The child listens attentively; divining, by a sort of intuition, the sense of these teachings, engraving themselves, in letters of fire, on her heart; and which she will understand, each day, more and more.
Little by little, lulled by the whispering of her father; refreshed, as if bathed in such admirable tenderness, she fell asleep. Her father held her in his arms, and, raising his eyes, he prayed.
Day has come. The aurora awakes in its humid splendor, and throws its first rays over the mountain violets. The bells of the town dance into the air their clear and joyous notes.
"My father," said Paganina in a low voice, and without opening her eyes, "what do those bells say? Their ringing sound makes me tremble with joy."
"My daughter, they celebrate, as they may, the day of the Ascension, when Christ ascended into heaven."
"To heaven! my father;" and she added, in so weak a voice that he could scarcely hear her, "It seems that I am there now—that I repose in your arms."
The organist looked at his daughter, whose closed eyes seemed to enjoy interior contemplation; while his pale face expressed his delight. He raised her; held her up, as if to offer her to God; then laid her quietly on her little bed, and let her sleep.
XV.
From that day, the organist possessed perfect control over his daughter. If she seemed disposed to escape from his influence, he recalled the night of the Ascension, and that sufficed. Paganina was still a little girl; but soon she would cease to be one. Her future beauty was crystallizing. The features could be seen; but they had not yet blended into their after harmony. There was something surprising about her.
Morally, the incomprehensible little creature was all dissonance and violent contrasts, promising to be equally powerful for good or evil, as she should be led by superior or inferior influences.
The distinctive character of her nature, habitually concentrated and sometimes impetuous to excess, was her passion for every thing beautiful. Music exercised an extraordinary influence over her. It was, properly speaking, her language; and she understood in it what others could not. Already she spoke in it wonderfully.
Her father taught her his instrument; and she gave herself with love to the study. However, it was easy to see that the demon of song would make her his; so Master Swibert hesitated to give her a master, restrained by his personal ideas on the subject. He had his theory, which appeared singular, no doubt, and he revealed it to his daughter, saying, "Too perfect an instrument is a snare for a musician; for when he has at his service an organ of this kind, he forgets too often to raise it to the ideal, and gives it to matter. Where are those who can disengage themselves from matter to arrive at an idea? Where are those who know that the beauty of the body is the shadow of the beauty of the soul? To pursue exclusively the first is to lose both.
"Look at the immortal composers of my country, whose genius will radiate unto the last of posterity. The shrill notes of the piano are the most common expression of their glorious thoughts. The musicians of this nation find voices neither pure nor powerful enough to express their pitiful imaginations. When I see such anxiety for the sign, I esteem poorly the thing signified, and I think that its beauty is, above all, material.
"I love the human voice. What an admirable instrument! But I tremble to see how it is used to express the passions of earth and the enchantments of pleasure. It is dangerous to possess it. I warn you of your danger, my daughter."
I have already said that this theory was singular. The word appears weak, perhaps; but it came from Germany.
However, it had no influence on the destiny of Paganina; for, having finished his reasoning, her father gave her a master. Happily, logic alone does not govern the world.
The little one then learned to sing. Her success in this study was rapid, and passed all foresight. Sometimes Master Swibert was confounded when he heard her, and trembled before this power which had come from himself.
XVI.
The moment came when André was to be submitted to the proof of a public education. His uncle considered such a course necessary to make him a man. It was decided that he should receive at the conservatory of Naples the classic traditions of Italian art. The organist and his daughter wished to accompany him to his destination.
They travelled by short stages. Master Swibert proposing, according to his habit, an elevated result, communicated to his children the riches of his erudition. They stopped wherever they could hope to gather some fruit, curious to visit every place of which they knew the history, and he desirous to give them a living knowledge which would be for ever impressed upon them.
His studies and affections induced him to neglect the mere vestiges of antiquity to seek with greater love the souvenirs of Christianity and the relics of the saints. We know if they abound on this illustrious earth.
Every day, then, the travellers turned a new leaf of the book which they had lisped from their childhood. The history of the martyrs particularly seized upon the imagination of Paganina. She never tired of listening to it on the very places they had sanctified by such sublime acts as the world rarely knows.
We may scoff at or disdain the wonders of interior sanctity, but indifference is arrested by the heroism of martyrdom.
The martyrs wear the double crown of divine and human glory. After their God, they are the vanquishers of death. Inspired courage burns on their faces; and when are added to their ranks the grace and beauty of woman and child, why refuse to their memory the homage of love and admiration, if even not to be Christian is considered worthy of worldly honor.
Paganina had the intelligence of greatness; she loved courage and true nobility. The recitals of her father drew tears from her eyes; and in traversing the arenas made memorable by some bloody triumph, she felt within her every inspiration to celebrate them. Here she was true to her Italian nature; but she spoke with an elevation of accent and depth of emotion which are the privileges of northern nations.
One evening she was at the Colosseum. She felt an enthusiasm within her, an inspiration unaccountable, and pictured in life-colors the crowd of excited people, watching and crying out to the poor Christian martyrs struggling and dying, in the brightness of a supernatural light. She entirely forgot herself.
Something like a hymn breathed from her oppressed heart; eloquence overflowed from her lips. The passers-by were attracted toward her, and her father listened overcome and astonished. While she appeared transfigured, standing in the light of the setting sun, which seemed to throw around her the bloody purple of which she chanted, a ray of the glory of her ancestors rested on the forehead of this grandchild of the martyrs.
That evening, her father, in taking her home again, said to her, "Go on, my little one; many have passed for eloquent who had not your inspiration; many have sought for poetry, and great they were; but they have not found the fruit your tiny hands have gathered. Mignon sang: you sing and speak; and if you use your power for good, Mignon may not compare with you."
Excuse the blindness of a father, if you please.
XVII.
When the time came for the children to part, André was overcome in a manner which seemed incompatible with his nature, so ordinarily tranquil. The father and daughter returned alone, and lived afterward with no other company than themselves. They felt no need to seek their diversion among their neighbors. The simple ties of friendship or convenience to them were unnecessary, and the organist preserved with the outside world only the acquaintance that strict politeness demanded.
Paganina's affection increased daily. A profound sentiment without display, and only recognizable by certain mute signs that might have escaped an indifferent eye. Her father, however, could not be deceived.
So these two beings were never separated. They worked together; the organist conducted his daughter into the highest regions of music, and was astonished, in teaching her, to discover horizons hitherto unknown. Paganina made wonderful progress.
Those who find in art their happiness in this world, and seek the depths of those mysterious tongues of which so many speak and know nothing—those alone can form an idea of the happy moments passed in their solitude.
At times these two souls rose together, mounted even to the pure heights where, to those who attain to them, is given a supernatural felicity.
To these joys Paganina aspired with an immoderate ardor; but in attaining them she experienced a reaction of extreme sadness. This disquieted her father; so, in the language of parable which he liked to use, and which sometimes proved more original than gracious, he said, "My daughter, my daughter, drink with precaution; at the bottom of the purest streams are hidden the most dangerous reptiles. Be prudent, or you will swallow the leech. There is only one fountain to quench your thirst, and where, with your impetuous humor, you may drink with safety: it is that which gushes toward eternal life."
To Be Continued.
[Transcriber's note: This discussion is impressive, considering that quantum theory and the internal structure of the atom appears many decades in the future.]
The hypothesis of an ethereal medium everywhere diffused, is still, in spite of some vague objections urged against it, universally received, and the most recent theories and researches have not suggested its abandonment or modification in any important respect. On the contrary, they point to its more exact establishment, and to its application to large classes of phenomena in which, until lately, it was hardly supposed to be involved. There is no longer any branch of natural philosophy which can dispense with it; and in the theory of heat as a mode of motion, which will soon be the basis of a new system of physics more full and clear than the previous one, the motion must probably be explained by the principle of ethereal undulations or vibrations.
These vibrations show themselves by three different effects, namely, heat, chemical action, and color. The first two were for a long time neglected, but the third offered quite a large field, in which many very beautiful discoveries were made. It was known, for instance, that the oscillations were made with prodigious rapidity. Thus, the red of the spectrum is produced by vibrations repeated four hundred and eighty-three trillions of times in a second; while for the violet, more than seven hundred and eight trillions are required. Between these limits all the visible rays are contained, and, taken successively, they produce all the shades of the spectrum, and, by their combination, all possible colors. But as there are vibrations in the air too rapid or too slow to give the sense of sound to the ear, so there are, in the ether, slower than the red, or quicker than the violet, and hence invisible. The first have been detected by their calorific, the second by their chemical effects. The spectrum has thus been considerably extended at both ends, and we cannot be sure that its true limits have even yet been found.
These facts have been known for some time, and are found in all treatises on physics. We only speak of them in order to explain better the theories proposed by modern science to explain the three effects of ethereal radiation.
The hypothesis of three essentially different kinds of rays has now been abandoned. The solar beam, for example, which causes six hundred and thirty trillion vibrations a second, has the three properties of producing in the eye the sensation of blue, of heating Melloni's thermo-electric pile, and of decomposing the chloride of silver used in photography; but it does not appear that three different rays vibrating with this velocity are sent to us, each the cause of a separate effect. Notwithstanding the most careful experiments, no one of these properties has ever been diminished in a ray without diminishing the rest in the same proportion. Of course, these properties are differently proportioned in the different rays of the spectrum; but in two rays from the same part, and hence having the same velocity of vibration, these properties always consist in the same relative intensity.At the red end of the spectrum, the heating power predominates; at the other extremity, the chemical; in the middle, the luminous. The reason of this seems to be merely the difference of vibratory velocities; and we shall see that this will suffice to account for it.
Let us first explain how we conceive the production of the phenomena of chemical action and of heat. For clearness, we must advert to a theory familiar to all, according to which ponderable matter is composed of excessively small volumes, called atoms, which, though perhaps theoretically divisible, are never divided by any physical or chemical action. In the constitution of bodies, these atoms are supposed to be grouped in some manner, each group forming what is called a molecule. These, unlike the atoms, are decomposed in chemical changes, though not in physical ones, by which we understand such as evaporation, melting, crystallization, heating, magnetizing, electrifying, etc., unless these happen to affect the chemical constitution as well as the physical condition of the substance. All these do not alter the arrangement of the atoms in the molecule, but only the position or distance of the molecules with regard to each other. A collection of molecules may be called a particle; physical action then alters the constitution of the particle as chemical does that of the molecule. It may be remarked that our senses give us no direct evidence of the existence of molecules, much less of that of atoms, and they are supposed to be so extremely small that it will probably never be possible to detect them in this way.
In the application of this chemical theory to that of light, a new hypothesis is made, namely, that the ethereal fluid, whether itself continuous or composed of separate elements, penetrates all the interstices between the atoms of a molecule, as well as those between the molecules. The motions of this fluid, and of the matter which it penetrates, are communicated to each other, according to laws not yet ascertained, but of which we already have some glimpses. Thus, in treating of the effects of the ethereal vibrations on ponderable bodies, great importance is probably due to what is calledisochronism, or equality of times; that is, the agreement of the rapidity of vibration of the ether with that of which the matter is susceptible; for in all known communications of vibratory movements, this isochronism plays a very notable part. If, for example, we place upon the same stand two clocks, having pendulums of the same length, and consequently swinging in the same time, and start one of them, the slight impulses communicated by this to the other will finally set the latter also in motion. If, on the other hand, the pendulums are not isochronous, no such effect will be produced. In the same way, a stretched cord will vibrate if one of the sounds of which it is capable is produced near by; but it will not be affected by other notes, even though much louder—showing that isochronism is more important than intensity. Another illustration of the same thing struck me forcibly some ten years ago. I had ascended with some photographic apparatus to the top of an old square tower, very high and massive, to take some views. The tower belonged to a church, the bells of which were rung several times while I was there. The great bell, though of a very considerable size, shook the building very slightly; it hardly caused any tremor in the image of the landscape.But a second and much smaller bell could not be rung without giving to the tower, after two or three minutes, a strong swaying movement like that of a tree shaken by the wind. This was owing to the isochronism between the oscillations of the tower and of the small bell, which more than compensated for the difference of mass.
We have here an explanation of the physical and chemical phenomena produced by the ethereal rays. A few vibrations of this medium, probably, would produce no perceptible effect on a mass of matter; but these movements are repeated hundreds of trillions of times in a second, and however feeble their influence at first, isochronism may finally give it great power. Let us consider, first, the molecules, which have some connection between them, as yet unknown, but probably only allowing a certain set of vibratory velocities, (as a cord will only vibrate so as to produce a definite series of musical notes.) If, then, these are isochronous with those of the surrounding ether, the movement of the latter will be communicated to the molecules; or, according to the new theory of heat, the body will be warmed. These movements may even become so violent as to permanently modify the manner of union of the molecules—that is, to change the state of the body from solid to liquid or gaseous; and, by this change of state, the molecules may become insensible to the vibrations which previously affected them; for the set which they can now perform may have been entirely altered. The phenomena of heat are then well accounted for by this theory. To explain similarly the chemical ones, we have only to suppose ethereal vibrations, such that the movement affects the atoms separately, instead of the whole molecule, so that, after they have been sufficiently prolonged, the connection between the atoms will be destroyed. According to this, the chemical action of light should always be one of decomposition; it is so undoubtedly in most cases, and in the rest, where a combination is produced—as, for instance, in the formation of chlorhydric acid by the action of the violet rays on a mixture of chlorine and hydrogen—we shall adduce hereafter some facts which explain them, and show that even here the real action of the rays is a decomposing one. It may be remarked that the introduction of these ethereal vibrations, whose dimensions and velocities are well known, into the region, still so mysterious, of atoms and of molecules, promises to lead to results long unhoped for. If, for example, the theory above stated is correct, it would appear that the union of the atoms is such that their necessary time of oscillation is shorter than that of the molecules; since the red rays, which have the greatest heating power, vibrate more slowly than the violet, which are the most active chemically, as stated some distance back.
The luminous action of the rays is no doubt the most important for us, but also the most difficult to study; we have, however, something to say about it, for real progress has lately been made in this department. In the first place, since we are speaking of sensations, it is necessary to notice that this subject has two very different parts, one of which belongs to natural science, and the other to psychology. We shall here speak only of the first, that is, of three classes of phenomena which are produced at the exterior extremities of the nervous fibres, on the line of the fibres, and in the brain respectively.It has been said, in a previous paper, that each of these requires a certain time, and the experimental results as to these times were there given. But this is all, or almost all, the knowledge, unfortunately, which we yet have as to what takes place in the brain. The conjecture has been made that the different kinds of sensations are due to different modifications of the cerebral extremities of the various nerves; or that at the interior extremity of the optic nerve, a different action occurs from that at the nerve of hearing, which seems probable, since there are good reasons for believing that the action of the main body of the nerve itself is precisely the same for all the sensations. In more than one way, our nervous system would then resemble the telegraph. All the wires are traversed by similar currents, but the registering apparatus is different in each. In one, the dispatch is read off upon a dial; in another, it is printed on a moving band; in a third, a facsimile is given of it, etc. The sending is also accomplished by different means; but in all cases the same agent, the electric current, is employed.
Since we are treating of the sensation of sight only in connection with the external vibrations, we need here only discuss the first of the three classes of phenomena mentioned above, those which correspond to the transmission of the dispatch. In explaining this, we shall follow the celebrated professor of Heidelberg, M. Helmholtz.
The use of the spectroscope, and the analysis of light as now made in physics, chemistry, and astronomy, might induce the idea that color is an intrinsic property of the rays, depending entirely upon the length of the undulation in each, and inseparably connected with it; but this is not the case. Color is an organic phenomenon, only produced in the living animal; and, in one sense, is very independent of the length of the wave, since it can even exist without the presence of any luminous ray. Its laws are admirably exhibited in a figure called Newton's circle. This circle has been modified by recent experiments, and has received three enlargements, which make it a sort of triangle with rounded corners; but it is very well to preserve its name, for, as yet, the claims of Newton in optics have not been contested in any "Commercium epistolicum." Let us briefly describe this figure. The red, green, and blue of the spectrum occupy the three corners respectively. Passing round the circumference, we go from red to green through yellow, from green to blue through greenish blue, and from blue to red through violet and purple. If we draw a straight line from any point of the circumference to the centre, we find the same color on all points of the line, but more and more diluted, so that the centre itself is perfectly white. This figure contains all possible shades of color, and has the following remarkable property, established by experiment. If we wish to know what color will be produced by the mixture of any others, we have only to mark upon this figure the points where the several colors are found, and place weights there proportional to the intensities in which the different colors are to be used in the combination; at the centre of gravity of these weights, that is, at the point on which the circle (supposed itself to be without weight) would balance when thus loaded, we shall find the resulting shade. This point does not need to be found by experiment, being more easily calculated mathematically.
Now it is evident from this that color is a mere matter of sensation; for it is obvious that the same centre of gravity can be obtained by an infinity of arrangements of the original colors, notwithstanding the diversity of their wave-lengths; and it will also be found that these various mixed rays, though having precisely the same color—that of the centre of gravity—will differ entirely in their other properties. They act variously upon the thermometer and on the sensitive photographic plate, and give different tinges to colored objects which they illumine. But upon the retina the action of all is the same. How is this result to be explained? We will answer without stating the proofs, which the limits of this article would forbid.
From what has been said, it will be seen that all colors can be produced by the mixture of the three fundamental or primary ones, red, green, and blue, which were placed at the three rounded corners of Newton's circle. It will also be supposed that, as in the theory of Thomas Young, nervous fibres of three kinds are found at every point of the retina. When these are excited in any way, whether by the vibrations of the ether, by lateral pressure on the ball of the eye, by a feeble electric current, or by any other means, they transmit the excitement to the brain; but the red fibres, (so to speak,) if they should act alone, would only produce, however they were irritated, the uniform sensation of a red such as we hardly ever actually see, moresaturatedthan the ordinary red, and which would be found in our figure at the extreme summit of the rounded corner. The two other kinds of fibres would, of course, act similarly, producing colors more pure than are usually seen; since, in our usual sensations, the three are always mixed, each predominating in its turn; and this is the case even in the spectrum itself. The effect of the pure colors in the latter may, however, be heightened as follows: Let us fix our eyes, for instance, for a few moments on the blue-green. This is the complementary of the red. The fatigue will produce a momentary insensibility in the fibres corresponding to the blue and green, and, turning the eyes to the red part of the spectrum, the slight admixture of these colors there present will fail to excite sensibly the corresponding nerves, so that the red will be seen for a few seconds in great purity. But to return. The stimulus of the first set of fibres, though found more or less in all parts of the spectrum, will predominate at the red end, where the vibrations are slowest; that of the second set in the middle, where the green is found; that of the third, at the blue extremity. Why these inequalities? Why, also, do the dark rays, preceding the red and following the violet, fail to act on the retina? No certain reason can be assigned, but there are two very plausible ones: first, the media which the rays have to traverse in the eye before reaching the nerves have, like all other transparent bodies, the power of absorbing the vibrations, not all uniformly, but some in preference to others. This elective absorption would destroy or diminish the effect of the rays on the nervous fibres. The second reason, as will readily be surmised, is the want of isochronism between the vibrations of the rays and those of the nervous fibres.
In confirmation of this theory, a remarkable anatomical fact, noticed among many birds and reptiles, may be cited. These actually have in the retina three kinds of fibres: the first terminated by a small, oily red drop, the second by a yellow one, while the third have no perceptible appendage.Evidently, the red rays will arrive most purely at the first, the central rays of the spectrum at the second, while the blue and violet ones will act freely only on the third. It must be granted that no such thing has been observed in man and the other mammalia; but something similar may be found in the singular pathological phenomenon to which the chemist Dalton has given his name. Daltonism is most frequently an inability to perceive red. For eyes thus affected, the chromatic triangle or circle just mentioned is considerably simplified; but sad mistakes are the consequence. "All the differences of color," says Helmholtz, "appear to them as mixtures of blue and green, which last they call yellow." This disorder would be, according to the above theory, a paralysis of the first, or red fibres. The simplicity of this explanation is certainly in favor of the theory which gives it. But we had determined not to bring up arguments. Let us, then, pass on; remarking, however, one respect in which the eye, otherwise so superior to the rest of the senses, is inferior to the ear. Sounds, though combined to any extent in harmonies or discords, can readily be separated by an experienced ear. The eye, on the other hand, only sees the result of mixed colors; it needs instruments to rival the ear; and it is only by means of the prism that it can separate and classify the various vibrations which reach it.
But, provided with this prism, orspectroscope, it has lately done wonders. It has discovered and measured a whole world of new phenomena, which, according to the theory just developed, must be attributed to reciprocal exchanges of movement between the ether and the ponderable molecules. The light given by these has disclosed to us many secrets of chemistry, and especially of astronomy.
Before specifying the most recent of these discoveries, we will profit by what has already been said to explain very briefly the fundamental principles of spectral analysis. Transparent bodies, whether solid, liquid, or gaseous, exercise upon the rays an absorption which is called elective, because some undulations are allowed to pass, while others are stopped, according to their velocities; and one of the effects of this absorption is the color of such bodies. This is to be explained by the principle of isochronism. Those vibrations which, for want of it, cannot be imparted to the surrounding matter, pass freely; the others are absorbed. But it is remarkable that gases and vapors only absorb a small number of them, while solids and liquids retain a great many. Thus, supposing that we have obtained, in any way, a continuous spectrum—that is, one with no breaks—containing all the known rays, not only the visible ones between the red and violet, but also the rest outside of these limits, a liquid or solid body intercepting this light will entirely destroy, or considerably weaken, large portions of this spectrum; whereas a gas or vapor generally will only efface a few small ones, whose absence is detected in the luminous part of the spectrum by the dark, transverse lines which have been so long known in that of the sun. This is certainly quite extraordinary, since it would suggest the inference that in gaseous bodies, the molecules, though less condensed, or further from each other, than in solids or liquids, have a much smaller range of possible vibrations. Besides this, the researches of Mr. Frankland on flames have lately shown that, even in gases, this range increases as the density augments. These results must undoubtedly be considered as strange; but what, after all, do we know of the connection of the elements of matter?Without dwelling further on this point, we will mention the most important fact learned by these experiments: that this elective absorption is a complete test of the chemical composition of gases. In given conditions of temperature and pressure, each gas is perfectly distinguished from all others by the special absorption which it exercises upon the luminous rays. The principle by which chemical analysis is performed spectroscopically is thus evident. To find if any particular gas is to be found on the path of the ray, it is only necessary to develop the latter into a spectrum, and to see, by the position of the particular dark lines produced in it, if the absorption due to this gas has been effected.
But this is not all. Bodies sufficiently heated become luminous. According to the theory, this means that the molecules of matter, in their turn, communicate their vibrations to the ether; and here again we should find the influence of isochronism. The ether, it is true, is susceptible of vibrations of any velocity within certain very wide limits; but the molecules can give it none which are not isochronous with their own. Let us see what will result. Evidently, that the light which is emitted will, when developed into a spectrum, be concentrated in brilliant lines at those points where the velocities of undulation are the same as those of which the gas is capable; and, further, these lines should also evidently be in the same places, as the dark lines which this gas produces, as explained above, in a continuous spectrum, by absorption. This actually takes place in most cases, but some exceptions must be expected; because variations of temperature and pressure change the mutual connections of the gaseous molecules, and hence should also change the velocities of their oscillations. Thus, it is often found that the same gases change their systems of brilliant lines as their temperature or pressure changes; and Mr. Frankland has even obtained gases giving continuous spectra, sometimes attaining this result by pressure alone. The influence of heat also explains why solid or liquid bodies, when incandescent, give continuous spectra; while, at a low temperature, their interposition produces an elective absorption. For it is known that transparent solids or liquids become opaque when heated sufficiently to shine; the reason apparently being that, like the ether, they are capable of vibrations of any degree of rapidity within the usual limits, and hence allow no ethereal ones—or, in other words, no light—to pass through them, but absorb them all. Most flames or incandescent vapors, on the contrary, do not entirely lose their transparency. This property is of inestimable value in our investigations of nature.
Gases, by the combination of their elective absorption with their equally elective emission, produce results which at first sight might appear singular, but which can now readily be explained. Suppose that a flame is situated on the path of some rays which, without this interposition, would give a brilliant continuous spectrum. This flame only absorbs the ray having vibrations isochronous with its own; on the other hand, it emits rays similar to those which it absorbs. The resulting spectrum will vary according to the relative intensity of the emitted and absorbed rays. If these two intensities are equal, the spectrum will remain continuous; but if the absorption predominates, there will be dark lines in it; if the emission, brilliant ones.Similar phenomena of reversal have been often met with in the recent examinations of different parts of the sun.
The principles just explained have been known for several years, and were sufficient for astronomy as long as it restricted its investigations to the chemical analysis of the atmospheres of the heavenly bodies. But it was soon perceived that much greater use could be made of the spectroscope. Information is now beginning to be acquired by means of it which had previously appeared to be unattainable, regarding, for instance, the rapidity of the motion of stars the distance of which is still unknown; the great movements which are continually taking place in the great masses of gas in the solar photosphere, and the pressure of these masses at different depths; and it is even hoped that a direct determination of their temperature may be made. Let us speak first of the observations of stellar velocities. Their possibility may easily be shown by means of an acoustic phenomenon which the reader must frequently have noticed. Let us suppose two trains of cars to be moving rapidly in opposite directions, and that one of them whistles as it passes the other. If we are seated in the latter, we shall perceive that the pitch of the whistle suddenly falls as it passes us. The reason is manifest. A certain time is necessary for the sound to reach us; and while the train is approaching, this time is sensibly shorter for each succeeding vibration, so that the interval between the vibrations is apparently diminished, and the note is higher than it would be were the trains at rest. On the other hand, as the whistle recedes after passing, its pitch is lowered for a similar reason. Of course, no such effect is produced by that of our own train, which always remains at the same distance from us. By the amount of flattening of the sound, it is quite possible to calculate the velocity of the train, as compared with that of sound. [Footnote 198]
[Footnote 198: Suppose the sum of the velocities of the trains to be one-ninth of that of sound, and that the whistle is, at a given moment, 1140 feet (which is about the distance travelled by sound in a second) from our ear. The vibrations emitted at this instant will reach us in one second; and all those emitted in the nine seconds required for the train to arrive will be condensed into the remaining eight. Their frequency will then be nine-eighths of what it would be without the motion. It will be diminished in nearly the same ratio after the passage; since the vibration emitted nine seconds afterward will require an additional second to reach us; thus, the frequency will now be nine-tenths of what it would be without the motion, or four-fifths of what it was before meeting; corresponding to a flattening of two whole musical tones. This would require a relative velocity of 127 feet a second, or 87 miles an hour; which gives the rule, that, for every half-tone of flattening, the sum of the velocities, or the velocity of the moving train, if we are at rest, is 22 miles an hour.]
It is very easy to apply what has just been said of the waves of sound to those of light. The motion of the sonorous body displaces its sounds on the acoustic scale; in the same way, the motion of the luminous body will displace its light on the optic, placing any particular line, dark or brilliant, in the spectrum nearer to the violet or rapid end, if the body is approaching; and nearer to the red, if it is receding. And we are not obliged to wait till the change has taken place in the character of the motion, as in the case of the train, since we can always obtain lines similar to those thus displaced, and having the same velocity of vibration, from some terrestrial substance, relatively at rest, and put the two side by side in the same field; and by this means we obtain at once the difference between the apparent number of vibrations in a second of the ray from the moving body, and the real number, and thus the velocity of the moving object. This observation has the advantage of being independent of the distance of the objects observed, being as accurate for the most distant stars as for the nearest.We may notice, in passing, also a singular consequence. If the motion were rapid enough, it would change the colors of objects; and, since outside the visible spectrum there are dark rays, it would even be possible for a luminous body to become invisible, by the mere effect of movement away from or to us. But the prodigious velocity of light places such a result among mere metaphysical possibilities. Indeed, it was thought, for a time, that the effect of motion on the spectral lines would never be perceptible. The first trials only gave negative results, either because the bodies observed were moving too slowly, or because the instruments used were not sensitive enough. This is no longer the case, as we shall soon see.
To conclude this explanation of principles, it only remains to say a few words on the spectroscopic observations of temperature and pressure. But here we shall indeed be obliged to be brief; since Messrs. Frankland and Lockyer, who have undertaken investigations on these important points, have not yet finished their labors; and what they have as yet communicated to the Royal Society of London, and the Academy of Sciences of Paris, is not sufficiently detailed. In 1864, Messrs. Plücker and Hittorf discovered that variations in temperature of some of the chemical elements, such as hydrogen, nitrogen, sulphur, and selenium, caused sudden changes in their spectra. At a certain degree of heat, their former lines instantly disappeared and were succeeded by new ones. This is evidently somewhat analogous to what takes place in a sonorous pipe when it is blown more forcibly. At first, the sound only becomes louder, then its pitch is suddenly raised. But here we know the relation of the new note to the old one; but the connection between the successive spectra has not yet been ascertained. As regards pressure, Messrs. Frankland and Lockyer inform us that one of the lines of hydrogen increases in breadth with increased compression of the gas. We have also already said that under very high pressures the gases have not only shown broader bright lines, but even continuous spectra. (It will be remembered that the usual spectrum given by a luminous gas consists of isolated bright lines.) Father Secchi, whose attention has lately been turned to composite rather than to simple substances, has observed, among other things, that the spectrum of benzine vapor is gradually modified with a gradual increase of density.
Let us pass to the recent applications which astronomers have made of these various principles. The eclipse of the 18th of August, 1868, and the beautiful discovery of M. Janssen, have naturally turned their attention to the sun, and some most interesting discoveries have been made. To study its various portions, an image of it is first produced in the focus of a large telescope, which image is afterward enlarged by a lens similar to those used for the objectives of microscopes; and its different parts are successively placed upon the slit of the spectroscope. (The slit is the small aperture of that shape through which the light enters before falling upon the analyzing prism.) This slit thus receives light from only a part of the sun's disc; for the light diffused in our atmosphere and falling upon it, although coming indeed from all parts of the sun, is too feeble to interfere with the observations. Suppose, then, that our eye is at the spectroscope, and that the slit is receiving rays from the centre of the sun.The movement of the heavens will bring all the points of the solar radius successively upon it, from the centre to the edge; and if the slit is placed perpendicular to this radius, it will come out, of course, tangent to the edge. Under these conditions, and if the atmosphere is steady, the phenomena will be as follows.
As long as we are upon the disc, we shall see nothing but the usual solar spectrum with its colors and its numerous dark lines. The region from which this light comes is called the photosphere; and its spectrum would be continuous were not its light absorbed by the interposed vapors of a great many substances. These vapors produce the dark lines; but where are they? It was for a long time supposed that they formed an immense atmosphere round the sun, only visible during total eclipses under the form of a brilliant aureola. This hypothesis seems now to have been abandoned, for reasons which will soon be given. It is generally thought that these absorbing vapors form the atmosphere in which the luminous clouds float, or, at least, that they are in immediate contact with the photosphere.
Secondly, when we have nearly arrived at the edge, the spectrum is covered with a number of bright lines. According to Messrs. Frankland and Lockyer, these probably indicate a very thin gaseous covering of the photosphere, the elective emission of which has no effect for want of sufficient thickness, except upon the borders of the sun, where it is seen very obliquely. Upon the rest of the surface it only acts by its elective absorption, and perhaps may be the only cause of the dark lines. This conjecture certainly agrees with the principles just developed.
Thirdly, at the moment of passing off the disc, the lines all disappear, and the spectrum becomes continuous. Father Secchi, who informs us of this fact, naturally ascribes it to a particular layer enveloping the photosphere. He adds that this layer is very thin, so that tremulousness in the air suffices to prevent its observation, on account of the mixture of lights. It is not found on the whole circumference of the disc; but we shall give an explanation of this. He supposes that it is the seat of the elective absorption which produces the dark lines; but how can this be reconciled with the continuity of the spectrum which it emits?
This spectrum soon disappears, and some brilliant lines take its place, particularly a red, a yellow, a green, and a violet one. At this moment the slit is illumined by the famous rose-colored layer, now called thechromosphere, upon which rest the protuberances, formerly so mysterious, seen in total eclipses. We cannot see it in the ordinary way, on account of the atmospheric light; but it comes out in the spectroscope, its light being concentrated in a few bright lines, while that of our atmosphere is spread out in a long spectrum, and consequently much weakened. It has been found that the mean thickness of this gaseous envelope of the sun is more than 5000 kilometres, (3107 miles,) or about four tenths of the earth's diameter, and that its contour is very variable; it is often agitated like the waves of a stormy sea, while in some places it sometimes has a very uniform level. It is now regarded as forming the outer limit or coating of the sun. The only reason which formerly supported the belief in a gaseous atmosphere outside of it, the elective absorption of which gave the dark lines of the solar spectrum, was the phenomenon of the aureola, already mentioned. But the thin layer discovered by F. Secchi will probably account for this; and there are, on the other hand, very strong reasons for rejecting the idea of such a vast exterior envelope.One is the appearance, mentioned above, of the numerous bright lines which Messrs. Frankland and Lockyer attribute to a thin, gaseous coating of the photosphere. The light of these ought seemingly to be absorbed by a thick atmosphere, and the lines reversed to dark ones. Besides, these same observers consider that the change of breadth of the lines shows that the pressure is insignificant at the summit of the chromosphere, and that even at the base it is less than that of our own air. Lastly, no traces have been found of the bright-line spectrum which this envelope ought itself to give in the vicinity of the disc.
To return to the chromosphere: of what gases is it formed? It certainly is principally composed of hydrogen, perhaps in many parts entirely so. When a series of electric sparks is passed through a tube containing pure hydrogen at a very low pressure, the tube is illumined with a light of the same color as that of the protuberances. If this light is examined with the spectroscope, it shows a fine spectrum with a number of brilliant and very fine lines, among which four are conspicuous, broader and brighter than the others. The first is red, the second green, the third and fourth are violet; but this fourth is much the faintest, and even the third is not so bright as the other two. The first is called C, the second F, because their positions exactly correspond to those of the two dark lines thus designated by Fraunhofer in the solar spectrum. The third is very near the dark line G of the sun, which is produced by the vapor of iron. Now, the two first are always found among the lines of the chromosphere; the third also is often visible; and M. Rayet has recently seen the fourth. Hydrogen, then, exists in this layer; for though its other lines are not seen, this may easily be ascribed to their faintness. But there is one line of the chromosphere which is still unexplained, the yellow one between C and F. It would at first seem to be the well-known double line of sodium, called D, which is so frequently met with in spectroscopic experiments; but it is certain that it is somewhat more refrangible than this; and it is not yet known to what substance it is due; it may, perhaps, also belong to hydrogen, under a different pressure or temperature from any under which it has been observed here.
It has been said that the outline of the chromosphere is generally very irregular. Immense columns rise from it, the celebrated protuberances, the height of which is sometimes as much as eleven diameters of the earth, (or 85,000 miles.) It must, therefore, be subject to great agitation, to which the spectroscope bears witness. Mr. Lockyer has observed several times that foreign substances were projected into it; for example, magnesium into one protuberance as far as the sixth part of its height; barium and sodium, and probably other bodies also, were seen, but at smaller elevations. We now understand the breaks in the thin layer detected by F. Secchi; it is probably torn by the upward movement of various substances toward the protuberances. It is, in fact, wanting near the bright spots on the sun, called faculae, and it is now known that these faculae are always covered by protuberances.
Near these bright spots are also usually found the dark spots which have been observed for more than two centuries. Some discoveries have just been made regarding these which are perhaps the most interesting of any yet made in the sun.Every one knows that they are composed of two distinct parts—the nucleus, which appears black in a telescope, but which is really quite bright, since it gives a spectrum of its own; and the penumbra, which surrounds this nucleus. The latter consists of portions of the photosphere, drawn out in the form of threads toward the centre of the nucleus; these threads sometimes unite with each other and form bridges, as it were, over the dark space. All the spectral observations confirm the idea previously entertained, that these spots are really cavities in the photosphere; also they indicate that these cavities are filled with absorbing vapors, whose high degree of pressure is manifest by the broadening of their lines. Mr. Lockyer has seen in them sodium, barium, and magnesium; F. Secchi, calcium, iron, and sodium. Above these spots the hydrogen of the chromosphere appears in quantities sufficient for its elective emission to destroy the black lines produced by its absorption upon other parts of the disc, and even sometimes to change them into bright ones. But there are many other peculiarities in the spectra of the spots; and F. Secchi, in examining them, has hit upon an idea which seems to us very suggestive. It was already known by observations of their frequency and size, that the sun is a slightly variable star, with a period of ten and one third years. We now find a new resemblance between it and the other variable stars. It may be remembered that the Roman astronomer has lately divided the stars into four classes, according to the general character of their spectra. He has just compared the different portions of the sun with these four groups, and finds that if its surface was all like the nuclei of the spots, it would have to be put in the class whose type is Betelgeux, all of which are more or less variable; that the penumbras are like Arcturus, and the general surface of the photosphere like Pollux. He has also concluded, from the presence of many of the dark lines in the nuclei, that the vapor of water exists in these regions of the sun; and the appearance of others not yet named has caused him to suspect the presence of many other compound bodies. Up to this time, hardly any thing but the simple substances has been looked for, as the heat of the sun would seem to be so great as to separate all the composite ones; but this temperature probably is not so high in the spots. It became, therefore, of interest to examine the faint red stars which form his fourth group; and in doing so, F. Secchi has obtained the surprising result that the vapor of a compound substance, namely, benzine, gives, when incandescent, a spectrum having bright lines exactly corresponding to the dark ones of one of the stars of this group. This star, then, appears to have an atmosphere of benzine.
Finally, the spectroscope has demonstrated the movement of at least one star. Mr. Huggins has found that the hydrogen lines in the spectrum of Sirius do not exactly coincide with those of this gas when at rest, but are displaced toward the violet; this observation was confirmed at Rome. It would follow from this that Sirius is rapidly approaching us. This is the only observation of this description which seems yet to be well established. But may it not be possible to make others, and even elsewhere than among the stars? The chromosphere is, as we know, the scene of very rapid movements; and may not these be visible by the displacement of the spectral lines?The following remark of Mr. Lockyer, in one of his communications to the Royal Society, would induce us to hope for this: "In the protuberance of which we are speaking, the line F was strangely displaced. It seemed that some disturbing cause altered the refrangibility of this line of hydrogenunder certain conditions and pressures." But is it really to pressure that this displacement is due, when we know that rapid movement produces this effect, which has never been known to follow from pressure? But let us hasten to acknowledge that, in a subsequent communication of the same author, we find a sentence much more to the point, and which only needs to be a little more developed to answer our question. Mr. Lockyer is here speaking of movements in the vapors which fill the cavities of the spots. "The changes of refrangibility," says he, "of the rays in question show that the absorbing matter is rising and falling relatively to the luminous matter, and that these movements can be determined with great precision." Let us hope that this will be verified by observation, and that exact measures will show the fertility of such a promising theoretical principle. [Footnote 199]
[Footnote 199: The rapidity of some of these movements has been said to be about one hundred miles a second.]
The length of this bulletin is beginning to alarm us; but since it should include all the last scientific developments concerning the subject of ethereal vibrations, a word must be added on some curious experiments of Mr. Tyndall. The chemical action of these vibrations had hardly been examined hitherto, except in the nutrition of plants, in the formation of chlorhydric acid, and in the transformation of various substances, principally used in photography. The successor of Faraday has recently studied their effects upon vapors, and has applied the curious results of his investigations to some as yet unexplained facts of meteorology and astronomy. Passing a cylindrical beam of light down a long glass tube full of the vapor which he wished to examine, he found that the vapor soon ceased to be completely transparent. An incipient cloud, as he calls it, soon appeared, so thin that it could only be seen by the light of the beam producing it, but became invisible in the full light of day. Some vapors undoubtedly will not produce it; but the experiment succeeds perfectly with many different ones, especially with nitrite of amyle, bisulphide of carbon, benzine, etc. The following explanation of this phenomenon seems quite probable. The vibrations of the ethereal medium, or at least some of them, are communicated to theatomsof which the compositemoleculesof the vapor are formed. Owing to isochronism, the movement becomes strong enough to break up the molecule, the atoms of which are formed into new combinations, which are better able to resist the action of light. If the new substance cannot remain under the given pressure and temperature in the gaseous state, it will be precipitated in liquid particles, which are at first extremely small, but gradually increase in size, so as to intercept the light and become visible. If the vapor employed satisfies these conditions, the experiment ought to succeed. The chemical analysis of the products has, we believe, in some cases confirmed this explanation; we will now confirm it by some facts of another kind.
In Mr. Tyndall's experiments, the vapor examined was never unmixed; when it was put into the tube, some other gas was also introduced, usually atmospheric air; but other gases were also employed. With hydrogen, a remarkable effect was produced. On account of its small density, it failed to sustain the liquid particles, and they slowly settled in the bottom of the tube.By a suitable diminution of the pressure of these mixtures of gas and vapor, the chemical action of the rays could be retarded at pleasure. The "incipient cloud" could then be seen to form gradually; and whatever was the character of the vapor used, the cloud had always at first a magnificent blue color. Continuing the experiment, the brilliancy of the cloud increased, but its blue tinge diminished, until it became as white as those usually formed. The natural explanation of this change is found in the gradual growth of the liquid particles.
The cloud was not usually formed all along the course of the rays. After having traversed a certain thickness of vapor, the rays, though seeming as bright as ever, lost their chemical power. This result might easily be predicted by the theory. Only a few of these rays had the proper length of wave to act by isochronism upon the atoms of the vapor. These would be absorbed shortly after entering; and the others, though vastly more numerous and escaping absorption, would produce no chemical effect. It was even probable that, by passing the light at the outset through a small thickness of the liquid, the vapor of which was contained in the tube, all its active rays could be taken out; and experiment confirmed this conclusion. It is to be regretted that the light was not examined with the prism before being employed; the wave-length of the active rays would then have been known. It is no doubt very probable that they are toward the violet extremity, either among the visible rays or beyond. But the colored glasses, which the English physicist interposed, only partially resolve the question. The prism would undoubtedly have shown that the wave-length of the active rays varies with the substance exposed to them.
Some vapors taken alone are almost insensible, while their mixture is immediately affected by the passage of the rays. Such is the case of that of nitrite of butyle with chlorhydric acid. This is very easily explained theoretically. The disturbance communicated to the atoms by the ethereal vibrations, though very decided, may be insufficient to break up the molecules. But if another cause, though itself insufficient alone, comes to its assistance, the atoms may be separated. Such another cause is that which chemists have long known asaffinity, the manifestations of which are very numerous; but which has not yet been submitted to a precise analysis. In the case just mentioned, the affinity of the elements of the nitrite of butyle for those of the chlorhydric acid conspires with the vibrations to destroy the molecules of the two substances and form a new one, which is precipitated. The phenomenon is like that observed in the growth of plants. Light alone is not sufficient to decompose the carbonic acid of the air; neither are the leaves when in the dark. But when the sun's rays fall upon them, the carbonic acid is decomposed, its oxygen uniting with the atmosphere and its carbon with the plant. It is now easy to justify what was said in the beginning as to the formation of chlorhydric acid by the action of the rays on a mixture of chlorine and hydrogen. It is only necessary that the molecules of these gases, or, at least, of one of them, should be composed of several atoms. Affinity alone could only break the union of these very slowly; but the light would shake them apart, and enable the affinity to act immediately.