Chapter III: Quod Erat Demonstrandum (5)

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. {819} 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, or _spectroscope_, 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? {820} 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. {821} 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. {822} 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. {823} 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 the _chromosphere_, 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. {824} 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. {825} 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? {826} 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 hydrogen _under 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 the _atoms_ of which the composite _molecules_ of 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. {827} 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 as _affinity_, 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.

{828}

So far Mr. Tyndall's experiments agree perfectly with the theory; they confirm it, but they do not extend it. He has, however, made others, which seem to disclose new points in the theory of exchange of movements between the ether and ponderable matter. It might no longer be the atoms or the molecules which would have to be considered in respect to the ethereal vibrations, but even the particles, if sufficiently small. In fact, these particles reflect the rays not absorbed, according to entirely new laws. In the first place, although belonging to colorless liquids, they reflect the blue rays much better than the others. This is true of all the vapors tried, without exception. This elective reflection only holds when their dimensions are small, since it disappears as the size of the particles increases. This is quite a new fact, and, it must be acknowledged, as yet quite unexplained. Secondly, they polarize light according to laws which must also be called new, being entirely different from those given by theory and experiment for polarization by reflection. In one respect these laws are not new; for they have been long observed in atmospheric polarization; but this has always been one of the knotty points of the undulatory theory. Evidently, Mr. Tyndall's experiments do not clear it up entirely; but they have made an important advance in that direction, by showing to what physical circumstance this polarization is probably due. It would appear, that is, that in the higher regions of our atmosphere there are vapors which, instead of condensing in particles large enough to form ordinary clouds, are precipitated like those used by Mr. Tyndall, and fill the air with extremely small particles and with incipient clouds. This hypothesis is certainly very probable. It accounts at once for the blueness of the sky, and for its polarization of light.

Here is, then, a problem for theorists, in a better condition than previously. We hope to return to it shortly, in a subsequent bulletin. In conclusion, let us point out a new application of these experiments to the physical theory of comets. Mr. Tyndall considers the cometary matter to be a vapor on which the sun's rays act physically and chemically. These two actions would be somewhat contrary to each other; for the first would tend to evaporate the liquid particles and expand the vapor, while the second would precipitate this vapor in the form of incipient cloud. As the comet approaches solar action, forming an immense volume, of which the visible part will be only a small fraction, the head being the most condensed portion. If, now, we suppose the head to absorb the heating rays more abundantly than the remaining ones, in the cool shadow behind it the chemical action may prevail, and form an incipient cloud, which will be the tail of the comet. Elsewhere, the calorific action will predominate, and the vapor will remain invisible. Such is substantially the new theory of comets. It certainly satisfies the general conditions of the problem, and especially it explains very naturally the enormously rapid movements observed in the tails of these bodies. But will what is still undetermined in it enable it to be accommodated to the numerous facts already observed, and hereafter to be so? Here, also, it may be regretted that the spectroscope was not employed by the English physicist. The spectra of the incipient clouds might have been compared with those of comets' tails; and would have given an excellent test of the theory. Perhaps, however, he has reserved this part of his researches for a future publication.

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{829}

St. Oren's Priory.
Or, Extracts From The
Note-book Of An American In A
French Monastery.

"Pour chercher mieux."
--Device of Queen Christina of Sweden.