CHAPTER XXX.

READING THE LIGHT.

[Illustration: THE PRISM AND SPECTRUM.]

Several times, in the course of this book, mention has been made of a wonderful new instrument called the spectroscope. We now proceed to give a brief account of the origin and achievements of this newest of scientific appliances. The first step towards its formation was made by Sir Isaac Newton, when he discovered the power of the prism to decompose light. This consists in the fact that a ray of light, after passing through a transparent prism, becomes expanded into an elongated spectrum, no longer white, but presenting an invariable succession of colors from red to violet. These are called the seven primary colors; namely, red, orange, yellow, green, blue, indigo, and violet. The rainbow is a familiar illustration of this spectrum with its various colors; and the raindrops are the prisms which reflect and decompose the light.

[Illustration: THE SPECTROSCOPE.]

Optical science was long satisfied with this glance into the interior constitution of light, occupying itself with the phenomena of the prismatic colors, and theorizing on the nature of white light. In 1802, Dr. W. H. Wollaston, in closely examining a spectrum, found it to be crossed by at least four fine dark lines. It is only when an extremely narrow slit is employed in admitting the sunlight that they become visible. Dr. Wollaston, supposing them to be merely “natural boundaries” of the different color-bands, inquired no further; and there for a while the matter rested. Not many years later, in 1815, the matter was taken up by a German optician and scientist of Munich, Joseph von Fraunhofer. Applying more delicate means of observation, he was surprised to find very numerous dark lines crossing the spectrum.

With patience he went into the question, using the telescope as well as a very narrow slit, and soon he discovered that the dark lines were to be numbered, not by units or by tens, but by hundreds--or, as we now know, by thousands. Some were in the red, some were in the violet, some were in the intermediate bands; but each one had, and has, its own invariable position on the solar spectrum. For, be it understood, these dark lines are constant, not variable. Where a line is seen, there it remains. Whenever a ray of sunlight is properly examined, with slit and with prism, that line will be found always occupying precisely the same spot in the spectrum.

Some of the chief and more distinct lines were named by Fraunhofer after certain letters of the alphabet, and by those letter-names they are still known. He began with A in the red and went on to H in the violet. Fraunhofer made a great many experiments connected with these mysterious lines, anxious to discover, if possible, their meaning. For although he now saw the lines, which had scarcely so much as been seen before, he could not understand them--he could not read what they said. They spoke to him, indeed, about the sun; but they spoke in a foreign language, the key to which he did not possess. He tried making use of prisms of different materials, thinking that perhaps the lines might be due to something in the nature of the prism employed. But let the prism be what it might, he found the lines still there. Then he examined the light which shines from bright clouds, instead of capturing a ray direct from the sun. And he found the lines still there. For cloudlight is merely reflected sunlight.

Then he examined the light of the moon, to see if perchance the spectrum might be clear of breaks. And he found the lines still there. For moonlight is only reflected sunlight. Next he set himself to examine the light which travels to us from some of the planets, imagining that a different result might follow. And he found the lines still there. For planet-light again is no more than reflected sunlight.

[Illustration: SPECTRA, SHOWING THE DARK LINES.]

Lastly, he turned his attention to some of the brighter stars, examining, one by one, the ray which came from each. And, behold! he found the lines _not_ there. For starlight is not reflected sunlight. That is to say, the identical lines which distinguish sunlight were not there. Each star had a spectrum as the sun has a spectrum, and each star-spectrum was crossed by faint dark lines, more or less in number. But the spectra of the stars differed from the spectrum of the sun. Each particular star had its own particular spectrum of light, different from that of the sun, and different from that of every other star. For now, Fraunhofer was examining, not sunlight, but starlight; not the light of our sun, either direct from himself or reflected from some other body, such as planet or moon or cloud, but the light of other suns very far distant, each one varying to some extent from the rest in its make. The fact of the stars showing numerous sets of black lines, all unlike those of our sun, showed conclusively that those lines could not possibly be due to anything in our earthly atmosphere. Sunlight and starlight travel equally through the air, and are equally affected by it. If our atmosphere were the cause of the black lines in sunlight, it would cause the _same_ lines in starlight. But the sun and each individual star has its own individual lines, quite irrespective of changeful states of the air.

So, also, the light from any metal sufficiently heated will give a spectrum, just as sunlight gives a spectrum, under the needful conditions. That is to say, there must be slit and prism, or slit and diffraction-grating, for the light to pass through. But the kind of spectrum is by no means always the same.

Putting aside for a few minutes the thought of sunlight and starlight, let us look at the kind of rays or beams which are given forth by heated earthly substances. Any very much heated substance sends forth its light in rays or beams, and any such rays or beams may be passed through a prism, and broken up or “analyzed,” just as easily as sunlight may be “analyzed.”

Suppose that we have a solid substance first--a piece of iron or of steel wire. If it is heated so far as to give out, not only heat, but also light, and if that light is made to travel through the slit and prism of a spectroscope, the ray will then be broken up into its sub-rays. They, like the sub-rays of a sunbeam, will form a continuous row of soft color-bands, one melting into another. This is the characteristic spectrum of the light which is given forth by a burning or glowing _solid_.

Next, suppose we take a liquid--some molten iron or some molten glass, for instance. If you have ever been to a great plate-glass manufactory, like that at St. Helens, not far from Manchester, you will have seen streams of liquid glass pouring about, carried to and fro in huge caldrons, bright with a living light of fire from its intensity of heat. If a ray of _that_ light had been passed through slit and prism, what do you think would have been the result? A continuous spectrum once more, the same as with the glowing iron or steel. The light-ray from a heated and radiant _liquid_, when broken up by a prism, lies in soft bands of color, side by side.

Both of them a good deal like the solar spectrum, you will say. Only here are no mysterious dark lines crossing the bright bands of color. But how about gases? Suppose we have a substance in the state of gas or vapor, as almost every known substance might be under the requisite conditions, and suppose that substance to be heated to a glowing brilliance. Then let its light be passed through the slit and prism of a spectroscope. What result shall we find this time? Entirely different from anything seen before. Instead of soft, continuous bands of color, there are _bright lines_, well separated and sharply defined.

How many bright lines? Ah, that depends upon which particular gas is having its ray analyzed. Try sodium first--one of the commonest of earthly substances. Enormous quantities of it are distributed broadcast in earth and air and water. More than two-thirds of the surface of our globe lies under an enfolding vesture of water saturated with salt, which is a compound of sodium. That is to say, sodium enters largely into the make of salt. Every breeze which sweeps over the ocean carries salt inland, to float through the atmosphere. Sir H. E. Roscoe writes: “There is not a speck of dust, or a mote seen dancing in the sunbeam, which does not contain chloride of sodium”--otherwise salt. The very air which surrounds us is full of compounds of sodium, and we can not breathe without taking some of it into our bodies. Sodium is an “elementary substance.” By which I mean that it is one of a number of substances called by us “simple,” because chemists have never yet succeeded in breaking up those substances, by any means at their command, into other and different materials.

Iron is, so far as we know, a simple substance. It is found as iron in the earth. No chemist has ever been able to make iron by combining other materials together. No chemist has ever managed to separate iron into other materials unlike itself. Iron it is, and iron it remains, whether as a solid, as a liquid, or as a gas. Gold is another simple substance, and so is silver. Sodium is another. All these we know best in the solid form. Mercury, another simple substance, we know best as a liquid.

Water is not a simple substance; for it can be separated into two different gases. Glass is not a simple substance; for it is manufactured out of other substances combined together.

We can speak quite positively as to such substances as are not simple; but with regard to so-called “elements,” we may only venture to assert that, thus far, nobody has succeeded in breaking them up. Therefore, at least for the present, they are to us “elementary.”

All the simple substances, and very many of the combinations of them, though often known to us only in the solid form, may, under particular conditions, be rendered liquid, and even gaseous. Every metal may be either in the solid form, or the liquid form, or the vapor form. Iron, as we commonly see it, is solid--in other words, it is frozen, like ice. Just as increase of warmth will turn ice into water, so a certain amount of heat will make solid iron become liquid iron. And just as yet greater warmth will turn water into steam, so a very much increased amount of heat will turn liquid iron into vapor of iron. A little heat will do for ice what very great heat will do for iron.

Whether iron and other metals exist in the sun, as on earth, in a hard and solid form, it is impossible to say. It is only in the form of gas that man can become aware of their presence at that distance. The intense, glowing, furnace-heat of the sun causes many metals to be present in large quantities in the sun’s atmosphere in the form of vapor.

Not only have we learned about some of the metals in the sun, but this strange spectrum analysis has taught us about some of the metals and gases in the stars as well. It is found that in Sirius, sodium and magnesium, iron and hydrogen, exist. In Vega and Pollux there are sodium, magnesium, and iron. In Aldebaran, these substances and many others, including mercury, seem to abound. These are merely a few examples among many stars, each being in some degree different from the rest.

But how can we know all this? How could the wildest guessing reveal to us the fact of iron in the sun, not to speak of the stars? We know it by means of the spectrum analysis--or, as we may say, by means of the spectroscope. This instrument may be looked upon as the twin-sister to the telescope. The telescope gathers together the scattered rays of light into a small spot or focus. The spectroscope tears up these rays of light into ribbons, sorts them, sifts them, and enables us to read in them hidden meanings.

When a ray of light reaches us from the sun, that ray is _white_; but in the white ray there are bright colors concealed. Newton was the first to discover that a ray of white light is really a bundle of colored rays, so mixed up together as to appear white. If a ray of sunlight is allowed to pass through a small round hole in a wall, it will fall upon the opposite wall in a small round patch of white light. But if a _prism_--a piece of glass cut in a particular shape--is put in the path of the ray, it has power to do two curious things. First, it bends the ray out of a straight course, causing the light to fall upon a different part of the wall. Secondly, it breaks up or divides the ray of white light into the several rays of colored light of which the white ray is really composed. This breaking up, or dividing, is called “analyzing.”

If in place of a round hole the ray of light is made to pass through a very narrow slit, it is proved that the bright bands of color do not overlap. Instead of this, dark lines, or gaps, show here and there. Now, these lines or gaps are always to be seen in the spectrum or image of bright colors formed by a broken-up ray of _sunlight_. There is always a certain number of dark lines in each colored band--some near together, some far apart; here one or two, there a great many. Where a simple ray of sunlight is concerned, the exact arrangement of the lines never changes.

When the stars were examined--when the rays of light coming from various stars were split up and analyzed--it was found that they too, like the sun, gave a spectrum of bright colors with dark lines. But the lines were different in number and different in arrangement from the sun’s lines. Each star has his own particular number and his own particular arrangement, and that arrangement and number do not change.

If a white-hot metal is burnt, and the light of it as it burns is allowed to pass through a prism, a row of bright colors appears as in the sun’s spectrum, only there are no dark lines. If a gas is burnt, and the light is allowed to pass through a prism, no bright color-bands appear, and no dark lines either; but instead of this, there are _bright lines_. Each gas or vapor has its own number of lines and its own arrangement. Sodium shows two bright lines, side by side. Iron shows sixty bright lines, arranged in a particular way.

Now you see how a row of bright colors without bands of color may appear. But what about color-bands and dark lines together? That discovery came latest. It was found that if a white-hot metal were burned, and if its light were allowed before touching the prism to shine through the flame of a burning gas, _then_ there were dark lines showing in the colored bands. These dark lines changed in position and number and arrangement with each different kind of gas, just as the bright lines changed if the gases were burned alone. If the light of the burning metal passed through a flame colored with gas of sodium, two dark lines showed on one part of the spectrum; but if it passed through a flame colored with vapor of iron, sixty dark lines showed on another part of the spectrum.

So now, by means of this spectrum analysis, we know with all but certainty that the sun and stars are solid, burning bodies, sending their light through burning, gas-laden atmospheres. By examining the little black lines which appear in the spectrum of one or another, it is possible to say the names of many metals existing as gas in those far-off heavenly bodies. Is not this a wonderful way of reading light?

The split-up rays tell us much more than the kinds of metals in different stars. When a nebula is examined, and is found to give no spectrum of bright bands and dark lines, but only a certain number of bright lines, we know it to be formed of gas, unlike stars and other nebulæ. Also, it is by means of the spectroscope that so much has lately been discovered about the motions and speed of the stars coming towards or going from us.