Popular Science Monthly/Volume 2/January 1873/Evolution and the Spectroscope
|EVOLUTION AND THE SPECTROSCOPE.|
By F. W. CLARKE.
MEN of science may be divided into two great classes—thinkers and observers. And, although both classes are often represented in one individual, the distinction between them is practically valid. For, in classifying mankind, no sharp boundaries can be drawn. The observer, on the one hand, contents himself with merely ascertaining facts, and rarely deduces more than the simplest and most obvious conclusions from them. He is in some measure an intellectual miser, who accumulates, but never uses. It is the thinker, however, who gives shape to science. His generalizations make true science possible. To him, a discovery amounts to something more than its mere self, and is valuable, like a choice seed, largely for what it may become. He ranges facts into series, gives each series its proper place in a science, clusters the sciences into groups, and, studying these groups with reference to each other, and to the grand problems with which thought is always busied, seeks to arrive at higher conceptions of the universe, and of the essential unity of all material things. At the present day this method of comparison has led to the announcement of the philosophy of evolution; a philosophy which places the physical world in a clearer light, and classifies a greater number of facts, than any other scheme that human earnestness and ingenuity ever devised. Surely it is worth while for us to study all great discoveries with reference to their bearings upon this philosophy.
Probably none of the many remarkable discoveries of the nineteenth century are more important or more striking than those achieved by means of the spectroscope. It is now less than fifteen years since this famous instrument was devised, and already it ranks in importance side by side with the telescope and the microscope. New fields of research have been opened, which, widening ever since, show as yet no signs of approaching limits. Chemical analysis has been simplified, many optical researches facilitated, and four new metals discovered. Our knowledge of the sun and stars has in some respects been more than doubled. Problems which were deemed insoluble, have been settled with the greatest ease. The magnitude of the discoveries already made leads us to expect still greater revelations in the future. Let us see what the spectroscope has to say for the philosophy of evolution.
Among the doctrines held by evolutionists, the all but proved Nebular Hypothesis occupies a very prominent position. Originating with Kant more than a century ago, and afterward furnished with secure foundations by Laplace, it has since striven for complete acceptance with ever-varying strength. According to this hypothesis, our solar system began existence as a nebulous cloud of incandescent vapor, which, rotating about a centre, and cooling as it revolved, cast off rings of matter that gathered into globes and became planets, while the central portion, undergoing less change, formed the sun. A vast weight of physical and mathematical evidence supported this theory, and the nebulæ seen in different parts of the heavens lent to it the confirmation of analogy. From the first, the hypothesis was strong.
But soon doubts began to arise. Larger and more powerful telescopes were constructed, and many nebulæ were resolved into clusters of stars. Astronomers began to hope that all these bodies might be similarly resolved, and the nebular hypothesis lost a little ground. But the spectroscope came apparently to the rescue. In the skilful hands of Mr. Hoggins, the narrow slit was made to receive the light of several unresolved nebulæ, and nebula after nebula gave up its secret to the observer. Some yielded spectra, consisting of from one to four bright lines, while others gave continuous bands of feeble light. The former class told the story. Spectra like theirs could belong only to the light emitted by incandescent gas, and therefore of such material, true nebulous vapor, these distant bodies consisted. But even more was revealed. The bright lines were characteristic of two well-known substances, nitrogen being the more distinct of the two, and hydrogen the less clearly visible. No other elements could be detected, nor could any good reason be found for supposing others to be present. But the main fact of the existence of genuine nebulæ was fairly demonstrated, and the nebular hypothesis received a great accession of strength. To-day it almost commands acceptance, although it is capable of being made much stronger. Even the evidence which analogy might offer in its favor is far from complete. We must look to the spectroscope for its completion.
In this connection a great variety of interesting questions suggest themselves. We assume that our planet originated from a gaseous cloud by a slow process of condensation and cooling, and point to the visible nebulæ to confirm our views. Now, in evolving a solar system from a nebula, a long series of changes would necessarily occur. We see the extremes of such a line of development, and also a few of the intermediate links. And we are at once led to ask whether we can hope to find existing to-day, among the heavenly bodies, examples of all the stages of evolution through which matter must pass in forming solid globes from shapeless clouds of incandescent vapor. The task will be a difficult one, but not hopeless. We have much material to begin upon, and can safely look to the spectroscope to furnish us with an abundance in the future. If the work can be done, the nebular hypothesis will become so well grounded that we are scarcely able to conceive of any possible arguments which could afterward disturb it.
In beginning upon such an inquiry, we must start with a consideration of the nebulæ themselves. And, at the outset, their varieties of form, and the visible changes which they undergo, offer strong suggestions of processes of evolution actually going on. The spiral nebulæ hint of rotary motion, and some annular forms speak to us of rings of vapor from which planets are yet to grow. In the double nebulæ we see future pairs of suns, companion stars; and in every true nebula are signs of condensation in the brighter portions. The nuclei which are so common may be the germs of central luminaries, around which systems like our own are yet to revolve. But all these observations are due to the telescope. We have to consider what the spectroscope has done.
Now, as regards spectroscopic work, the nebulæ may be divided into three classes: First, those which give spectra consisting only of bright lines. Secondly, nebulæ whose spectra are continuous. And, in the third place, the nebulæ described by Lieutenant Herschel, which are apparently intermediate between the other two classes, and furnish spectra of bright lines upon a continuous background.
The nebulæ of the first class I have partly described. They consist mainly, if not wholly, of two common gases, nitrogen and hydrogen. But gases give somewhat different spectra under different circumstances of temperature and pressure; and the spectrum of a nebula indicates that the gases of which it is composed are in a highly-rarefied condition, and at a temperature considerably lower than that of our sun! Of this we are tolerably sure, though perhaps not absolutely certain.
The nebulæ whose spectra are continuous speak to us with less certainty. Lord Oxmantown has shown that the resolved nebulæ—those which are known to be mere star-clusters—give this kind of spectrum, as do also most of those which appear to be resolvable. Accordingly, it is reasonably inferred that all the nebulæ of this class probably belong to the resolvable order; but here is where a slight doubt may arise: gases, under great pressure and at a high temperature, give continuous spectra; possibly, then, some of these nebulæ may consist of gases under just such conditions. Here is a problem yet to be solved. The third class of nebulæ may, perhaps, strengthen this latter view. Their spectra are intermediate between those of the other classes. It may be that a more careful study will show them to be gaseous, with their spectral lines in a state of transition to the full continuous spectrum; but this is little more than bare conjecture at present; for the published descriptions of these nebulæ are too incomplete to admit of very satisfactory discussion.
This consideration of nebular spectra plunges us at once into a sea of difficulties. We say that the sun and planets were formed by condensation and cooling from incandescent vapors, and hail the nebulæ as confirming this opinion. But could a sun be evoked, by cooling, from a body less hot than itself? Moreover, the sun is known to contain at least sixteen elements and probably many more. Were these developed from a nebula containing only nitrogen and hydrogen? Or did the original nebulæ differ in constitution? All those which the spectroscope has analyzed are chemically alike. We know nothing of any whose constitution differs in this respect from theirs; and, therefore, if we point to them as confirmatory of the nebular hypothesis, we are compelled to ask this portentous question: Did our planet, with all its chemical complexity, arise, by a slow process of evolution, from a glowing cloud of but two familiar gases? Upon our answer to this question depends largely the value of our spectroscopic confirmation of the great hypothesis. The safety of the hypothesis itself is not involved; merely that of this one argument in its favor. We can easily conceive of more complex nebulæ, which could give rise to systems like ours, although we know nothing of them. And, if we interpret the spectra of some nebulæ of the second class as due to gases at very high temperature and pressure, the difficulty regarding the heat of our sun will be easily gotten over.
Let us consider the question suggested, as to the possible evolution of complex from simple matter. It is easy to speak out boldly, in an ex-cathedra manner, and say that an affirmative answer to such a question would be absurd; but dogmatism of this sort is, in the highest sense, unphilosophical and foolish. We do not know but that the evolution of one element from another may be possible, under circumstances over which we have as yet no mastery; indeed, such a view would have many points of probability about it. Although unsupported, it is quite strongly suggested by evidence. The demonstrated unity of force leads us, by analogy, to expect a similar unity of matter; and the many strange and hitherto unexplained relations between the different elements tend to encourage our expectations. These elements, which seem to-day so diverse in character, may be, after all, one in essence. This idea is philosophically strong, but waits for experimental evidence to support it. At present, it can neither be discarded as false, nor accepted as true. But what an addition the proof of such a doctrine would bring to the philosophy of evolution!
Now, although questions like these cannot be settled by any evidence which we are likely to obtain for many years to come, speculation upon them is not altogether unprofitable. The time spent in conjectures and surmises is not wholly wasted; for it is impossible to follow up any of the lines of thought thus opened, without reaching some valuable suggestions, which may pave the way to new discoveries. New truth, in one direction or another, is sure to be reached in the long-run. So, then, we may proceed to theorize in the most barefaced manner, without entirely quitting the legitimate domain of science.
It is plain that the nebular hypothesis would be doubled in importance, and our views of the universe greatly expanded, if it could be shown that an evolution of complex from simple forms of matter accompanied the development of planets from the nebulæ. Evolution could look for no grander triumph. For the evidence to support such a theory, we must depend mainly upon the spectroscope. Let us continue upon our task of finding the intermediate links between the two extremes of planetary growth, and see whether, as we ascend in the line of change, an increased chemical complexity can be observed. Upon this theory, the planets should contain more elements than the sun; the sun more than some of the less advanced among the fixed stars; and these, in turn, should be more highly organized than the nebulæ?. But we must not fail to remember that we are merely speculating, and that the spectroscope, in telling us of the presence of certain substances, does not give us accurate information with regard to the absence of others. In this investigation, we can look to the spectroscope only for hints, not certainties. Difficulties will abound in our path, and, in a paper of this length, we cannot stop to scrutinize them closely. We must bridge many chasms with guesses.
The evidence concerning the constitution of the fixed stars has been furnished chiefly by Secchi and Huggins. The former observer, favored by Italian skies, has done, perhaps, the major portion of the work, and has given us a classification of these bodies. According to Secchi, the stars may be divided into four classes, as follows:
In the first class, which is by far the largest, we find most of the white stars, Sirius, Altair, Vega, Regulus, and Rigel, being especially prominent. These give spectra characterized by the intense development of the four hydrogen lines, which stand out with great distinctness upon a background of the seven primary colors. Lines belonging to some of the metals, particularly to sodium, magnesium, and iron, are also visible, but are exceedingly faint in comparison with those of hydrogen. The distinctness of this element, as compared with the faintness of the metallic lines, is characteristic of the stars of this type. The absence of bands, indicating an absorptive atmosphere, is also noteworthy.
In the second class of stars we find our sun, Arcturus, Aldebaran, Capella, Pollux, Procyon, and many others. Here we have spectra in which the lines of the metals are apparently more numerous, and certainly more distinct, the hydrogen being less conspicuous. In Aldebaran, Mr. Huggins detected sodium, magnesium, calcium, iron, bismuth, antimony, tellurium, mercury, and hydrogen.
The third class, in which are some stars of a red color, is comparatively small in numbers. Alpha Orionis or Betelgeux, Alpha Herculis, Beta Pegasi, Mira beti, and Antares, are good examples of this type. Their spectra, as a rule, resemble the spectrum of a solar spot, and sometimes contain bright lines. Hydrogen is still present, but so difficult to detect that, at first, it was supposed to be wanting in the spectrum of Betelgeux. But, in a state of combination, as aqueous vapor, it has been found in the stars of this order, and, most notably, in Antares. In the spectrum of Betelgeux, Mr. Huggins observed lines belonging to magnesium, sodium, iron, calcium, and bismuth.
The stars of the fourth type are very inconspicuous, but give quite peculiar spectra, consisting chiefly of three bright bands, separated by dark spaces. Such a spectrum suggests that of carbon, but really tells us nothing, as yet, of the constitution of these stars. We must, therefore, leave them out of account in our speculations. It would be easy to theorize about them, only the theories would find no place in our argument.
Now, taking the spectra of stars of the first, second, and third classes as a basis for our speculations, we have quite decent evidence of a gradual increase in chemical complexity. And, if we bring the nebulæ into line, we can devise a very neat progressive series of development up to the solid planet. Beginning with a nebula consisting mainly of nitrogen and hydrogen at low temperature and pressure, we can easily conceive of several ways by which it might gain great accessions of heat, and give a bright, continuous spectrum. A collision with meteoric or cometary matter would account for such an increase of temperature. But, given a nebula which is sufficiently hot, and from which a sun might be evolved by cooling, what shape will our speculations assume? This intensely-heated body undergoes a certain condensation, rings are thrown off from it, and a nucleus appears, which soon becomes a star or sun of the first type. Hydrogen still predominates in its constitution, but metals begin to show themselves, though very faintly. But the cooling continues, and gradually the hydrogen lines become fainter, the metallic lines stronger, a larger number of substances are detected, and we have a sun of the second class. By another slow transition, chemical action, as we recognize it, begins to set in. The hydrogen lines disappear; aqueous vapor is formed; spots, like those of the sun, which are probably centres of chemical activity, become more and more abundant, and the star enters the third order. As the spots accumulate, the star becomes more decidedly a "variable," and, after violent and prolonged convulsions of its surface, solidity is reached, the emission of light ceases, and a planet is formed. Some volcanic heat, however, yet remains; but this slowly dies away, the volcanoes become extinct, and, at the end of the line of change, we have a body like our moon, dead and sterile. And here our speculations end. What next ensues, no one can say. We are seeking the past history of our planet, not looking into its future.
Now, a paper of this sort should always contain a summary of the steps by which its conclusions have been reached. Beginning with the nebular hypothesis, as it is commonly understood, we saw that it was philosophically strong, was supported by much evidence, and opposed by none. Bringing the spectroscope to bear upon it, we found that true nebula? undoubtedly exist, and that there is tolerably good proof of different degrees of complexity among the fixed stars. Notwithstanding these differences, however, we know that the universe is built throughout of essentially the same materials. In order to bring unity out of this diversity in the constitutions of the heavenly bodies, we arranged a series of development, from nebula to planet. This made it apparent that an evolution of matter from lower to higher stages might have accompanied the formation of planets and suns; an idea which was suggested also by physical analogies, and which had decided elements of philosophical strength. And thus we gave to the nebular hypothesis the somewhat novel form which it has received in our speculations. Without our additions, it could derive no real support from the spectroscopic evidence adduced in its behalf. The known nebulæ are simple, our systems of suns complex. By assuming the evolution of matter, these difficulties cease to exist, and we have a coherent hypothesis, in which the evidence offered by the spectroscope is used to good advantage. To be sure, although it is in harmony with many observed facts, it is open to many objections. And yet we can admit its probability, to a certain extent, without giving it the adherence of actual belief. Such theorizing is profitable, partly because it aids us in making out the limits of our present knowledge, suggests to us new paths of investigation, and, by uniting masses of different ideas, helps the mind to handle more easily the facts and conceptions with which it has to deal.
But, when one is fairly started on a line of thoughts, it is hard to come to an end. Problem after problem, theory after theory, law after law, crowd forward for inspection. If we assume one hypothesis to be true, a hundred others rush in upon the mind, and demand consideration. From every one of these a host of interesting conclusions can be drawn, each suggesting another, until the brain grows weary of action. The present case is no exception to the rule. Objections must be answered, consequences foreseen, demonstrations sought. In an article of this scope few points can receive due attention. Let it then suffice, in closing, to say that science has done so much in the past that we can justly expect almost any achievement in the future. And perhaps, in days yet to come, an evolution of matter may be experimentally be brought about, and our speculations of to-day proved to be not altogether foolish.