Chapter II: Physical and Chemical Condition of the Sun

Physical Condition of the Sun.

143. _The Sun Composed mainly of Gas._--It is now generally believed 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.

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."

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

(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.

(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.

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.

(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_.

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.

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.

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.

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.

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.

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.

166. _Change of Pitch caused by Motion of Sounding Body._--When a sounding body is moving rapidly towards us, the pitch of its note becomes somewhat higher than when the body is stationary; and, when such a body is moving rapidly from us, the pitch of its note is lowered somewhat. We have a good illustration of this change of pitch at a country railway station on the passage of an express-train. The pitch of the locomotive whistle is considerably higher when the train is approaching the station than when it is leaving it.

167. _Explanation of the Change of Pitch produced by Motion._--The pitch of sound depends upon the rapidity with which the pulsations of sound beat upon the drum of the ear. The more rapidly the pulsations follow each other, the higher is the pitch: hence the shorter the sound-waves (provided the sound is all the while travelling at the same rate), the higher the pitch of the sound. Any thing, then, which tends to shorten the waves of sound tends also to raise its pitch, and any thing which tends to lengthen these waves tends to lower its pitch.

When a sounding body is moving rapidly forward, the sound-waves are crowded together a little, and therefore shortened; when it is moving backward, the sound-waves are drawn out, or lengthened a little.

The effect of the motion of a sounding body upon the length of its
sonorous waves will be readily seen from the following illustration:
Suppose a number of persons stationed at equal intervals in a line
on a long platform capable of moving backward and forward. Suppose
the men are four feet apart, and all walking forward at the same
rate, and that the platform is stationary, and that, as the men
leave the platform, they keep on walking at the same rate: the men
will evidently be four feet apart in the line in front of the
platform, as well as on it. Suppose next, that the platform is
moving forward at the rate of one foot in the interval between two
men's leaving the platform, and that the men continue to walk as
before: it is evident that the men will then be three feet apart in
the line after they have left the platform. The forward motion of
the platform has the effect of crowding the men together a little.
Were the platform moving backward at the same rate, the men would be
five feet apart after they had left the platform. The backward
motion of the platform has the effect of separating the men from one
another.

The distance between the men in this illustration corresponds to the
length of the sound-wave, or the distance between its two ends. Were
a person to stand beside the line, and count the men that passed him
in the three cases given above, he would find that more persons
would pass him in the same time when the platform is moving forward
than when it is stationary, and fewer persons would pass him in the
same time when the platform is moving backward than when it is
stationary. In the same way, when a sounding body is moving rapidly
forward, the sound-waves beat more rapidly upon the ear of a person
who is standing still than when the body is at rest, and less
rapidly when the sounding body is moving rapidly backward.

Were the platform stationary, and were the person who is counting
the men to be walking along the line, either towards or away from
the platform, the effect upon the number of men passing him in a
given time would be precisely the same as it would be were the
person stationary, and the platform moving either towards or away
from him at the same rate. So the change in the rapidity with which
pulsations of sound beat upon the ear is precisely the same whether
the ear is stationary and the sounding body moving, or the sounding
body is stationary and the ear moving.

168. _Change of Refrangibility due to the Motion of a Luminous Body._--Refrangibility in light corresponds to pitch in sound, and depends upon the length of the luminous waves. The shorter the luminous waves, the greater the refrangibility of the waves. Very rapid motion of a luminous body has the same effect upon the length of the luminous waves that motion of a sounding body has upon the length of the sonorous waves. When a luminous body is moving very rapidly towards us, its luminous waves are shortened a little, and its light becomes a little more refrangible; when the luminous body is moving rapidly from us, its luminous waves are lengthened a little, and its light becomes a little less refrangible.

169. _Displacement of Spectral Lines._--In examining the spectra of the stars, we often find that certain of the dark lines are _displaced_ somewhat, either towards the red or the violet end of the spectrum. As the dark lines are in the same position as the bright lines of the absorbing vapor would be, a displacement of the lines towards the red end of the spectrum indicates a lowering of the refrangibility of the rays, due to a motion of the luminous vapor away from us; and a displacement of the lines towards the violet end of the spectrum indicates an increase of refrangibility, due to a motion of the luminous vapor towards us. From the amount of the displacement of the lines, it is possible to calculate the velocity at which the luminous gas is moving. In Fig. 185 is shown the displacement of the _F_ line in the spectrum of Sirius. This is one of the hydrogen lines. _RV_ is the spectrum, _R_ being the red, and _V_ the violet end. The long vertical line is the bright _F_ line of hydrogen, and the short dark line to the left of it is the position of the _F_ line in the spectrum of Sirius. It is seen that this line is displaced somewhat towards the red end of the spectrum. This indicates that Sirius must be moving from us; and the amount of the displacement indicates that the star must be moving at the rate of some twenty-five or thirty miles a second.

170. _Contortion of Lines on the Disk of the Sun._--Certain of the dark lines seen on the centre of the sun's disk often appear more or less distorted, as shown in Fig. 186, which represents the contortion of the hydrogen line as seen at various times. 1 and 2 indicate a rapid motion of hydrogen away from us, or a _down-rush_ at the sun; 3 and 4 (in which the line at the centre is dark on one side, and bent towards the red end of the spectrum, and bright on the other side with a distortion towards the violet end of the spectrum) indicate a _down-rush_ of _cool_ hydrogen side by side with an _up-rush_ of _hot and bright_ hydrogen; 5 indicates local _down-rushes_ associated with _quiescent_ hydrogen.

The contorted lines, which indicate a violently agitated state of the sun's atmosphere, appear in the midst of other lines which indicate a quiescent state. This is owing to the fact that the absorption which produces the dark lines takes place at various depths in the solar atmosphere. There may be violent commotion in the lower layers of the sun's atmosphere, and comparative quiet in the upper layers. In this case, the lines which are due to absorption in the lower layers would indicate this disturbance by their contortions; while the lines produced by absorption in the upper layers would be free from contortion.

It often happens, too, that the contortions are confined to one set of lines of an element, while other lines of the same element are entirely free from contortions. This is undoubtedly due to the fact that different layers of the solar atmosphere differ greatly in temperature; so that the same element would give one set of lines at one depth, and another set at another depth: hence commotion in the solar atmosphere at any particular depth would be indicated by the contortion of those lines of the element only which are produced by the temperature at that particular depth.

A remarkable case of contortion witnessed by Professor Young is shown in Fig. 187. Three successive appearances of the _C_ line are shown. The second view was taken three minutes after the first, and the third five minutes after the second. The contortion in this case indicated a velocity ranging from two hundred to three hundred miles a second.

171. _Contortion of Lines on the Sun's Limb._--When the spectroscope is directed to the centre of the sun's disk, the distortion of the lines indicates only vertical motion in the sun's atmosphere; but, when the spectroscope is directed to the limb of the sun, displacements of the lines indicate horizontal motions in the sun's atmosphere. When a powerful spectroscope is directed to the margin of the sun's disk, so that the slit of the collimator tube shall be perpendicular to the sun's limb, one or more of the dark lines on the disk are seen to be prolonged by a bright line, as shown in Fig. 188. But this prolongation, instead of being straight and narrow, as shown in the figure, is often widened and distorted in various ways, as shown in Fig. 189. In the left-hand portion of the diagram, the line is deflected towards the red end of the spectrum; this indicates a violent wind on the sun's surface blowing away from us. In the right-hand portion of the diagram, the line is deflected towards the violet end of the spectrum; this indicates a violent wind blowing towards us. In the middle portion of the figure, the line is seen to be bent both ways; this indicates a cyclone, on one side of which the wind would be blowing from us, and on the other side towards us.

The distortions of the solar lines indicate that the wind at the surface of the sun often blows with a velocity of _from one hundred to three hundred miles a second_. The most violent wind known on the earth has velocity of a hundred miles an hour.