magnitude, and so small as to be hardly known, was observed to suddenly blaze out, and attain an apparent magnitude equal to that of the largest stars. Examined by the spectroscope it was found that, in addition to the usual dark lines, there were the bright lines of hydrogen, remarkably clear. The star, however, soon began to fade, and the bright lines to dwindle, and after the lapse of twelve days, when it had fallen to the eighth magnitude, these lines had totally disappeared. It seemed like the outburst of prominences upon our own sun, though on a far more stupendous scale.

210. Marvelous Delicacy of the Investigation.-In the case of the sun, the spectroscopist has to deal with light of overpowering brilliancy, his meridian rays being many times more intense than can be produced by any artificial means; but the light of the stars is at the opposite extreme. We shall appreciate the difficulty of these observations by remembering that the light of a star emanates from a mere point—that is, it has no sensible magnitude, and has to be kept steadily upon a slit only the part of an inch in breadth, and which is constantly altering its position with the motion of the earth. Moreover, this faint line of light has to be still further weakened by being spread out into a band. The air, besides, is so unsteady as to cause flickering, and confusion of the spectrum. Yet over all these embarrassments skill and patience have proved victorious. Roscoe says the spectrum of the star Sirius has been photographed by Huggins. The intensity of the light of this star is, according to the best measurements, the 6,000,000,000 part of that of the sun; and, although probably not less in size than sixty of our suns, it is estimated to be at the enormous distance of more than 130,000,000,000,000 miles; and yet even this immense distance does not prevent us registering the chemical intensity of the rays which left Sirius twentyone years ago (MILLER).

.

211. The Double Solar Spectrum.-Another remarkable result remains to be noticed, the spectroscopic proof of the motions of celestial masses; and to explain this we must refer again to the sun. As from his photosphere we get dark lines, and from his chromosphere bright ones, how are they related to each other? The lines from the same elements having the same positions, if the bright and dark spectra are brought together they should be continuous, and such is the fact. If the spectroscope be placed radially (Fig. 118), so that the slit s s covers the photosphere p and the chromosphere c, a double spectrum will be seen, and the dark lines will coincide with the bright lines. In Fig.

P

s

FIG. 118.

H

The Two Solar Spectra.

119 the dark Fraunhofer line Cis continuous with the scarlet hydrogen-line, and the same continuity is observed. with the lines of other elements.

212. Variations of the Bright Solar Lines.-It has been stated that the changes in the aspects of the lines may indicate physical alterations in the substances producing them, the hydrogen-lines, for example, being widened when the gas is under pressure. The bright solar hydrogen line H (Fig. 119) is generally more slender than the dark line C, which is

explained by the greater rarity of The Spectrum of the Sun's Disk

the hydrogen in the higher region

of the chromosphere. At the base,

below. and that of the Chromosphere above, near the Cline.

however, it is seen to be widened, an effect due to the pressure of the superincumbent mass. But it is also observed that the bright hydrogen-lines are often bent,

FIG. 121.

FIG. 120.
H

F

Changes in the F-line.

widened, twisted, and displaced, in a very remarkable way. Fig. 120 represents F, as pictured by Lockyer, strongly bulged and contorted; and Fig. 121 shows it as affected by a solar cyclone. These alterations of the positions of lines, in the spectrum, are simply changes of refrangibility, and, as the corresponding dark lines suffer no disturbance, at the same time we have to seek the cause of the altered refrangibility in some change of the hydrogen-mass above. An illustration from sound will help us to understand the cause of this. When in a railway-train we listen to the whistle of a rapidly-approaching engine, as it passes, the pitch of the sound falls. This is because, with the advance of the engine, the rate of air-vibrations striking upon the ear is increased. In the same way, if a luminous body is very rapidly approaching the eye, the ethereal waves that enter it are increased in number, and, as color depends upon their rate, the pitch of color, so to speak, will be altered. In the spec trum the effect would be to change the refrangibility of the rays, and consequently the position of the lines. With the swift approach of the body the more rapid wave-beats would displace the lines toward the violet, while the reces sion of the body would shift them toward the red. From this cause of variation in the lines, it becomes possible to trace the direction of gas-streams, cyclones, and the course of eruptive masses, and to account for the otherwise inexplicable mutations of the bright solar lines.

213. Motions of the Stars.-Perhaps the most splendid

triumph of spectrum analysis is the application of this principle to the determinations of the motions of the stars. Hitherto observations have been limited to movements across the field of vision. Spectrum analysis proves the approach and retreat of the stars by the displacement of the hydrogen-lines. Mr. Huggins first established this by a series of observations upon Sirius of the most consummate delicacy. A powerful spectroscope being applied, a slight displacement of H toward the red was discovered and verified by numerous observations. Fig. 122 shows the sition of this line in Sirius, as compared with its other determined positions. The normal A position is obtained by sealing up pure hydrogen in a vacuumB tube, free from pressure, and passing through it a stream of electric sparks. It will be

red

FIG. 122.
H

F

violet

A, Hydrogen-line in Vacuum-tube; B. F-line in the Spectrum of Sir ius; C, F-line in Solar Spectrum.

C

po

seen that the F-line of Sirius is started toward the red, as compared with both its normal and its solar positions. This displacement exactly measured corresponds to a receding motion of the star of twenty-nine miles per second. Later observations by Mr. Huggins, with instruments of still higher power, confirm and extend these results. Arcturus is shown to be approaching us at the rate of fifty-five miles per second, and the motions of various other stars have been established.

Only a meagre outline of spectrum analysis has here been given, and it can convey but an imperfect idea of the extent, precision, and surprising harmony, of the knowledge that has so quickly arisen upon this interesting subject. Those who care to pursue the subject further, are referred to the works of Schellen, Roscoe, and Lockyer.

CHEMICAL

PART II.

PRINCIPLES.

CHAPTER VIII.

GENERAL CHARACTER OF CHEMICAL ACTION.

214. FROM the study of those molecular forces which determine the forms of matter, and variously influence chemical phenomena, we now pass to the consideration of chemical changes themselves. There are, however, certain elementary facts and principles of the subject, leading to important theoretical views, which it is necessary to consider before stating the peculiar language of chemistry, or enter ing upon the detailed description of chemical substances.

215. The Chemical Force.-Affinity, chemism, or chemical force, are names given to that power in Nature which produces transformations of matter by altering its composition. It acts only at insensible distances, or when differ ent substances are brought into the closest relation with each other; but its effects are conspicuous, numberless, and of the highest importance. It is an inherent and universal energy of the natural world, from which no form of matter is exempt, and causes incessant and innumerable changes everywhere, around and within us. In the production of all its effects, the chemical force conforms to exact and inflexible laws, forming a science equally remarkable for the beauty of its principles, the depth of its philosophy, and the practical value of its applications.