appertains to him alone among all the stars of our world. Of himself
he burns, and sheds abroad light and heat. The other stars are neither
warm nor luminous, and if the sun did not exist, they would be plunged
into eternal darkness and eternal cold. This privilege alone ought to
make us comprehend the immense importance of the central star.
The light and heat which emanate from the sun are constant; they are
never interrupted, and they never lose their force. Thus, a second
characteristic--constancy of illumination--separates the sun from all
the other celestial bodies of our world.
The intensity of the real heat of the sun has been measured by the
physicists. This result was attained in an endeavour to determine by
experience the quantity of heat which accumulates in a given time,
upon a certain portion of the earth's surface, exposed to the sun's
rays, and adding to that element the quantities of heat which would be
absorbed by the atmospheric air, the ethereal spaces, and the soil.
Pouillet, the French physicist, who undertook this critical
investigation, arrived at certain results, which he states as follows:
"If the total quantity of heat emitted by the sun was exclusively
employed to melt a layer of ice applied to the solar globe, and
covering it completely in all its parts, that quantity of heat
would be able to melt, in one minute, a layer of eleven metres,
eighty centimetres, and in one day a layer of seventeen kilometres
in thickness."
"'This same quantity of heat,' says Professor Tyndall, 'would boil
2900 milliards of cubical kilometres of water, at the temperature
of ice.'"
The astronomer Herschel found, that, in order to extinguish the sun, to
prevent his "giving out caloric," according to the scientific phrase,
it would be necessary to dash a stream of iced water, or a cylindrical
column of ice, eighteen leagues in diameter, against its surface, at
a rate of speed of 70,000 leagues per second. A comparison adopted
by Professor Tyndall gives us an amazing view of the intensity of
the calorific force of the sun. "Imagine," says he, "that the sun is
surrounded by a layer of peat, seven leagues in thickness, the heat
produced by its combustion would be the same as that produced by the
sun in one year." The physicists have measured the intensity of the
sun's light with exactitude, as they had previously measured his heat.
It is known that the solar light is 300,000 times stronger than that
of the full moon, and 765,000,000 stronger than that of Sirius, the
most brilliant of the stars.
Bouguer discovered, by experiments made in 1725, that the sun, at a
height of 31° above the horizon, gives a light equal to that of 11,664
candles, placed within 43 centimetres of the object to be lighted, and
equal to 62,177 candles placed within one metre.
According to this result, if we take account of atmospheric absorption,
and of the law of the variation of the intensity of light, which
decreases in inverse ratio to the square of distance, the light given
by the sun at its zenith would be 75,200 times greater than that of
a single candle, placed within one metre. Wollaston had arrived at
a similar conclusion. By means of experiments of another kind, made
during the months of May and June, 1799, Wollaston found that 59,882
candles, at one metre, give as much light as the sun. Supposing the sun
to be in the zenith, the lightening power of that great star would be
equivalent to 68,009 candles.
There is but little difference between this valuation and that of
Bouguer, who states the result at 75,200 candles.
Whatever may be the intensity of the light of the sun, we now possess
other sources of light which approach to it. Such is the oxhydric
light, produced by burning hydrogen gas by means of a current of
oxygen gas, or air, a method of lighting which has recently been
employed in Paris and in London. This light is equal in power to more
than 200 candles. A thread of magnesium burning in the air, develops
a prodigious quantity of light, which may be taken as equivalent to
that of 500 candles. The electric light produced by a voltaic battery
of from 60 to 80 coils, produces a luminous arc equal to the light of
800 or 1000 candles. In the latter instance the voltaic arc, according
to Bouguer and Wollaston, would give 75 times less light than the sun,
supposing the luminous electric point to be placed at a distance of one
metre.
With very powerful batteries, it has been possible to go further, and
produce a light not much inferior to that of the sun. Messieurs Fizeau
and Foucault, by comparing the light of a voltaic arc, produced by the
action of three series of Bunsen's coils, of forty-six couples each,
with the light of the sun in a clear sky in April, have established
that the light-giving power of the sun is not more than twice and a
half that of the electric light.
The preceding numbers represent the light-giving power of the sun
upon our globe, taking into account atmospheric absorption. Arago, on
endeavouring to determine the intrinsic light-giving power of the sun,
found that the intensity of the solar light is 52,000 times greater
than that of a candle placed at one metre. But, according to more
recent researches for which we are indebted to Mr. Edmond Becquerel,
the result obtained by Arago is greatly inferior to the truth, and
the light of the central star is 180,000 times greater than that of a
candle placed at one metre.
All the planets, attended by their satellites, and all the comets which
accidentally manifest themselves to us, turn round the sun. The sun
remains motionless in the midst of this imposing procession of stars,
which circulate around him, like so many courtiers paying him homage.
Thus, the sun is the heart of our planetary system; everything is
drawn, everything converges towards him.
Half-informed persons will exclaim, "What can be more simple! The sun
being six hundred times the size of all the other stars put together,
the phenomenon of the condition of all those stars around the sun is
explained by the law of attraction, which prescribes that bodies shall
attract in proportion to their mass. If the sun attracts the stars of
our world to itself, it is because his mass is greater than that of all
the other stars collectively." But such an answer would be erroneous,
involving the common error of taking a word for a thing, an hypothesis
for an explanation, of putting a term of language in the place of a
logical consideration. When Newton conceived the hypothesis (and the
phrase) of _reciprocal attraction of matter_, he was careful to state
that he only proposed to characterise by a name a phenomenon which in
itself is entirely inexplicable, and of which we know nothing but the
exterior mode of its manifestation, that is to say, the mathematical
law. We know that bodies go towards each other in the ratio of their
masses, and in the inverse ratio of the square of their distances; but
why do they go towards each other? This is what we do not know, and
what we probably never shall know. If, for the word _attraction_ we
were to substitute the word _electrization_, or, as Keppler did, the
words _affection_, _sympathy_, _obedience_, &c., we should have a new
hypothesis, with a new name, but the mathematical law would remain the
same, the hypothesis only would be changed. The real cause which makes
small bodies rush towards large ones, and the stars of lesser magnitude
revolve round the stars of greater magnitude, is an impenetrable
mystery to mankind.
Whatever may be the hypothesis by which we seek to explain the fact,
it is certain that the sun holds the planets with their satellites,
the asteroids and the comets, suspended above the abysses of space,
and that they journey through the heavens in unintermitting obedience
to his guiding influence. The sun draws with him all the stars which
follow and surround him, like flatterers of his power, like humble
slaves of his universal preponderance. Like the father of a family
in the midst of his progeny, the sun peacefully governs the numerous
children of sidereal creation. Obedient to the irresistible impulsion
which emanates from the central star, the earth and the other planets
circulate, roll, gravitate, around him, receiving light, heat and
electricity from his beneficent rays, which are the first agents of
life. The sun marks out for the planets their path through the heavens,
and distributes to them their day and night, their seasons and their
climate.
The sun is, then, the hand which holds the stars above the unfathomable
abysses of infinite space, the centre from which they obtain heat, the
torch which gives them light, and the source whence they derive the
principle of life.
From all time the immense and unique task fulfilled by the sun in the
economy of nature has been understood. But this great truth has only
been deeply studied in our days. Science has gone far beyond all the
imagination the poets had conceived relative to the preponderance of
the sun in our world. By means of numerous experiments and abstruse
calculations, modern physicists have proved that the sun is the first
cause of almost all the phenomena which take place on our globe, and
that, without the sun, the earth and no doubt all the other planets
would be nothing but immense wastes, gigantic corpses, rolling about,
frozen and useless, in the deserts of infinite space.
Professor Tyndall, who has added largely to the discoveries of physics
and mechanics, has brought out this truth very strongly, and the
results to which he has been led may be said to form the most brilliant
page of contemporary physical science.
We shall now endeavour to explain how it is that everything on the
earth, and no doubt on all the other planets also, is derived from the
sun, so entirely, that we may affirm that vegetables, animals, man, in
short, all living beings, are but the productions, the children of the
sun; that they are, so to speak, woven out of solar rays.
In the first place, the sun is the primary cause of all those movements
which we observe, in the air, in the water, or in the ground under our
feet, and which keep up life, feeling, and activity on the surface of
our globe.
Let us consider the winds, which have such important relations with
all the physical phenomena of our globe. Whence proceed the winds?
From the action of the sun. The sun heats the different portions of
the earth very unequally, bestowing much more warmth on the tropical
and equatorial regions than on the other latitudes, which he leaves
exposed to cold. On each point of the earth which is struck by the rays
of the sun, the layers of air near the ground are dilated and raised,
and immediately replaced by colder layers from the temperate regions.
Thus the periodical winds are produced. Across the hemispheres two
great aërial currents are perpetually blowing, going from the equator
to each of the poles; one, the upper current, towards the north-east
in the northern hemisphere, and towards the south-east in the southern
hemisphere; the other, the lower current, in a contrary direction.
The movement of the earth gives rise to other regular winds. The action
of heat and of evaporation, added to the unequal distribution of the
continents and the seas, produce others, which are irregular. Thus,
for example, in the great valleys of the Alps, as in those of the
Cordilleras, the warmth of the air regulates the afflux of the cold
air of the mountains, and brings on tumultuous winds, and, in fact,
hurricanes.
The sea breezes arise from the difference in the temperature of the
shore during the day and the night. By day, the sun has warmed the
shore and produced a considerable dilatation of the air. When the sun
quits the horizon, this hot air is replaced by cool currents from the
inland. The same phenomenon is reversed in the morning, when the sun
returns; the shore is warmed, the hot air rises, and is replaced by
the colder air of the sea, which then goes inland. Thus, the evening
breeze comes from landward, and the morning breeze from seaward.
We see, therefore, that the great atmospheric movements which we call
the winds, are due to the successive appearances and disappearances
of the sun, as are also the lesser movements which we call breezes.
The position of the sun, constantly varying according to the period
of the year, and the hour of the day, explains the inequality and the
continuous existence of the aërial current.
The general cause of the winds which preserve the homogeneity of the
air in all the terrestrial regions, is the heat of the sun dilating the
atmospheric air; its absence, on the other hand, causes that gaseous
mass to contract.
The _watering of the globe_, that is to say the rain, an element
indispensable to the exercise of life, is another consequence of solar
heat. The waters of the seas, the rivers, and the lakes, those which
steep the soil, or are exhaled from vegetable matter, are gradually
transformed into vapour by the action of the sun's heat, and form
clouds and invisible vapour. When the sun has quitted the horizon,
these vapours grow cold in the bosom of the atmosphere in which they
floated, and fall down upon the earth again in the form of dew, of fog,
and of rain.
When the cooling of the watery vapour in the bosom of the atmosphere
is more intense, instead of rain we have snow, that is to say, a fall
of congealed water. It is chiefly on the summit of mountains that snow
falls and accumulates, because the temperature of elevated places is
always cold. In very great altitudes the snow, remaining for long
periods on the tops of the mountains, passes into an intermediate
condition, between snow and pure ice, and ends by forming those great
expanses of congealed water which are called glaciers. During the
hot seasons the glaciers melt by degrees; the water resulting from
this melting process, flows down the slopes of the mountains into the
valleys, and gives rise to springs, rivers, and streams. These streams
and rivers run into the ocean, from which they are again evaporated by
the action of solar heat, and reconstitute clouds and invisible vapour.
Thus is established and maintained that incessant circulation of
the waters which lie on the surface of the earth, their continual
exchange with the aërial masses, whose effect is to water the globe,
a phenomenon necessary to the exercise of the functions of organized
beings.
The regular currents which furrow the waters of the ocean are also the
result of the action of solar heat. From the poles to the equator the
waters of the sea are unequally heated, and this absence of equilibrium
in the temperature of the sea occasions a regular furrow, or line
from the poles to the equator, resulting from the displacement of the
waters, the cold waves rushing in to replace the hot. The unequal
evaporation caused by the unequal distribution of heat at the equator
and the poles, concurs to produce a similar result, by augmenting the
degree of saltness at the equator, without augmenting it at the poles,
occasioning a certain difference in density, and finally displacement
for want of equilibrium. The currents of the sea are thus entirely
produced by the action of the sun.
We see, therefore, that the winds, the watering of the globe, and the
currents of the sea are the consequence of solar heat.
The movement of the magnet is another physical result of the action
of the sun, if it be true, as Ampère says, that the magnetic currents
which traverse the terrestrial globe are nothing but _thermo-electric_
currents engendered by the unequal distribution of heat on the surface
of the globe.
In addition to being the agent of powerful physical forces, the sun is
a valuable agent of chemical forces,--indeed, this is the greatest part
which he plays in the phenomena of nature. The light and heat of the
sun produce the most important chemical actions on the earth's surface;
those on which the exercise of vegetable and animal functions depend.
If the sun did not exist, life would be banished from the terrestrial
globe. Life is the child of the sun, as I shall endeavour to prove to
you.
The operations of photography serve to make us understand how it is
that the sun presides over chemical action in the vegetable world. What
is photography? What does that curious phenomenon which fixes a drawing
formed by light upon a sheet of paper, consist of? A paper steeped in
chloride or iodide of silver is placed in the focus of the lens of a
dark camera, and the image formed by the lens is made to fall upon
paper sprinkled with water. The portions of the picture not exposed to
light produce no effect upon the salt of silver, which is incorporated
with the paper, but the portions exposed to light decompose the salt
of silver, and turn it black, or dark violet colour. On withdrawing
this paper from the apparatus, where the operations have been carried
on in darkness, we have a drawing which reproduces, in black, the
luminous image formed by the lens. By certain means this image, solely
produced by the chemical action of light, is rendered fixed and
unalterable.
All the salts of silver thus exposed to light undergo an analogous
decomposition. Nor are they the only salts which light modifies.
Compounds of gold, platinum, and cobalt, properly prepared, may also be
altered under the influence of direct or indirect rays, when exposed to
the sun, or to his diffused light.
The light of the sun possesses the power of bringing about the
combination of several other bodies. This is the case with hydrogen
and chloric gas. If you mix equal parts of chloric gas and hydrogen in
a bottle, and expose the mixture to the sun, an immediate combination
will take place between the two gases, and chlorohydric acid gas will
be formed. The combination will take place with so much force that it
will be attended by a considerable escape of heat. If you throw the
bottle containing the mixture up into the air, towards a space where
the sun is shining, the bottle will break before it falls, with a
violent explosion, at the moment of its contact with the light.
We might multiply examples of the chemical action produced by light
only on substances belonging to the mineral kingdom, but it is
sufficient for our purpose to say that the chemical action of light
is still more powerful and more general in the vegetable than in the
inorganic realm. This is a phenomenon of such importance that it is
impossible to believe it otherwise than a premeditated design of nature.
One of the most fruitful discoveries of modern science is the
recognition of the fact, that the respiration of plants depends upon
the presence and the direct action of light, that is to say, that the
decomposition of the carbonic acid which circulates in the tissue of
vegetables, and which has been breathed up from the soil by the roots,
takes place only when the plants are exposed to the sun. The labours of
Priestley, Charles Bonnet, Ingenhouz and Sennebier, have taught us that
the decomposition of carbolic acid into carbon, which remains fixed in
the tissue of the plant, and into oxygen, which disengages itself from
it, can take place only under the direct or indirect influence of the
sun's rays. Our readers may easily convince themselves of this fact.
Place a handful of green leaves in a glass full of water, and expose
the glass to the sun. At the close of the day the upper portion of the
glass will be filled with gas, which is nothing but pure oxygen, the
result of the breathing of the leaves.
All the importance, all the value of such a phenomenon will be evident,
if we reflect that it takes place over the whole extent of the globe,
and that the respiration, which means the life of all the vegetable
masses which cover the earth, depends solely upon the light of the
sun. It is by means of the respiration of the plants, which restores
oxygen to the atmospheric air, that nature makes up for the withdrawal
of oxygen by the respiration of animals, by the continual absorption
of that gas by numerous mineral substances, and by the frequent
combustions, natural and artificial, which occur in the world. The
result of these combustions would be the disappearance of the greater
portion of the oxygen contained in the air, if there did not exist a
permanent machinery for the restitution of that oxygen. This permanent
machinery is the respiration of plants, produced by solar light. So
absolute is the dependence of plants for their respiration on the
action of the sun's light, that if it be intercepted by clouds, the
escape of oxygen from them suffers a marked diminution. If the light of
the sun be suddenly stopped, which occurs during a total solar eclipse,
the escape of oxygen ceases, and the plants transpire carbonic acid
only, as they always do during the night.
It is for this reason that a plant kept in complete darkness loses its
colour, and becomes white. It does not respire, it emits carbonic acid
gas without retaining carbon, it becomes etiolated, according to the
scientific phrase, which means that the plant no longer lives at the
cost of the external air, or of gas furnished by the soil, but consumes
its own substance. The whitened salads which we prefer are not green
only because they are grown in darkness, and the mushrooms brought to
table are white only because they are reared in cellars.
M. Boussingault, who has studied vegetation in darkness, finds that
the leaves of a vegetable which has never had any light at all, in
its first appearance and development, never exhales oxygen, its
respiration furnishes carbonic acid gas only. The plant, therefore,
breathes just as an animal does. We must observe in this case that the
substance of the seed only supplies this product. The plant borrows
nothing from without, consumes nothing but the elements which were
contained in the seeds, and dies when those nutritive elements are
exhausted. The duration of its existence depends entirely on the
weight of the seed whence it has sprung. If a well-developed plant be
kept in darkness, the same fact may be observed. The plant gives out
nothing but carbonic acid, and, as it borrows nothing from without,
it perishes when it has thus devoured its own substance. M. Sachs
says, in his _Physiologie Végétale_, that the movements proper to the
leaves of many vegetables cannot take place if the plant is kept in
darkness. Plants so kept remain always in the condition which Linnæus
defined as _sleep_. Flowers contained in natural coverings, which in a
great measure debar them from the light of the sun, do indeed produce
colours, but those flowers are formed inside their natural coverings,
at the expense of substances contained in their leaves, which could not
be produced except under the influence of light. The same truth applies
to fruits.
Leaves, flowers, fruits, are then, as the German physiologist,
Moleschott, has said, "beings woven of air by light." The same author
adds: "When we contemplate the brilliant colours of the flowers, and
when their delicious perfume gives serene satisfaction to that poetic
faculty which exists, though it may slumber deeply, in the soul of
every man it is still the light which is the mother of colour and of
perfume."
The influence of the sun on vegetation is of fundamental importance.
Without the sun no plant would grow upon our globe. In those regions
which are permanently deprived of the powerful and beneficent torch
of nature, towards the extreme north, all vegetation is stunted, and
higher still, it does not exist. Absence of light, and cold, are the
causes of the complete disappearance of the natural adornment, and the
useful tribute, which elsewhere vegetation furnishes to the earth. In
the hot regions, vegetation is vigorous and extensive, in proportion
to the abundance of sunshine poured upon them. There is nothing to be
compared to the luxuriant vegetation of the tropical countries in both
hemispheres. The vegetation of Brazil, of equatorial Africa, and the
inter-tropical regions of India, is renowned for its abundance and
variety.