THE SPECTROSCOPE.
We now come to speak of an instrument which may
fairly rank, after the telescope and microscope, as one
of the most wonderful discoveries of modern optical
science. By its means we have not only discovered
four new elementary bodies, which are found in certain
minerals in inconceivably small quantities, but we have
also determined the chemical composition of some of
the remotest stars and nebulæ.
In 1701 Newton discovered that if an ordinary ray of white light was admitted through a small hole into a dark chamber, and thence passed through a triangular prism, it became decomposed into a coloured band, known as the solar spectrum. As we have already explained that this decomposition is caused by the different coloured rays that make up white light being bent unequally by the action of the prism, we trust the following explanations will be readily understood. In 1802 Dr. Wollaston, an English philosopher, discovered that by using a narrow slit, instead of a round hole, the resulting spectrum was no longer continuous, but was divided at intervals by dark lines extending across it in a direction parallel to the edges of the prism. These lines attracted considerable attention at the time, but it was not until 1815, that Fraunhofer, an optician of Munich, investigated them with accuracy. He mapped and counted no less than six hundred of them, identifying eight of the most conspicuous by the first eight letters of the alphabet. Their positions are as follow:—
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The designations of these lines have been retained to the present day, and they have been named after the Munich philosopher, being known as Fraunhofer's lines. They are to be seen in all parts of the spectrum, and increase in number and fineness according as the width of the slit through which the light passes is diminished. It may be asked, how it happens that they increase in proportion to the narrowness of the aperture admitting the light? A little consideration will soon show the reason of this.
When a beam of light is passed through a hole of, let us say, the eighth of an inch in diameter and decomposed by a prism, the spectrum so produced is imperfect, inasmuch as an infinite number of spectra are thus superposed, and for this reason, that the rays of light entering on the right side of the aperture will give a spectrum falling in a different place to that formed by the rays entering on the left. In order, therefore, to diminish the confusion caused by the superposition of a number of spectra, the aperture ought to be reduced to a narrow slit. When the thin slice of light passing through the slit is decomposed by the prism, we find that not only is the purity of the colours greatly increased, but the lines in question make their appearance more or less in all parts of the coloured band.
These lines are very unequally distributed, some being crowded together in masses, while others are extremely faint, and are separated by large intervals. Their position is well marked and determined, no matter from what source we obtain our beam of sunlight. Whether the spectrum be produced from the sun itself, or from the reflected light proceeding from the moon or planets, they are still found in the same place; only that in the latter case they are not so numerous, on account of the light being much fainter. For many years the cause of these lines remained a complete mystery, and it was not until Bunsen and Kirchhoff undertook their investigation that a satisfactory explanation of their origin was arrived at. In order to explain this, we must consider briefly the properties of the spectra of flames, and other luminous bodies.
If, instead of the light of the sun, we examine prismatically the light given off by an incandescent body, such as a white-hot piece of platinum, we shall find that the lines seen in the solar spectrum are absent, and that we have a continuous band of coloured light quite uninterrupted by dark spaces or bands. The same absence of lines is seen in the spectra of the electric light and the flame of an ordinary candle, the light in each of these cases being produced by particles of carbon in a state of vivid incandescence. But if we examine the flame of incandescent gases, we shall find a spectrum of an entirely new kind. Thus if we examine an ordinary gaslight through a slit with a prism, we shall obtain a continuous spectrum, in consequence of the luminous portion of the flame consisting of solid carbon in a state of incandescence; but if we turn down the flame, so as to lessen the amount of carbon to be burned, we shall find the whole of that body is converted into feebly luminous gas, giving off a faint reddish blue light. If we now again examine it in the same manner, we shall find that the spectrum produced consists of black spaces, here and there crossed by a few faint coloured lines or bands. The reason of this is obvious: in the faint flame caused by the carbon and hydrogen in a state of luminous vapour, which only have a few of the colours of the spectrum, which, when passed through the prism, fall into their proper places. All substances with which we are acquainted are capable of being converted into luminous vapour by means of heat, and when thus burnt produce flames of more or less faint luminosity, generally characteristically coloured. A piece of soda inserted in the wick of a spirit lamp gives a yellow tinge to the flame; a morsel of saltpetre (nitrate of potash) or nitrate of strontia will give a purple and crimson tint respectively. These hues are caused by the metals sodium, potassium, and strontium contained in these salts being converted into luminous vapour. On analyzing these coloured flames with a prism, as before, we should find in the case of the soda a single broad yellow line, situated just in the middle of the yellow portion of the spectrum, the rest of the space where the spectrum should be being perfectly dark. The reason of this is pretty simple. Sodium burns with a pure yellow flame, consequently when passed through a prism it cannot split into any other colours, but takes its place in the position belonging to yellow of that particular hue. Were it a little more orange or green in tint, it would take its place nearer to the red or violet end of the spectrum. The light from saltpetre, which contains potassium may next be examined. It will be found to tinge the flame with the spirit-lamp of a beautiful purple. We can almost guess what will happen when this flame is submitted to the action of the prism. We shall find that the purple light emitted will split into red and violet, which will immediately arrange themselves in their proper positions according to their hues. If in like manner we substitute nitrate of strontia for saltpetre, we shall get a splendid crimson flame which is decomposed by the prism into red, orange, or blue. On submitting the compounds of the other elements to the same tests, we shall find that each of them, when converted into luminous gas, is capable of producing coloured lines of various kinds when the light of their flames is passed through a prism. If, therefore, we had a number of salts of whose composition we were ignorant, all we need do is to burn them in a spirit-lamp, and by the number and position in the lines of their spectra we should be able to tell immediately of what they were composed.
The spectra of nearly all the elements capable of being connected with luminous gas have been determined with great accuracy. Perhaps the number and position of the lines of a few spectra will be interesting to the student.
Sodium.—This is the metallic base of soda salts, and gives a double bright yellow line in the middle of the yellow.
Potassium.—The base of the various salts of potash. It gives one line in the extreme red, one in the middle of the red, one in the violet, and a peculiar glow in the centre of the spectrum.
Strontium.—The base of the strontia salts, of which the nitrate is used as the principal ingredient in the red fire of the theatres. It gives a group of lines in the red and orange, and a beautiful blue one in the middle of the blue.
Barium.—The base of the baryta salts, one of which is used in making green fire. It gives several strong lines in the green, and a few in the red, orange, and yellow.
After the position of the spectral lines of most of the elements had been discovered, Messrs. Bunsen and Kirchhoff were one day examining the saline deposit of a spring which issues from the earth near Durkheim, in the Palatinate, and were surprised to find that a blue line belonging to no known metal made its appearance in addition to the potassium, sodium, and other lines produced by the saline ingredients of the water. These philosophers immediately concluded that the unknown line was caused by an unknown metal, and they at once set to work to obtain a larger quantity of the saline residue from the spring. They evaporated down no less than forty tons of water, and succeeded in isolating the new substance, which turned out to be a metal resembling potassium. While examining the residue more carefully, a new, dark red line, beyond that belonging to potassium, was discovered, pointing to the existence of a second new element, which was also afterwards obtained in the pure state. These two new metals, which closely resemble potassium in their properties, were named in accordance with the lines given by them when converted into luminous gas. The first was called cæsium, from cœsius, Lat. light blue; and the other, rubidium, from rubidus, Lat. dark red. Since the publication of MM. Bunsen and Kirchhoff's experiments, these two elements have been found in comparatively large quantities in various minerals, and these properties have been closely studied.
Spectrum analysis has yielded us two more new metals since first these philosophers applied the prism to the determination of the chemical composition of various bodies. Mr. W. Crookes, F.R.S., an English chemist of eminence, while examining the flame of a deposit obtained during the manufacture of sulphuric acid from a certain sulphur mineral found in the Hartz mountains, perceived a brilliant green line with which he was previously unacquainted, which quickly flashed into view, and then disappeared. After numerous experiments on various other minerals (for the deposit he had first experimented upon only yielded him a few grains of the new body), Mr. Crookes succeeded in discovering a comparatively large quantity of it in a sulphur mineral found in Belgium. The new element was found to be a heavy metal, closely resembling lead in its properties. It was named by the discoverer, thallium, from the Greek word thallos, a green twig, from the brilliancy of the single green line that indicates its presence. In like manner, Messrs. Reich and Richter have discovered a fourth new metal, which has been named indium, from its principal lines being found in the centre of the indigo of the spectrum.
The delicacy of spectrum analysis may be imagined from the fact that a quantity of sodium amounting to less than the two-millionth of a grain can be detected by its means. Indeed, it has taught us that sodium in one form or other exists almost everywhere. This mode of analysis is only serviceable to indicate the composition of any salt or other substance, the quantities of the different elements found by its use having no influence on the appearances brought out by the prism. Thus, a substance which has only been contaminated with sodium from being handled by warm fingers, will show the yellow bands as strongly as if it contained a large proportion of that metal.
For ordinary experiments in spectrum analysis the apparatus used is very simple. It consists of a tube with a fine slit at one end, and a convex lens at the other, for concentrating the light from the coloured flame upon the centre of the prism. After the light passes through the prism, it is examined by a small telescope of low magnifying power. The lamp used may be either a spirit-lamp or a colourless gas flame into which the substance to be examined is introduced upon a platinum wire.
We now come to another very important discovery, made by means of our prism and narrow slit—the determination of the composition of the photosphere or mass of luminous vapour surrounding the body of the sun.
A simple experiment will show how this brilliant discovery was arrived at. The light of a candle or other flame containing incandescent solid matter is passed through the spectroscope, and is found to decompose into a continuous spectrum, uninterrupted by dark lines. Between the light and the slit a spirit-lamp is placed, but no difference in the appearance of the spectrum is perceived. Introduce, however, the smallest portion of a soda salt into the non-luminous flame of the second-lamp, and a broad black line is immediately seen, crossing the middle of the yellow portion of the band of colour. Remove the sodium flame and the band disappears; but do the same with the lamp producing the spectrum, and the spectrum of course disappears, and the dark band caused by the sodium flame is changed to the yellow line produced by that metal. The same experiments may be tried with potassium, strontium, and other metals; and we shall always find that when a coloured flame is introduced between an incandescent solid and its continuous spectrum, it produces a series of black lines corresponding to the substances by which it is coloured. Thallium, in like manner, would give a black band in the middle of the green, and indium a similar one in the indigo. (Fig. 6, Frontispiece.)
The exact position of the black band in the middle of the yellow is shown in the coloured figure of the spectrum so beautifully printed in the frontispiece of this book, and it has been found to correspond exactly with the dark line D of the solar spectrum. The inference from this fact is obvious. The incandescent portion of the sun gives off light corresponding in its properties to that emitted by the solid matter contained in the candle flame, but the photosphere containing the vapour of so-dium cuts off that portion corresponding to the sodium line. Accurate measurements prove that numberless other lines occurring in the solar spectrum are due to the vapours of other well known metals existing on the earth. Amongst these may be mentioned potassium, calcium (the base of lime), iron, nickel, chromium, and several others. This discovery with regard to the sun has resulted in the spectral examination of a large number of the fixed stars and nebulæ. For centuries the fixed stars refused to answer all questions put to them by mortals. The telescope showed them merely as bright points. Their nature and origin remained a beautiful mystery, until Dr. Miller, Mr. Huggins, Father Secchi, and a few other philosophers interrogated them in a manner that could not fail to draw forth an answer. They brought their light within range of their prisms, and forthwith they declared themselves to be suns like our own. It is true that before this they were looked on by most astronomers as bodies analogous to our own sun, but it was only reasoning from analogy, after all; but we are now able to assert with all the certainty that is compatible with human fallibility that many of these heavenly bodies are possessed of an incandescent centre, surrounded by a photosphere or envelope of gaseous matter in a luminous condition. It would be impossible to give a list of all the stars that have been examined up to the present time; the composition of the photospheres of a few must therefore suffice. It is singular that the elements hitherto discovered in the stars are those which are more or less abundant on the earth. Amongst them we may name hydrogen, nitrogen, sodium, magnesium, barium, iron, antimony, bismuth, tellurium, and mercury. The bright star in the constellation of Orion known as Betelgeux is one of the most singular in composition, the lines of its spectrum indicating the absence of hydrogen. If, as Messrs. Huggins and Miller suggest, the worlds revolving round this star are also deficient in this element, they would be without water, like our moon.
Upon a very clear night it may be noticed that the stars are not all of the same colour, but that many of them appear to be of a ruddy or yellowish tint. The cause of this is plainly seen when they are submitted to spectral analysis. Thus, Sirius, which is a brilliant white star, shows but three dark lines, while one of the stars in the constellation of Hercules shows several groups of bands in the red, blue, and green portions of its spectrum, fully accounting for its orange tint.
The double star Cygni is a very beautiful example of the distribution of colour between two members of a stellar group. One star shows a strong spectrum with the blue and violet portions almost totally blotted out, while its companion is similarly circumstanced with respect to the yellow and orange portions of its spectrum. The colour of one is consequently orange, while the other is of a delicate blue. If these stars are the principal members of a system, the alternation of blue and orange days must be indeed a singular phenomenon to those who inhabit their planets.
In some of the stars lines have been discovered which do not possess any equivalent amongst those produced by terrestrial matter; they consequently contain elements of which we know nothing; at the same time, however, it has been found that terrestrial elements exist in some of the remote nebulæ, which are so distant that their light takes many thousands of years to reach our earth.
Spectrum analysis has decided the grand question of the physical composition of the nebulæ. Those bodies were supposed, with some reason, to be aggregations of stars, like our Milky Way, which only required telescopes of sufficient power to resolve them. That they partly consist of gaseous matter in a luminous condition is evidenced by their showing a series of bright lines in the spectroscope, exactly like those produced by terrestrial gases. Their light is therefore not emitted by a solid or liquid incandescent body, but by a glowing gas. The lines mentioned by Messrs. Huggins and Miller showed that the nebula in the sword-handle of Orion consists of hydrogen and nitrogen in a state of luminous incandescence. Not the slightest trace of a continuous spectrum can be detected in the light emanating from this body; consequently, according to present hypotheses, it contains no solid matter at all. A number of other nebulæ have given similar results.
There are numerous star clusters which, unlike the true nebulæ, give continuous spectra when their light is submitted to the action of the prism. Of these may be specially mentioned the great clusters in Andromeda and Hercules, which give continuous spectra, interrupted by dark bands on the red and orange. The light thrown by these experiments upon the nebular hypotheses of Sir William Herschel, who considered that true nebulæ consisted of the primordial gaseous matter out of which suns and stars have been elaborated, is very great, and will be appreciated even by those whose knowledge of astronomy is small.
Spectral analysis has also been the means of our witnessing a celestial conflagration, and understanding the cause of this marvellous event. It is well known to most people that from time to time stars have suddenly burst upon us, and have almost as suddenly disappeared. The theories advanced to account for these singular celestial visitors, have been more numerous than satisfactory. In May 1866, a star of the second magnitude suddenly burst forth in the Northern Crown, and was almost immediately noticed by Mr. Huggins who brought every power of prism and telescope to bear upon this extraordinary celestial phenomenon. He found the spectrum of the star to consist of two distinct spectra, one being formed by four bright lines, the other analogous to the spectra of the sun and stars. Consequently two kinds of light were given off by this star; one forming a series of bright lines indicative of luminous gas, the other consisting of a continuous spectrum, crossed by dark lines, showing the existence of a solid body in a state of incandescence, surrounded by a photosphere of luminous vapours. Two of the bright lines undoubtedly showed the presence of hydrogen in a state of illumination, the great brightness of the lines indicating that the burning gas was hotter than the photosphere. These facts taken in conjunction with the suddenness of the outburst in the star, and its immediate decline in brightness from the second down to the eighth magnitude in twelve days, suggest the startling speculation that the star had become suddenly wrapped in the flames of burning hydrogen, consequent possibly on some violent convulsion in the interior of the star having set free enormous quantities of this gas. As the free hydrogen became exhausted, the spectrum showing the bright lines gradually waned until the star decreased in brilliancy. It must not be forgotten that the event seen by Mr. Huggins occurred many years ago, and that the light emitted by this marvellous celestial convulsion has been travelling to us ever since.
Comets and meteors have been submitted to the test of spectral analysis. The former erratic visitors have been but few and small since stellar spectrum analysis has been perfected. In January 1866, Mr. Huggins brought his apparatus to bear upon a small comet, which gave a somewhat unexpected result. When the object was viewed in the spectroscope, two spectra were distinguishable—a very faint continuous spectrum of the tail, showing that it reflected solar light, and a bright space towards the centre of the spectrum, indicating that the nucleous was self-luminous and gaseous.
Mr. Alexander Herschel—the nephew and the grandson of Sir John and Sir William Herschel—has recently succeeded in obtaining indications of the composition of the meteors that people the heavens in the months of August and November. The principal result of his observations appears to be, that sodium in a state of luminous vapour is present in the trains left behind these singular bodies.
Lightning has also been similarly examined, and lines showing that hydrogen and nitrogen were rendered luminous during the electrical discharge, were seen with great distinctness. In fact, the applications of the prism to scientific discovery are almost endless, and in describing them it is difficult to tell where to draw the line.
Before quitting this subject, it will be as well to say a few words on the fluorescent rays of the spectrum, to which allusion has already been made towards the end of Chapter IV., Part II. It was there said that the chemical power of the spectrum extends to some distance beyond the extreme violet, a fact that may be readily proved by exposing a piece of photographic paper to the action of the dark portion of the spectrum. Professor Stokes found that there were means of rendering these rays visible to the eye by altering their rate of vibration. This he found was possible by passing them through the solutions of certain substances, such as sulphate of quinine, horse-chestnut bark, &c. We have already said, that light vibrating at the rate of from 458 to 727 billion times a second, was capable of exciting luminous sensations upon the optic nerve. The latter is the rate of vibration of the extreme violet ray, and it has been found that the eyes of many persons are not sufficiently sensitive to be influenced by it; it is, therefore, just probable that there are animals whose eyes are so much more sensitive than ours, that they can see rays that exist far beyond those seen by us. Now, as difference of colour is produced by difference in the rate of vibration, it follows that those whose eyes are sensitive enough to perceive the extreme violet rays, see tints of violet that are inappreciable by others.
The power of sulphate of quinine in reducing the luminous vibrations is easily seen by passing a tube filled with the solution successively through each of the colours of the spectrum formed by a quartz prism; the ordinary colours will pass through the liquid as if it were simply water, but on arriving near the violet extremity a gleam of pale blue light will shoot across the tube, and continue to increase. As it is moved onwards the light will gradually die away, until a point is reached nearly equal in length to the whole of the visible spectrum, when it will disappear altogether. It is somewhat singular that no substance has yet been found that will increase the refrangibility of the dark rays beyond the red end of the spectrum. There are many artificial flames which produce this dark light (if we may use such a paradoxical expression) in greater quantity than the sun, whose light is no doubt greatly deteriorated in this respect during its passage through the atmosphere. The substance of which the prism is made also greatly influences the length of the invisible portion of the spectrum. By using a quartz prism and lenses of the same material Professor Stokes, found that the spectrum of the electric light could be traced for a distance equal to six times that of the visible portion.
The action of certain substances in rendering the invisible rays of light perceptible may be easily shown by any one possessing a horse-chestnut tree. A weak decoction of the inner portion of the bark having been made and filtered through blotting-paper, or at any rate allowed to settle, the room is made quite dark and a piece of common brimstone is ignited. The pale blue light given off is comparatively feeble, but it is very rich in the ultra-violet rays; consequently, when the infusion of horse-chestnut bark is poured into a tall jar of water, beautiful waves of phosphorescent light are seen flashing backwards and forwards as the two liquids mingle. The tincture of stramonium is also possessed of this property, and characters traced on paper with it, although nearly invisible by ordinary daylight, appear distinctly when examined by the light of burning sulphur.