the temporary degree of fixedness
which has been obtained, we may fairly hope that in a short time colour
may be rendered as permanent on the productions of the photographer
as on those of the painter. It is a curious and striking fact, that
in the preparation of these plates, salts are used which give colours
to flame; and according to the colour which is produced by them when
burning, so, on the photographic plate, is that colour impressed
with greater intensity than the others. To what is this leading us?
Mysteries surround our advances on the domain of truth. We dare not
speculate upon them: the time of their full development will arrive.
By the aid of this beautiful art, we are enabled to preserve the
lineaments of those who have benefited their race by their intellect,
or their heroism. We can hand down to future ages portraits of our own
Wellington, and the illustrious Arago, unerring in their truthfulness.
How great would be the joy of all, could we now obtain a daguerreotype
portrait of a Greek poet, or of a Roman philosopher, of a Sophocles,
or of a Seneca! How much discussion would be prevented did we possess
a calotype portrait of the Bard of Avon, or of the Philosopher of
Grantham!
By the agency of those very rays which give life and brilliancy to the
laughing eye and the roseate cheek, we can at once correctly trace the
outline of the features we admire, with all those shadowy details which
give a reality to the “presentment.” The objects of our love may be for
ever present with us in these self-painted pictures. The vicious, whom
we would avoid, may be made known to us by this unerring painter. The
process which nature employs is perfect; the imperfections are those of
man, and these being few, he may soon learn to remedy.
To the traveller, how valuable are the processes of photography! He
secures representations of those remains of temples which were in their
glory when Moses wrote. He copies by one operation a tomb at Karnac,
covered with myriads of hieroglyphics, or an inscribed stone in Arabia,
which it would occupy him days to trace. These he can carry to his home
and read at his leisure. The relics of hoar antiquity speaking to the
present of the past, and recording the histories of races which have
fleeted away like shadows, are thus preserved to tell their wondrous
tales.
The admirer of nature may copy her arrangements with the utmost
fidelity. Every modulation of the landscape, each projecting rock or
beetling tor--the sinuous river in its rapid flow--the meandering
stream, “gliding like happiness away;” and the spreading plains
over which are scattered the homes of honest industry and domestic
peace, intermingled with the towers of those humble temples in which
simple-hearted piety delights to “bow the head and bend their knee;”
these, all of these, may, by the sunbeam which illuminates the whole,
be faithfully pencilled upon our chemical preparations.
Our art enables us to do more even than this; we have but to present
our sensitive tablet to the moon, and she, by her own light, prints her
mountains and her valleys, and indicates with all truth the physical
conditions of her surface.
Any reference to the _chemical agency_ of LIGHT--_the luminous rays_ as
distinguished from the _chemical and calorific rays_--has been avoided
until we came to the consideration of this particular question of
chemical change.
Upon organic compounds, as, for instance, upon the colouring matter
of leaves and flowers, _light_ does exert a chemical power: and it
is found that vegetable colours are bleached, not by rays of their
own colour, but by those which are _complementary_ to them. A red dye
fades under the influence of a green ray, and a yellow under that of a
violet one, much more speedily than when exposed to rays of any other
colour; and this, it must be remembered, is due to the coloured ray
itself, and not to any _actinic_ power masked, as it were, behind the
colour, as is generally believed.[125] It was long a question whether
the decomposition of carbonic acid by plants was due to the luminous or
the chemical rays. It is now clearly established that the luminous rays
are the most active in producing this effect; which they do indirectly,
by exciting the vital powers of the organized structures. Therefore we
would refer this phenomenon of gaseous decomposition to a vital power
quickened by luminous excitement.[126]
We have already noticed some chemical phenomena due to heat,
particularly those experiments of Count Rumford’s, which appeared to
him to prove that the chemical agency of the sun’s rays was due to its
calorific power. Certain chemical phenomena, we know, may be produced
by thermic action; but the only variety of thermo-chemical action which
connects itself immediately with the solar radiations, belongs to a
class of rays to which the name of _Parathermic_ has been given, and to
which the scorching, as it is called, of plants, the browning of the
autumnal leaves, and the ripening of fruits, appear to be due.[127]
When we come to the consideration of those physical phenomena which
belong to the growth of plants, all these peculiarities of solar action
must be attended to in detail.
The manner in which we find the actinic power influencing electrical
action, also shows us that the equilibrium of forces is continued
through all the great principles of nature. If a galvanic arrangement
is made, by which small quantities of metals may be slowly precipitated
at one of the poles in the dark, and a similar arrangement be exposed
to sunshine, it will be found that no metal is deposited: the sun’s
rays have interfered with the decomposing power of the electrical
current. At the same time we learn, that by throwing a beam of light
upon a plate of copper which forms one of a galvanic pair, whilst
it is under the influence of an acidulated solution, an additional
excitation takes place, and the galvanometer will indicate the passage
of an increased current of electricity. These two dissimilar actions
appear enigmatical; but they may, there is no doubt, receive some
solution from the influence of different rays on the contrary poles
of the battery. One thing is quite evident,--electricity suffers a
disturbance of one order, by light; and an excitement of another by its
associated principles in the sunbeam. If a yellow glass is interposed
between the galvanic arrangement and the sun, the electro-chemical
precipitation goes on in the same manner as it would in perfect
darkness, and no extra excitement is produced upon the plates of the
battery. From this it would appear that actinism and not light is to be
regarded as the disturbing power.[128] It has already been shown that
yellow media possess the power of stopping back the chemical agent.
We have already, detailed many of the peculiarities of the different
varieties of Phosphori, which would seem to be the result of light.
Phosphorescence is probably excited by those rays which produce
no direct effect upon the eye. If we spread sulphuret of calcium
upon paper, and expose it to the action of the solar spectrum, it
is found to glow (in the dark) only over those spaces occupied by
the violet rays and the ordinarily dark rays beyond them; proving
that the excitation necessary to the development of the phenomena
of phosphorescence is due to a class of rays distinct from the true
light-giving principle, and more nearly allied to that principle or
power which sets up chemical decomposition. Whether the fluorescent
rays, before mentioned, which are found so abundantly over the space
which produces the greatest phosphorescent effect, are active in
producing the phenomena, is as yet an unsolved problem.
Vision and colour, calorific action, chemical change, molecular
disturbance, electrical phenomena, and phosphorescent excitation, all,
each one with a strange duality, are connected with the sunbeam.
We find, when we receive solar spectra upon iodized plates, or on
several kinds of photographic paper, that a line, over which no action
takes place, is preserved at the top and bottom of the impressed image,
and in many cases along the sides also. The only way in which this can
be accounted for, as the spectrum represents the sun in a distorted
form, is by supposing that rays come from the edges of the sun of a
different character from those which proceed from the centre of that
orb.[129]
Light from the centre of the solar disc is under different conditions
from that which comes from the edge of the sun: this is due to the
varying angle, which is presented to us by a circular body: calorific
action seems to be more strongly manifested when the envelope of
light, extending like an atmosphere to the sun, is thrown into great
agitation, and waves, and great hollows--solar spots--are produced.
There is some indication of the existence of a third condition on the
sun’s surface, to which probably belongs the mighty chemical power
which we call actinism. Electricity may be, as some have speculated,
the exciting agent; a constant and violent Aurora Borealis may exist on
the sun, and under the excitation of this force the others named may be
quickened into full activity.
That actinism is one of the great powers of creation we have abundant
proof. Nearly all the phenomena of chemical change which have been
referred to light, are now proved to be dependent upon actinic power;
and beyond the influence which has been ascertained to be exerted by
it upon all inorganic bodies, we shall have occasion to show still
further the dependence of the vegetable and animal worlds upon its
agency. The influence of the solar beams on vegetation is proved by
common experience; the closer examination of its action on vegetable
life is reserved for the chapter devoted to its phenomena. Of its
influence on animals nothing is very correctly known; but some early
experiments prove that they, like other organised bodies, are subject
to all the radiant forces, as indeed, independent of experiment, every
observation must teach. Certain it is, that organisation can take place
only where the sun’s rays can penetrate: where there is unchanging
darkness, there we find all the silence of death. Prometheus stole
fire from heaven, and gave the sacred gift to man, as the most useful
to him of all things in his necessities: by the aid of it he could
temper the severities of climate, render his food more digestible and
agreeable, and illuminate the hours of darkness. So says the beautiful
fiction of the Grecian mind,--which appears as the poetic dream or
prophetic glance of a gifted race, who felt the mysterious truth
they were yet unable to describe. Pheaton and Apollo are only other
foreshadowings of the creative energies which dwell in the glorious
centre of our universe. The poetry of the Hellenic people ascended
above the littlenesses of merely human action, and sought to interpret
the great truths of creation. Reflective, they could not but see that
some mysterious powers were at work around them; imaginative, they gave
to fine idealisations the government of those inexplicable phenomena.
Modern science has shown what vastly important offices the solar rays
execute, and that the principles discovered in a sunbeam are indeed the
exciters of organic life, and the disposers of inorganic form.
It must not be forgotten that we have already alluded to a speculation
which supposes this actinic influence to be diffused through all
nature, to be indeed the element to which chemical force in all its
forms is to be referred, and that it is merely excited by the solar
rays. This hypothesis receives some support from the very peculiar
manner in which chemical action once set up is carried on, independent
of all extraneous excitement, after the first disturbance has been
produced. If any of the salts of gold are exposed in connection with
organic matter, as on paper, to sunshine for a moment, an action is
begun, which goes on unceasingly in the dark, until the gold is reduced
to its most simple state.[130] The same thing occurs with chromate of
silver, some of the salts of mercury, argentine preparations combined
with protosulphate of iron or gallic acid, and some other chemical
combinations. These progressive influences point to some law not yet
discovered, which seems to link this radiant actinism with the chemical
agent existing in all matter.
This problem also connects itself with another class of facts which,
although due, in all probability, to a great extent, to calorific
radiations, and hence known under the general term of Thermography,
appear to involve both chemical and electrical excitation. From the
investigations of Moser and of others, we learn the very extraordinary
fact, that even inanimate masses act and react upon each other by
the influence of some dark radiations, and seem to exchange some of
the peculiarities which they possess. This appears generally in the
curious experiments which have been referred to, as confined merely
to form or structure. Thus an engraved plate will give to a polished
surface of metal or glass placed near it, after a very little time,
a neat distinct image of itself; that is, produce such a structural
disturbance as will occasion the plate to receive vapour differently
over those spaces opposite to the parts in cameo or in intaglio, from
what it does over the opposite. If a piece of wood is used instead of a
metal, there will, by similar treatment, be produced a true picture of
the wood, even to the representation of its fibres.[131]
It is also probable that chemical decomposition is produced by the
mere juxtaposition of different bodies. Iodide of gold or silver,
perfectly pure, has been placed upon a plate of glass, and a plate of
copper covered with mercury suspended over it: a gradual decomposition
of those salts is said to have been observed, iodide of mercury to
be formed, and the gold or silver salts reduced to a finely divided
metallic state.[132]
A body whose powers of radiating heat are low, being brought near
another whose radiating powers are more extensive, will, in the course
of a short time, undergo such an amount of molecular disturbance as
will effect a complete change in the arrangement of its surface, and
an impression of the body having the highest radiating powers will be
made upon the other. This impression is dormant, but may be developed
under the influence of vapour, or of oxidation.[133] A body, such
as charcoal, of low conducting power, being placed near another,
such as copper, which is a good conductor, will, in a very short
time, produce, in like manner, an impression of itself upon the metal
plate. Thus any two bodies, whose conducting or radiating powers are
dissimilar, being brought near each other, will occasion a molecular
disturbance, or impress the one with the image of the other. However
small the difference may be, an effect is perceived, and that of the
most extraordinary kind, giving rise to the production of actual images
upon each surface exposed. It is thus that a print on paper may be
copied on metal, by merely suspending it near a well-polished plate of
silver or copper for a few days. The white and black lines radiate very
differently; consequently an effect is produced on the bright metal in
the parts corresponding to the black lines, dissimilar to that which
takes place opposite to the white portions of the paper; and, on the
application of vapour, a true image of the one is found impressed upon
the other.[134]
Bodies which are in different electrical states act upon each other in
an analogous manner. Thus arsenic, which is highly electro-negative,
will, when placed near a piece of electro-positive copper, readily
impart to its surface an impression of itself, and so in like manner
will other bodies if in unlike conditions. Every substance physically
different (it signifies not whether as it regards colour, chemical
composition, mechanical structure, calorific condition, or electrical
state,) has a power of radiation by which a sensible change can be
produced in a body differently constituted.
Fable has told us that the magicians of the East possessed mirrors in
which they could at will produce images of the absent. Science now
shows us that representations quite sufficient to deceive the credulous
can be produced on the surface of polished metals without difficulty.
A highly polished plate of steel may be impressed with images of any
kind, which would remain invisible, the polished surface not being in
the least degree affected, as it regards its reflecting powers; but
by breathing over it, the dormant images would develope themselves,
and fade away again as the condensed moisture evaporated from the
surface.[135]
These, which are but a few selected from a series of results of an
equally striking character, serve to convince us that nature is
unceasingly at work, that every atom is possessed of properties by
which it influences every other atom in the universe, and that a most
important class of natural phenomena appear to connect themselves
directly with the radiant forces.
The alchemists observed that a change took place in chloride of silver
exposed to sunshine. Wedgwood first took advantage of that discovery to
copy pictures. Niepce pursued a physical investigation of the curious
change, and found that all bodies were influenced by this principle
radiated from the sun. Daguerre produced effects from the solar pencil
which no artist could approach to; and Talbot and others extended the
application. Herschel took up the inquiry; and he, with his usual
power of inductive search and of philosophical deduction, presented
the world with a class of discoveries which showed how vast a field of
investigation was opening for the younger races of mankind,--a field
in which a true spirit may reap the highest reward in the discovery of
new facts, and to which we must look for a further development of those
great powers with which we have already some slight acquaintance, and
for the discovery of higher influences which are not yet dreamed of in
our philosophy.
If music, with its mysteries of sound,
Gives to the human heart a heavenward feeling;
The beauty and the grandeur which are found
Spread like a vesture this fair earth around,
Creation’s wond’rous harmonies revealing,
And to the soul in truth’s strong tongue appealing,
With all the magic of those secret powers,
Which, mingling with the lovely band of light,
The sun in constant undulation showers
To mould the crystals, and to shape the flowers,
Or give to matter the immortal might
Of an embracing soul--should, from this sod,
Exalt our aspirations all to God.
FOOTNOTES:
[117] See _Researches on Light_, by the Author.--Reference to any of
the works of the alchemists will prove the prevalence of the idea
expressed in the text. We find that gold was considered to be always
under the influence of light and solar heat.--“It is said of gold that
it waxeth cold towards daylight, insomuch that they who wear rings of
it may perceive when the day is ready to dawn.”--_Speculum Mundi, or a
Glass representing the face of the World_. Cambridge, 1643.
[118] Daguerre’s Report to the Academy of Sciences: _La Daguerréotype
Historique, et description des procédés du Daguerréotype et du Diorama_
(Paris, 1839); particularly the description of _Heliography_, by M.
Niepce. See also the letters by Niepce, published for the first time in
_Researches on Light_.
[119] “If a solution of peroxalate of iron be kept in a dark place,
or if it be exposed to 212° of Fahr. for several hours, it does not
undergo any sensible change in its physical properties, nor does it
exhibit any phenomenon which may be considered as the result of any
elementary action.
“If, however, it be exposed to the influence of solar light in a glass
vessel provided with a tube, the concentrated solution of oxalate
of iron soon presents a very interesting phenomenon: in a short
time the solution receiving the solar rays, developes an infinite
number of bubbles of gas, which rise in the liquor with increasing
rapidity, and give the solution the appearance of a syrup undergoing
strong fermentation. This ebullition always becomes stronger, and
almost tumultuous, when an unpolished glass tube is immersed in it
with a small piece of wood; the liquid itself is afterwards thrown
into ascending and descending currents, becomes gradually yellowish,
turbid, and eventually precipitates protoxalate of iron, in the form
of small brilliant crystals of a lemon-yellow colour, gas continuing
to evolve.” _Chemical action of light, and formation of Humboldtine
by it_; Phil. Mag., 1832, second series.--“When a solution of
platinum in nitro-muriatic acid, in which the excess of acid has been
neutralized by the addition of lime, and which has been well cleared
by filtration, is mixed with lime-water in the dark, no precipitation
to any considerable extent takes place for a long while,--indeed, none
whatever, though after very long standing a slight flocky sediment
is formed, after which the action is arrested entirely. But if the
mixture, either freshly made or when cleared by subsidence of this
sediment, is exposed to sunshine, it instantly becomes milky, and a
copious formation of a white precipitate (or a pale yellow one, if the
platinic solution be in excess) takes place, which subsides quickly
and is easily collected. The same takes place more slowly in cloudy
daylight.”--_On the action of light in determining the precipitation of
Muriate of Platinum by Lime water_; being an extract from a letter from
Sir John F. W. Herschel, K.H., F.R.S., &c., to Dr. Daubeny.--Phil. Mag.
1832.
[120] _On a change produced by Exposure to the Beams of the Sun, in
the properties of an elementary substance_, by Professor Draper;
_On the changes which bodies undergo in the dark_, by Robert Hunt:
Report of the Thirteenth Meeting of the British Association, vol.
xii,--_Description of the Tithonometer, an instrument for measuring
the chemical force of the Indigo-tithonic rays_: by J. W. Draper,
M.D.--Philosophical Magazine, Dec. 1843, vol. xxiii.
[121] For several illustrations of this remarkable phenomenon, see _On
the Action of the Rays of the Solar Spectrum on Vegetable Colours,
and on some new Photographic Processes_; by Sir John F. W. Herschel,
Bart., K.H., F.R.S.--Phil. Trans. June, 1842, vol. cxxxiii.; _On
certain improvements on Photographic Processes described in a former
communication, and on the Parathermic Rays of the Solar Spectrum_;
by Sir John F. W. Herschel, Bart., K.H., F.R.S., &c., in a letter
addressed to S. Hunter Christie.--Phil. Trans. 1843, vol. cxxxiv.
[122] Sir J. F. W. Herschel; see also _Researches on Light_, by the
Author.
[123] Attention has been directed to the protecting action of certain
rays of the spectrum by Sir John Herschel and others. See the
Eighteenth Report of the British Association for an experiment by
the Author, in which it was proved that all the LIGHT rays protected
photographic papers from chemical change, and, therefore, convincingly
show that light and actinism were not similar powers.
[124] “Having noticed, one densely foggy day, that the disc of the sun
was of a deep red colour, I directed my apparatus towards it. After
ten seconds of exposure, I put the prepared plate in the mercury box,
and I obtained a round image perfectly black;--the sun had produced no
photogenic effect. In another experiment, I left the plate operating
for twenty minutes; the sun had passed over a certain space of the
plate, and there resulted an image seven or eight times the sun’s
diameter in length; it was black throughout, so that it was evident,
wherever the red disc of the sun had passed, not only was there a
want of photogenic action, but the red rays had destroyed the effect
produced previous to the sun’s passage. I repeated these experiments
during several days successively, operating with a sun of different
tints of red and yellow. These different tints produced nearly the same
effect; wherever the sun had passed, there existed a black band.”--Mr.
Claudet, _On different properties of Solar Radiation, modified by
coloured glass media, &c._: Phil. Trans. 1847. Part 2.
[125] “It may also be observed that the rays effective in destroying
a given tint are, in a great many cases, those whose union produces a
colour complementary to the tint destroyed, or at least one belonging
to that class of colours to which such complementary tint may be
referred. For example, yellows tending towards orange are destroyed
with more energy by the blue rays; blue by the red, orange, and yellow
rays; purples and pinks by yellow and green rays.”--Sir J. F. W.
Herschel, _On the action of the rays of the Solar Spectrum on Vegetable
Colours_: Phil. Trans., vol. cxxxiii. 1842.
[126] The following memoirs and works are necessary to a complete
history of the inquiry:--_Experiments and observations relating
to various branches of natural philosophy, with a continuation of
the observation on air_: by Dr. Priestley. London, 1779. _Mémoires
Physico-chimiques, &c._: by J. Senebier. _Expériences sur les
végétaux_, by De la Ville: Paris, 1782; and Phil. Trans. 1782.
_Observations sur les expériences de M. Ingenhousz_: by De la Ville;
Roz. obs. 23, 290. _Expériences propres à développer les effets de la
lumière sur certaines plantes_: by Tessier; Mém. de l’Ac. des Sc. de
Paris, 1783, p. 132; Licht. Mag. iv. 4, 146. _Sur la vertu de l’eau
impregnée d’air fixe pour en obtenir, par le moyen des plantes et de
la lumière du soleil, de l’air déphlogistiqué_: by Ingenhousz; Roz.
obs. 24, 337. _Expériences sur l’action de la lumière solaire dans la
végétation_: by Senebier; Genève et Paris, 1788, p. 61. _Extrait des
expériences de M. Senebier sur l’action de la lumière solaire dans la
végétation_: by Hasenfratz; Ann. Chim. iii. 2nd. ser. 266. _Expériences
relatives à l’influence de la lumière sur quelques végétaux_: by De
Candolle; Jour. de Ph. lii. 124: Voigt’s Mag. ii. 483; Gilb. Ann.
xiii. 372; Mém. des Sav. Etr. i. 329. _Recherches chimiques sur la
végétation_: by Saussure; Ann. Chim. l. 225; Jour. de Ph. lvii. p.
393; Gilb. Ann. xviii 208. _Recherches sur la respiration des plantes
exposées à la lumière du soleil_; by Ruhland; Ann. Ch. Ph. iii. 411;
Jour. de Ph. 1816. _On the action of light upon plants, and of plants
upon the atmosphere_: by Dr. Daubeny; Phil. Trans. cxxvii January,
1836. _On the action of yellow light in producing the green colour, and
of indigo light on the movements of plants_: by P. Gardner; Phil. Mag.
xxiv.; Bibl. Univ. xlix. p. 376, and lii. p. 381. _On the influence of
light on plants_: by R. Hunt; Phil. Mag. xxiv. p. 96; Bibl. Univ. xlix.
p. 383; Athen. 1844. _Note on the decomposition of carbonic acid by
the leaves of plants, under the influence of yellow light_: by Draper;
Phil. Mag. xxv. p. 169. _On the action of the yellow rays of light on
vegetation_: by Harkness; Phil. Mag. xxv. p. 339. _Influence des rayons
solaires sur la végétation_: by Zantedeschi; Inst. No. 541, p. 157.
[127] Sir John Herschel’s Memoirs already referred to; and _Reports on
the influence of the Solar Rays on the growth of Plants_, by Robert
Hunt: Report of the British Association for the Advancement of Science,
for 1847.
[128] _Memoir on the Constitution of the Solar Spectrum_, presented at
the meeting of the Academy of Sciences, 1842, by M. Edmond Becquerel;
_Des effets produits sur les corps par les rayons solaires_, par M.
Edmond Becquerel, aide au Muséum d’Histoire Naturelle: Mémoire présenté
à l’Académie des Sciences, le 23 Octobre, 1843.--“Dans le courant de
ce mémoire, j’ai employé les noms de rayons lumineux, chimiques, et
phosphorogéniques, pour désigner, dans chaque cas, la portion des
rayons solaires qui agit pour produire, en particulier, les effets
lumineux, chimiques, et phosphorogéniques; mais cela est sans préjudice
de l’opinion que je viens d’émettre touchant l’existence d’un seul et
même rayonnement.”
“My reply is this,” says M. Arago, in his paper entitled
_Considerations relative to the action of Light_: “It is by no means
proved that the photogenic modifications of sensitive substances
result from the action of the solar light itself. The modifications
are, perhaps, engendered by invisible radiations mixed with light
properly so called, proceeding with it, and being similarly refracted.
In this case, the experiment would prove not only that the spectrum
formed by these invisible rays is not continuous, that there are
solutions of continuity as in the visible spectrum, but also that in
the two superposed spectra these solutions correspond exactly. This
would be one of the most curious, one of the most strange results of
physics.”--Taylor’s Scientific Memoirs.
[129] The chemical evidence of this will be found in Sir John
Herschel’s Memoir _On the Solar Spectrum_, and particularly as
exemplified in the changes produced on the tartrate of silver. Similar
influences are described as observed on a Daguerreotype plate, in
a paper entitled _Experiments and Observations on Light which has
permeated coloured media, and on the Chemical Action of the Solar
Spectrum_; by Robert Hunt.--Philosophical Magazine, vol. xxvi. 1840.
[130] This peculiar continuance of an effect has frequently been
observed in many of the photographic processes. In a note to a memoir
_On certain improvements in Photographic processes_, Sir John Herschel
thus refers to this property:--“The excitement is produced on such
paper by the ordinary moisture of the atmosphere, and goes on slowly
working its effect in the dark, apparently without other limit than is
afforded by the supply of ingredients present. In the case of silver it
ultimately produces a perfect _silvering_ of all the sunned portions.
Very singular and beautiful photographs, having much resemblance to
Daguerreotype pictures, are thus produced; the negative character
changing by keeping, and by quite insensible gradations to positive,
and the shades exhibiting a most singular _chatoyant_ change of colour
from ruddy-brown to black, when held more or less obliquely. No doubt,
also, gold pictures with the metallic lustre might be obtained by the
same process, though I have not tried the experiment.”
[131] The details of this curious subject may be studied in the
following memoir and communications:--_On vision and the action of
light on all bodies_: by Professor Ludwig Moser, of Königsberg; from
Poggendorff’s Annalen, vol. lvi. p. 177, No. 6, 1845. _Some remarks
on Invisible Light_: by Professor Ludwig Moser, of Königsberg; from
Poggendorff’s Annalen, vol. lvi. p. 569, No. 8. _On the power which
light possesses of becoming latent_: by Professor Ludwig Moser, of
Königsberg; from Poggendorff’s Annalen, vol. lvii. No. 9, p. 1. 1842.
_On certain spectral appearances, and on the discovery of latent
light_: by J. W. Draper, M.D., Professor of Chemistry in the University
of New York; Phil. Mag. p. 348, Nov. 1842. _On a new imponderable
substance, and on a class of chemical rays analogous to the rays of
dark heat_: by Professor Draper; Phil. Mag., Dec. 1842. _On the action
of the rays of the solar spectrum on the Daguerreotype plate_; by Sir
J. F. W. Herschel, Bart.; Phil. Mag., Feb. 1843. See remarks in this
paper on the use which Moser has made of coloured glasses: also a
communication by Professor Draper, _On the rapid Detithonizing power of
certain gases and vapours, and on an instantaneous means of producing
spectral appearances_: Phil. Mag., March 1843; and _On the causes which
concur in the production of the images of Moser_: Comptes Rendus, Nov.
1842. See _Scientific Memoirs_, vol. iii.
[132] This fact was first observed by myself, and described in the
paper already referred to, Philosophical Magazine, vol. xxii. p. 270.
It does not, however, appear to have attracted the attention of any
other observer.
[133] _On Thermography, or the Art of copying Engravings or any printed
characters from paper or plates of metal, and on the recent discovery
of Moser, relative to the formation of images in the dark_, by Robert
Hunt: Reports of the Royal Cornwall Polytechnic Society for 1842, and
Philosophical Magazine, vol. xxi. p. 462.--_On the Spectral Images of
M. Moser_, by Robert Hunt: Philosophical Magazine, vol. xxiii. p. 415.
[134] _Catalytic force, or attraction of surface concerned in the
diffusive power of gases: an occult energy or power in saturated saline
solutions_; Prater.--Mechanic’s Magazine, vol. xlv. p. 106. _Ueber
elektrische Abbildungen_; by G. Karsten.--Poggendorff’s Annalen, vol.
lvii. p. 402.--Melloni and Brewster may be consulted for much that is
most remarkable connected with radiation from coloured surfaces.
[135] Cornelius Agrippa is said to have possessed such a mirror.
The Chinese make mirrors which, when placed in a particular light,
show upon their polished faces the pattern on the back of the metal,
although it is invisible in every other position. This is effected by
giving different degrees of hardness to the various parts of the metal.
In _Natural Magic_, by Sir David Brewster, several curious experiments
belonging to this class are named.