Among the many discoveries that have made the nineteenth century famous, none have been more interesting than those which relate to the physical constitution of the universe, and which tell us of what the stars are made.

This subject has always been a fascinating one to mankind, and was much discussed by the old philosophers, who offered various theories to account for the formation of the universe, and wrote many long treatises to make the facts agree with their theories. Air, fire, and water, together or singly, were regarded by some as the primal substances from which all things were made, while others held a more elaborate and intellectual creed.

Plato, the great Greek philosopher, taught that the universe was an animal in the form of a sphere, the most perfect of figures, made of an imperishable material, and with a circular motion. To the universe was then given a soul, and within its boundaries were placed gods, mortals, and the animals of the air, the earth, and the sea. The gods were made of fire, were circular in form, and were scattered over the heavens among the stars. Each star had a soul, which at some time entered a human body, forming its immortal part, and after living a certain time on the earth might return to its home if the years had been righteously spent.

This theory, which was perhaps but an allegory veiling some belief that might have been considered impious by the vacillating Greeks, is important from the fact that it embodies the idea that had existed from the earliest times among the mystics, that there was a certain unity and identity among the various phenomena of nature, and that the universe should be considered as a whole made up of many diverse parts.

The discovery of the law of gravitation, and its application by Herschel to the star systems, established the harmony of the motions of the heavenly bodies, and brought the earth into relationship with the most distant stars. It was the first convincing proof that the earth was but a member of the one great system which is called the universe, and it brought with it a suggestion that was full of meaning to those who were interested in the question of the physical constitution of the universe.

The nebular hypothesis answered this question partly, but left a wide field for speculation, and it is to the German physicist, Kirchoff, that we are indebted for the discovery of a method by which the nature of the substances which compose the sun and stars may be determined.

To understand the work of Kirchoff we must start with Newton's discoveries as to the nature of light.

The beautiful colors displayed in the rainbow, as well as in drops of dew, in glass prisms, precious stones, and other substances, had always been of great interest to philosophers, and many fanciful reasons were given for these appearances, but Newton was the first to explain the phenomena as due to the nature of light, and not to some quality in the substance through which the light passed or on which it rested.

Starting with the well-known fact that the white or colorless light from the sun would separate into rays of different colors corresponding to the hues of the rainbow when made to pass through a prism, he carried on a number of experiments which finally led to one of the most interesting discoveries in science. He found that the rays always arranged themselves in the same order—violet, indigo, blue, green, yellow, orange and red—no matter what substance they passed through, and from this he deduced the theory that white light consists of rays of different colors which are simply separated by the action of the prism. This theory, which would account for all the prismatic colors shown in various substances, was conclusively proved by Newton's collecting the different rays which had been separated and bringing them together again to a common focus by passing them through a lens, when a band of white light was produced.

From this discovery Newton claimed that all color arises from the arrangement of the particles of bodies in such a manner that certain rays of light will be reflected, and certain others absorbed by them.

Important as this discovery was it attracted little notice, and it was more than a hundred years afterward before the subject received any particular attention. But, in 1815, the German physicist, Fraunhofer, made a discovery in relation to the composition of white light which led to the most important results.

The band of rainbow colors which is produced by causing a ray of sunlight to pass through a prism, is called the solar spectrum. When the beam of sunlight that falls on the prism comes through a large opening the colors seen in the spectrum overlap each other, so that often the middle of the spectrum, where all the colors overlap, appears white, only the two ends showing colors, one end being red and yellow and the other end blue and violet. But when the opening through which the sunlight streams upon the prism is made narrower the colors overlap less, and if it is a very narrow slit there is scarcely any overlapping at all, so that there will be a continuous change in the color from one end to the other, each different ray having its own place. In this case if rays of any one color are absent the part of the spectrum which they would occupy if present will appear black. Fraunhofer made use of this arrangement and allowed a beam of sunlight that came through a very narrow crack or slit to fall on the prism, and on examining the spectrum with a telescope observed that the different colors were crossed transversely by a great number of fine dark lines. Fraunhofer counted over five hundred of these lines, but their number has since been raised to thousands.

By a series of careful experiments Fraunhofer came to the conclusion that these dark lines always occurred in the same order when the solar spectrum was shown, whether the light came directly from the sun or was reflected from the moon or planets; and another set of experiments proved that the light from the fixed stars gave a spectrum in which the dark lines were seen to differ in position and number from those in the solar spectrum, and from this he was lead to believe that the dark lines were caused by some special property of the sun's light, which thus differed from the light of the stars. The attention of the scientific world was at once turned toward this new field of investigation, and the science of spectroscopy, or the study of the colored rays of light, was pursued with much eagerness.

Sunlight, direct and reflected, the light of the stars, the electric spark, the flame of a candle, and the colored flames produced by burning different metals, together with the light from gases and vapors, were all subjected to the most careful study. The results were marked in a set of tables which indicated the spectrum of each substance, and thus a knowledge of the spectra of many different metals and vapors was attained. The dark and bright lines which crossed the spectra of the different substances were also marked according to their number and position, and in this manner it became as easy to recognize a certain mineral by its spectrum as to distinguish a flower by its perfume.

It was found that the spectra of glowing hot solid or liquid bodies are continuous and show no trace of the fine black lines. The spectra of vapors and gases, on the other hand, showed simply a number of fine bright lines of different colors according to their position in the spectrum. Thus the spectrum of white-hot iron is simply a continuous colored band with no dark lines in it, and so does not differ from that given by any other hot solid or liquid substance. But the spectrum of the vapor of iron consists of an immense number of fine bright lines in all parts of the spectrum, which are seen on a dark background, while in the sun spectrum we see a series of dark lines on a colored ground. These spectra of vapors are highly characteristic.

Now the spectrum of sodium vapor consists of two fine yellow lines; it is also observed that in the sun spectrum there are two fine black lines in the yellow part of the spectrum that exactly match the two yellow lines of sodium. Kirchoff discovered that when the light from a glowing solid body which shows no dark lines in its spectrum is made to pass through the vapor of sodium it will then have two dark lines exactly like those in the solar spectrum. Further experiments established the fact that whenever light which had passed through the vapor of a substance was examined, dark lines were found in its spectrum corresponding to the bright lines which the vapor would give if it were itself the source of light.

On the basis of these splendid results Kirchoff built up his theory of the physical constitution of the sun.

Taking the bright lines in the spectra of iron, nickel, copper, zinc and many other metals, he found that they were identical with the dark lines in the solar spectrum, as regarded number and position, and he was therefore led to the conclusion that the sun was a glowing solid or liquid mass whose light passed through an atmosphere of luminous vapors, which contained many of the substances which compose the earth.

This theory, which seemed to be upheld by the most convincing proofs, was immediately perceived to be the most reasonable that had yet been offered as to the nature of the sun, and scientists at once set to work to see whether its acceptance might not lead to a true knowledge of the physical constitution of all the heavenly bodies.

The light of the stars was found to give a similar spectrum to that of the sun, as regarded the appearance of dark lines, and, after many interesting experiments, the conclusion was reached that many of the stars have nearly the same physical structure as the sun, and the dark lines in their spectra indicate that many of the metals that we are familiar with on the earth exist also in them.

Further study of the light of the stars has resulted in placing them in groups according to the appearance of their spectra. Thus the stars which shine with a white light, and give spectra crossed by a few broad dark bands form one group, to which belong the great star Sirius and many of the brightest stars in the heavens. The red-colored stars give different spectra and form another group. Those stars which shine with a yellow light, and whose spectra are crossed by many fine dark lines, form another group, and to this it is supposed that the sun belongs.

It was thought by many astronomers that nebulae would be seen to be merely groups of very fine stars, if our telescopes were only powerful enough to discover them; but the spectroscope has given a decisive answer to the idea. The spectra of nebulae are found to be made up of bright lines, showing that they are simply masses of glowing vapors or gases.

Spectrum analysis, or the study of the colored rays of light, has had an effect upon the study of the universe second only to that of the discovery of the law of gravitation, and it is impossible to foresee the great results it may lead to. Already it has brought a knowledge of the nature of the remotest stars and the scarcely discernible nebulæ, and with the increased facilities which more delicate optical instruments may bring, we can hardly calculate the importance of its powers.

And it is the more remarkable that this wonderful agent may be of as much use in the mechanic arts as in solving the great problems of astronomy. The use of the spectroscope, in connection with the microscope, has led to the detection of certain substances in a drop of blood the size of the head of a pin which could never have been discovered by any other process, and the same subtle power has given to the world several new metals whose presence had never been suspected until the lines in the spectrum indicated their existence.

These metals have, for the most part, been named for the colors of the lines in the spectrum. Rubidium, which is found in many plants, as cocoa, tea, coffee, oak and others is shown by two dark-red lines; cæsium gives two intense blue lines; indium is marked by two characteristic lines of an indigo blue; and thallium, which gives a vivid green color, is named from the Greek word which means a green branch.

Thus the spectroscope reveals the unseen and unsuspected in nature, and brings to light forces as subtle as those which paint the flowers and give music to the winds.

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