that the sun is mainly a ball of gas, or vapor, powerfully condensed at
the centre by the weight of the superincumbent mass, but kept from
liquefying by its exceedingly high temperature.
The gaseous interior of the sun is surrounded by a layer of luminous
clouds, which constitutes its visible surface, and which is called its
_photosphere_. Here and there in the photosphere are seen dark _spots_,
which often attain an immense magnitude.
These clouds float in the _solar atmosphere_, which extends some
distance beyond them.
The luminous surface of the sun is surrounded by a _rose-colored_
stratum of gaseous matter, called the _chromosphere_. Here and there
great masses of this chromospheric matter rise high above the general
level. These masses are called _prominences_.
Outside of the chromosphere is the _corona_, an irregular halo of faint,
pearly light, mainly composed of filaments and streamers, which radiate
from the sun to enormous distances, often more than a million of miles.
In Fig. 161 is shown a section of the sun, according to Professor Young.
The accompanying lithographic plate gives a general view of the
photosphere with its spots, and of the chromosphere and its prominences.
144. _The Temperature of the Sun._--Those who have investigated the
subject of the temperature of the sun have come to very different
conclusions; some placing it as high as four million degrees Fahrenheit,
and others as low as ten thousand degrees. Professor Young thinks that
Rosetti's estimate of eighteen thousand degrees as the _effective
temperature_ of the sun's surface is probably not far from correct. By
this is meant the temperature that a uniform surface of lampblack of the
size of the sun must have in order to radiate as much heat as the sun
does. The most intense artificial heat does not exceed four thousand
degrees Fahrenheit.
[Illustration: Fig. 161.]
145. _The Amount of Heat Radiated by the Sun._--A unit of heat is the
amount of heat required to raise a pound of water one degree in
temperature. It takes about a hundred and forty-three units of heat to
melt a pound of ice without changing its temperature. A cubic foot of
ice weighs about fifty-seven pounds. According to Sir William Herschel,
were all the heat radiated by the sun concentrated on a cylinder of ice
forty-five miles in diameter, it would melt it off at the rate of about
a hundred and ninety thousand miles a second.
Professor Young gives the following illustration of the energy of solar
radiation: "If we could build up a solid column of ice from the earth to
the sun, two miles and a quarter in diameter, spanning the inconceivable
abyss of ninety-three million miles, and if then the sun should
concentrate his power upon it, it would dissolve and melt, not in an
hour, nor a minute, but in a single second. One swing of the pendulum,
and it would be water; seven more, and it would be dissipated in vapor."
[Illustration: Fig. 162.]
This heat would be sufficient to melt a layer of ice nearly fifty feet
thick all around the sun in a minute. To develop this heat would require
the hourly consumption of a layer of anthracite coal, more than sixteen
feet thick, over the entire surface of the sun; and the _mechanical
equivalent_ of this heat is about ten thousand horse-power on every
square foot of the sun's surface.
146. _The Brightness of the Sun's Surface._--The sun's surface is a
hundred and ninety thousand times as bright as a candle-flame, a hundred
and forty-six times as bright as the calcium-light, and about three
times and a half as bright as the voltaic arc.
The sun's disk is much less bright near the margin than near the centre,
a point on the limb of the sun being only about a fourth as bright as
one near the centre of the disk. This diminution of brightness towards
the margin of the disk is due to the increase in the absorption of the
solar atmosphere as we pass from the centre towards the margin of the
sun's disk; and this increased absorption is due to the fact, that the
rays which reach us from near the margin have to traverse a much greater
thickness of the solar atmosphere than those which reach us from the
centre of the disk. This will be evident from Fig. 162, in which the
arrows mark the paths of rays from different parts of the solar disk.
The Spectroscope.
[Illustration: Fig. 163.]
147. _The Spectroscope as an Astronomical Instrument._--The
_spectroscope_ is now continually employed in the study of the physical
condition and chemical constitution of the sun and of the other heavenly
bodies. It has become almost as indispensable to the astronomer as the
telescope.
148. _The Dispersion Spectroscope._--The essential parts of the
_dispersion_ spectroscope are shown in Fig. 163. These are the
_collimator tube_, the _prism_, and the _telescope_. The collimator tube
has a narrow slit at one end, through which the light to be examined is
admitted, and somewhere within the tube a lens for condensing the light.
The light is dispersed on passing through the prism: it then passes
through the objective of the telescope, and forms within the tube an
image of the spectrum, which is examined by means of the eye-piece. The
power of the spectroscope is increased by increasing the number of
prisms, which are arranged so that the light shall pass through one
after another in succession. Such an arrangement of prisms is shown in
Fig. 164. One end of the collimator tube is seen at the left, and one
end of the telescope at the right. Sometimes the prisms are made long,
and the light is sent twice through the same train of prisms, once
through the lower, and once through the upper, half of the prisms. This
is accomplished by placing a rectangular prism against the last prism of
the train, as shown in Fig. 165.
[Illustration: Fig. 164.]
[Illustration: Fig. 165.]
149. _The Micrometer Scale._--Various devices are employed to obtain an
image of a micrometer scale in the tube of the telescope beside that of
the spectrum.
[Illustration: Fig. 166.]
One of the simplest of these methods is shown in Fig. 166. _A_ is the
telescope, _B_ the collimator, and _C_ the micrometer tube. The opening
at the outer end of _C_ contains a piece of glass which has a micrometer
scale marked upon it. The light from the candle shines through this
glass, falls upon the surface of the prism _P_, and is thence reflected
into the telescope, where it forms an enlarged image of the micrometer
scale alongside the image of the spectrum.
[Illustration: Fig. 167.]
150. _The Comparison of Spectra._--In order to compare two spectra, it
is desirable to be able to see them side by side in the telescope. The
images of two spectra may be obtained side by side in the telescope tube
by the use of a little rectangular prism, which covers one-half of the
slit of the collimator tube, as shown in Fig. 167. The light from one
source is admitted directly through the uncovered half of the slit,
while the light from the other source is sent through the covered
portion of the slit by reflection from the surface of the rectangular
prism. This arrangement and its action will be readily understood from
Fig. 167.
[Illustration: Fig. 168.]
151. _Direct-Vision Spectroscope._--A beam of light may be dispersed,
without any ultimate deflection from its course, by combining prisms of
crown and flint glass with equal refractive, but unequal dispersive
powers. Such a combination of prisms is called a _direct-vision_
combination. One of three prisms is shown in Fig. 168, and one of five
prisms in Fig. 169.
[Illustration: Fig. 169.]
[Illustration: Fig. 170.]
A _direct-vision spectroscope_ (Fig. 170) is one in which a
direct-vision combination of prisms is employed. _C_ is the collimator
tube, _P_ the train of prisms, _F_ the telescope, and _r_ the comparison
prism.
[Illustration: Fig. 171.]
152. _The Telespectroscope._--The spectroscope, when used for
astronomical work, is usually combined with a telescope. The compound
instrument is called a _telespectroscope_. The spectroscope is mounted
at the end of the telescope in such a way that the image formed by the
object-glass of the telescope falls upon the slit at the end of the
collimator tube. A telespectroscope of small dispersive power is shown
in Fig. 171; _a_ being the object-glass of the telescope, _cc_ the tube
of the telescope, and _e_ the comparison prism at the end of the
collimator tube. A more powerful instrument is shown in Fig. 172. _A_ is
the telescope, _C_ the collimator tube of the spectroscope, _P_ the
train of prisms, and _E_ the telescope tube. Fig. 173 shows a still more
powerful spectroscope attached to the great Newall refractor (18).
[Illustration: Fig. 172.]
[Illustration: Fig. 173.]
153. _The Diffraction Spectroscope._--A _diffraction_ spectroscope is
one in which the spectrum is produced by reflection of the light from a
finely ruled surface, or _grating_, as it is called, instead of by
dispersion in passing through a prism. The essential parts of this
instrument are shown in Fig 174. This spectroscope may be attached to
the telescope in the same manner as the dispersion spectroscope. When
the spectroscope is thus used, the eye-piece of the telescope is
removed.
[Illustration: Fig. 174.]
Spectra.
154. _Continuous Spectra._--Light from an incandescent solid or liquid
which has suffered no absorption in the medium which it has traversed
gives a spectrum consisting of a continuous colored band, in which the
colors, from the red to the violet, pass gradually and imperceptibly
into one another. The spectrum is entirely free from either light or
dark lines, and is called a _continuous spectrum_.
155. _Bright-Lined Spectra._--Light from a luminous gas or vapor gives a
spectrum composed of bright lines separated by dark spaces, and known as
a _bright-lined spectrum_. It has been found that the lines in the
spectrum of a substance in the state of a gas or vapor are the most
characteristic thing about the substance, since no two vapors give
exactly the same lines: hence, when we have once become acquainted with
the bright-lined spectrum of any substance, we can ever after recognize
that substance by the spectrum of its luminous vapor. Even when several
substances are mixed, they may all be recognized by the bright-lined
spectrum of the mixture, since the lines of all the substances will be
present in the spectrum of the mixture. This method of identifying
substances by their spectra is called _spectrum analysis_.
The bright-lined spectra of several substances are given in the
frontispiece. The number of lines in the spectra of the elements varies
greatly. The spectrum of sodium is one of the simplest, while that of
iron is one of the most complex. The latter contains over six hundred
lines. Though no two vapors give identical spectra, there are many cases
in which one or more of the spectral lines of one element coincide in
position with lines of other elements.
156. _Methods of rendering Gases and Vapors Luminous._--In order to
study the spectra of vapors and gases it is necessary to have some
means of converting solids and liquids into vapor, and also of
rendering the vapors and gases luminous. There are four methods of
obtaining luminous vapors and gases in common use.
[Illustration: Fig. 175.]
(1) _By means of the Bunsen Flame._--This is a very hot but an almost
non-luminous flame. If any readily volatilized substance, such as the
compounds of sodium, calcium, strontium, etc., is introduced into this
flame on a fine platinum wire, it is volatilized in the flame, and its
vapor is rendered luminous, giving the flame its own peculiar color. The
flame thus colored may be examined by the spectroscope. The arrangement
of the flame is shown in Fig. 175.
[Illustration: Fig. 176.]
(2) _By means of the Voltaic Arc._--An electric lamp is shown in Fig.
176. When this lamp is to be used for obtaining luminous vapors, the
lower carbon is made larger than the upper one, and hollowed out at the
top into a little cup. The substance to be volatilized is placed in this
cup, and the current is allowed to pass. The heat of the voltaic arc is
much more intense than that of the Bunsen flame: hence substances that
cannot be volatilized in the flame are readily volatilized in the arc,
and the vapor formed is raised to a very high temperature.
(3) _By means of the Spark from an Induction Coil._--The arrangement of
the coil for obtaining luminous vapors is shown in Fig. 177.
[Illustration: Fig. 177.]
The terminals of the coil between which the spark is to pass are brought
quite close together. When we wish to vaporize any metal, as iron, the
terminals are made of iron. On the passage of the spark, a little of the
iron at the ends of the terminals is evaporated; and the vapor is
rendered luminous in the space traversed by the spark. A condenser is
usually placed in the circuit. With the coil, the temperature may be
varied at pleasure; and the vapor may be raised even to a higher
temperature than with the electric lamp. To obtain a low temperature,
the coil is used without the condenser. By using a larger and larger
condenser, the temperature may be raised higher and higher.
By means of the induction coil we may also heat gases to incandescence.
It is only necessary to allow the spark to pass through a space filled
with the gas.
[Illustration: Fig. 178.]
(4) _By means of a Vacuum Tube._--The form of the vacuum tube commonly
used for this purpose is shown in Fig. 178. The gas to be examined, and
which is contained in the tube, has very slight density: but upon the
passage of the discharge from an induction coil or a Holtz machine,
through the tube, the gas in the capillary part of the tube becomes
heated to a high temperature, and is then quite brilliant.
157. _Reversed Spectra._--If the light from an incandescent cylinder of
lime, or from the incandescent point of an electric lamp, is allowed to
pass through luminous sodium vapor, and is then examined with a
spectroscope, the spectrum will be found to be a bright spectrum crossed
by a single _dark_ line in the position of the yellow line of the sodium
vapor. The spectrum of sodium vapor is _reversed_, its bright lines
becoming dark and its dark spaces bright. With a spectroscope of any
considerable power, the yellow line of sodium vapor is resolved into a
double line. With a spectroscope of the same power, the dark sodium line
of the reversed spectrum is seen to be a double line.
It is found to be generally true, that the spectrum of the light from an
incandescent solid or liquid which has passed _through a luminous vapor_
on its way to the spectroscope is made up of a bright ground crossed by
dark lines; there being a dark line for every bright line that the vapor
alone would give.
158. _Explanation of Reversed Spectra._--It has been found that gases
absorb and quench rays of the same degree of refrangibility as those
which they themselves emit, and no others. When a solid is shining
through a luminous vapor, this absorbs and quenches those rays from the
solid which have the same degrees of refrangibility as those which it is
itself emitting: hence the lines of the spectrum receive light from the
vapor alone, while the spaces between the lines receive light from the
solid. Now, solids and liquids, when heated to incandescence, give a
very much brighter light than vapors and gases at the same temperature:
hence the lines of a reversed spectrum, though receiving light from the
vapor or gas, appear dark by contrast.
159. _Effect of Increasing the Power of the Spectroscope upon the
Brilliancy of a Spectrum._--An increase in the power of a spectroscope
diminishes the brilliancy of a _continuous_ spectrum, since it makes the
colored band longer, and therefore spreads the light out over a greater
extent of surface; but, in the case of a _bright-lined_ spectrum, an
increase of power in the spectroscope produces scarcely any alteration
in the brilliancy of the lines, since it merely separates the lines
farther without making the lines themselves any wider. In the case of a
_reversed_ spectrum, an increase of power in the spectroscope dilutes
the light in the spaces between the lines without diluting that of the
lines: hence lines which appear dark in a spectroscope of slight
dispersive power may appear bright in an instrument of great dispersive
power.
160. _Change of the Spectrum with the Density of the Luminous
Vapor._--It has been found, that, as the density of a luminous vapor is
diminished, the lines in its spectrum become fewer and fewer, till they
are finally reduced to one. On the other hand, an increase of density
causes new lines to appear in the spectrum, and the old lines to become
thicker.
161. _Change of the Spectrum with the Temperature of the Luminous
Vapor._--It has also been found that the appearance of a bright-lined
spectrum changes considerably with the temperature of the luminous
vapor. In some cases, an increase of temperature changes the relative
intensities of the lines; in other cases, it causes new lines to appear,
and old lines to disappear.
In the case of a compound vapor, an increase of temperature causes the
colored bands (which are peculiar to the spectrum of the compound) to
disappear, and to be replaced by the spectral lines of the elements of
which the compound is made up. The heat appears to _dissociate_ the
compound; that is, to resolve it into its constituent elements. In this
case, each elementary vapor would give its own spectral lines. As the
compound is not completely dissociated at once, it is possible, of
course, for one or more of the spectral lines of the elementary vapors
to co-exist in the spectrum with the bands of the compound.
It has been found, that, in some cases, the spectra of the elementary
gases change with the temperature of the gas; and Lockyer thinks he has
discovered conclusive evidence, in the spectra of the sun and stars,
that many of the substances regarded as elementary are really resolved
into simpler substances by the intense heat of the sun; in other words,
that our so-called elements are really compounds.
Chemical Constitution of the Sun.
162. _The Solar Spectrum._--The solar spectrum is crossed transversely
by a great number of fine dark lines, and hence it belongs to the class
of _reversed_ spectra.
These lines were first studied and mapped by Fraunhofer, and from him
they have been called _Fraunhofer's lines_.
[Illustration: Fig. 179.]
A reduced copy of Fraunhofer's map is shown in Fig. 179. A few of
the most prominent of the dark solar lines are designated by the
letters of the alphabet. The other lines are usually designated by
the numbers at which they are found on the scale which accompanies
the map. This scale is usually drawn at the top of the map, as will
be seen in some of the following diagrams. The two most elaborate
maps of the solar spectrum are those of Kirchhoff and Angström. The
scale on Kirchhoff's map is an arbitrary one, while that of Angström
is based upon the wave-lengths of the rays of light which would fall
upon the lines in the spectrum.
[Illustration: Fig. 180.]
The appearance of the spectrum varies greatly with the power of the
spectroscope employed. Fig. 180 shows a portion of the spectrum as
it appears in a spectroscope of a single prism: while Fig. 181 shows
the _b_ group of lines alone, as they appear in a powerful
diffraction spectroscope.
[Illustration: Fig. 181.]
163. _The Telluric Lines._--There are many lines of the solar spectrum
which vary considerably in intensity as the sun passes from the horizon
to the meridian, being most intense when the sun is nearest the horizon,
and when his rays are obliged to pass through the greatest depth of the
earth's atmosphere. These lines are of atmospheric origin, and are due
to the absorption of the aqueous vapor in our atmosphere. They are the
same lines that are obtained when a candle or other artificial light is
examined with a spectroscope through a long tube filled with steam.
Since these lines are due to the absorption of our own atmosphere, they
are called _telluric lines_. A map of these lines is shown in Fig. 182.
[Illustration: Fig. 182.]
164. _The Solar Lines._--After deducting the telluric lines, the
remaining lines of the solar spectrum are of solar origin. They must be
due to absorption which takes place in the sun's atmosphere. They are,
in fact, the reversed spectra of the elements which exist in the solar
atmosphere in the state of vapor: hence we conclude that the luminous
surface of the sun is surrounded with an atmosphere of luminous vapors.
The temperature of this atmosphere, at least near the surface of the
sun, must be sufficient to enable all the elements known on the earth to
exist in it as vapors.
[Illustration: Fig. 183.]
165. _Chemical Constitution of the Sun's Atmosphere._--To find whether
any element which exists on the earth is present in the solar
atmosphere, we have merely to ascertain whether the bright lines of its
gaseous spectrum are matched by dark lines in the solar spectrum when
the two spectra are placed side by side. In Fig. 183, we have in No. 1 a
portion of the red end of the solar spectra, and in No. 2 the spectrum
of sodium vapor, both as obtained in the same spectroscope by means of
the comparison prism. It will be seen that the double sodium line is
exactly matched by a double dark line of the solar spectrum: hence we
conclude that sodium vapor is present in the sun's atmosphere. Fig. 184
shows the matching of a great number of the bright lines of iron vapor
by dark lines in the solar spectrum. This matching of the iron lines
establishes the fact that iron vapor is present in the solar atmosphere.
[Illustration: Fig. 184.]
The following table (given by Professor Young) contains a list of
all the elements which have, up to the present time, been detected
with certainty in the sun's atmosphere. It also gives the number of
bright lines in the spectrum of each element, and the number of
those lines which have been matched by dark lines in the solar
spectrum:--
Elements. Bright Lines Reversed. Observer.
Lines.
1. Iron 600 460 Kirchhoff.
2. Titanium 206 118 Thalen.
3. Calcium 89 75 Kirchhoff.
4. Manganese 75 57 Angström.
5. Nickel 51 33 Kirchhoff.
6. Cobalt 86 19 Thalen.
7. Chromium 71 18 Kirchhoff.
8. Barium 26 11 Kirchhoff.
9. Sodium 9 9 Kirchhoff.
10. Magnesium 7 7 Kirchhoff.
11. Copper? 15 7? Kirchhoff.
12. Hydrogen 5 5 Angström.
13. Palladium 29 5 Lockyer.
14. Vanadium 54 4 Lockyer.
15. Molybdenum 27 4 Lockyer.
16. Strontium 74 4 Lockyer.
17. Lead 41 3 Lockyer.
18. Uranium 21 3 Lockyer.
19. Aluminium 14 2 Angström.
20. Cerium 64 2 Lockyer.
21. Cadmium 20 2 Lockyer.
22. Oxygen a 42 12 ± bright H. Draper.
Oxygen b 4 4? Schuster.
In addition to the above elements, it is probable that several other
elements are present in the sun's atmosphere; since at least one of
their bright lines has been found to coincide with dark lines of the
solar spectrum. There are, however, a large number of elements, no
traces of which have yet been detected; and, in the cases of the
elements whose presence in the solar atmosphere has been
established, the matching of the lines is far from complete in the
majority of the cases, as will be seen from the above table. This
want of complete coincidence of the lines is undoubtedly due to the
very high temperature of the solar atmosphere. We have already seen
that the lines of the spectrum change with the temperature; and, as
the temperature of the sun is far higher than any that we can
produce by artificial means, we might reasonably expect that it
would cause the disappearance from the spectrum of many lines which
we find to be present at our highest temperature.
Lockyer maintains that the reason why no trace of the spectral lines
of certain of our so-called elements is found in the solar
atmosphere is, that these substances are not really elementary, and
that the intense heat of the sun resolves them into simpler
constituents.
Motion at the Surface of the Sun.