AFTER NEWTON missed the spectroscope, in 1666, over a century and a quarter passed before any one made the great discovery. In 1802, Wollaston used the slit that was to make the spectroscope possible.
Basically, the prism spectroscope to-day is the same instrument that Wollaston discovered, refined, however, almost infinitely. A ribbon of light, defined by the edges of an adjustable slit, passes through a series of lenses that make it sharp-edged and narrow as possible. It enters the prism, which bends the light, each wave length to a slightly different degree, and so forms a long series of images of the ribbon of light that entered, each a different color.
After the original discovery, accuracy in the spectroscope came to be synonymous with greater definition, thinner and still thinner wedges of colored light, so that ever more strips of color, more slit images, could lie side by side in the rigidly limited width of the rainbow pattern the prism gave. But there was one great difficulty: the thinner the slit, the better the results so far as the color spread went -- but, if you try to illuminate a five-inch-long area one inch deep by the light of a slit a hundredth of an inch across by one inch tall, the illumination becomes so diluted you can't see the beautiful, sharp lines the slit would be throwing.
It is no wonder that in 1810, 1820 and those first years of the spectroscope most spectroscopic work was done in connection with sunlight. You could dilute sunlight a whole lot and still have enough to see, but when artificial illumination consisted of candles and oil lamps, artificial light sources simply gave the struggling spectroscopist grief. Artificial light, the development of the electric arc and the electric furnace, the gas-blow tube, have all been immensely helpful to him. An even greater thing was still in the future when those pioneers went to work: photography.
But for all their besetting troubles, they must have had a glorious time with that new instrument! Fraunhofer was the first to record the strange, inexplicable black lines that crossed the brilliant spectrum of the Sun. They'd tried all sorts of illumination: bits of metal heated in the hottest flames they could get, the fierce glow of their furnaces. Those all seemed much like the light from the Sun -- somewhat redder, of course, but much the same otherwise -- yet those sources never gave the black lines Fraunhofer recorded.
Then Bunsen, with a gas flame and a bit of table salt, showed the origin of those black lines. Then came Angström, and the measurement of the wave length of light, and the naming of the Angström unit of wave length.
IN the middle of the 1800s Kirchoff did for the accumulated mass of data produced by many workers what Kepler did for Tycho Brahe's accurate astronomic observations. He integrated all the findings, deduced from them certain principles, and set them forth as the laws of spectroscopy. Briefly, the data and the laws were these:
Incandescent, molten iron, molten copper, the fire brick of the furnace that holds them, all give exactly the same result in a spectroscope; no matter how great the dispersion, how great the resolving power, the result is a continuous spectrum, a smooth blending of one color light into another. Some parts of that spectrum will be brighter than others; with molten antimony metal, which glowed a dull red, the spectrum was strong in the red end and scarcely visible in the blue. With molten iron, the spectrum was stronger yet in the red, but the strongest part came toward the yellow, with the blue quite visible now. Molten platinum gave a still bluer spectrum.
But that meant little, because molten iron heated to the temperature of molten platinum would give exactly the same result. And even gases under very great pressure did the same, any gas.
So the first law of the spectroscope was: a solid, liquid, or gas under great pressure, radiates a continuous, featureless spectrum, the position of whose maximum intensity shifts toward the blue with increasing temperature.
Further, and much more interesting, they had found that if sodium, for instance, were dropped in a flame, the flame was colored yellow, and in the spectroscope the burning gas gave two intense yellow lines which were produced by any sodium gas burning, however it was derived, whether from table salt or from animal tissue. And no other substance ever gave exactly those lines.
The second law of the spectroscope was: a radiating gas under low pressure gives a spectrum consisting of bright lines, the position of which, and the relative intensity of which are characteristic of the chemical constitution of the radiating gas.
Bunsen had explained Fraunhofer's lines, and had shown what caused them; a cool gas under low pressure between the incandescent photosphere of the Sun -- which was a radiating gas under terrific pressure, and as is to be expected, gave a continuous spectrum -- and the spectroscopes on Earth. It was easy to show on Earth by boiling a bit of sodium metal in a closed glass vessel so that no burning took place, thus giving a layer of sodium gas at a fairly low temperature, and focusing a spectroscope through it at an incandescent solid. The result was a smooth, continuous spectrum, save for two black lines exactly where the bright yellow lines of sodium would have been, had the sodium gas been the radiator.
The third law of spectroscopy is: if, between a source of a continuous spectrum and the observing spectroscope, there is a gas under low pressure, there will be black lines crossing the bright spectrum in exactly the position the gas would give bright lines were it radiating.
The fourth law of spectrum analysis is based on literally un-Earthly data. It is the Doppler principle applied to light; if a radiating source and a spectroscope are relatively approaching each other, the spectrum is displaced toward the blue; if they are relatively receding from each other, the spectrum is displaced toward the red. The degree of displacement depends on the relative speed.
That highly important principle applies in astronomy most noticeably. If a star is approaching the Earth, and the spectroscope shows the typical calcium spectrum, but displaced well toward the violet, we know the star is moving in our general direction at a fast clip.
BOHR gave a general picture with his solar-systemlike atom, a nucleus with electrons circling it like planets circling a sun, in orbits far out. But the atom he pictured was a rigid structure, more like a model made up of steel balls on springs. The shorter the radius of the electron's orbit, the stiffer the spring that carried it, the more massive the nucleus, the more electric charge it carried and the stiffer grew the orbit electron springs. Thus light hydrogen, with one electron at a short distance, gave a spectrum in the visible range, while enormously massive uranium, heaviest of the known atoms, gave a spectrum in the visible range only with the electrons which were far out from the nucleus; electrons at the distance from the uranium nucleus that hydrogen's electron is from the hydrogen nucleus acted as though on such enormously stiff springs that they vibrated far too rapidly even for ultraviolet, far in the X-ray range.
Further, an atom robbed of an electron was left with an unbalanced electric charge, and stiffened up as though to defend itself against further loss. Thus a neutral atom of calcium would give a red spectrum, but robbed of one electron, the stiffening resulting would make the next electron vibrate far more swiftly in the violet, or ultra-violet.
But light had at last begun to tell its story. However, almost as soon as photography had made it possible to get the ultra-violet range of the spectrum, men found out that while light had an enormously interesting and important story to tell, the telephone line was faulty. The Earth's atmosphere, high above the stratosphere, contains a layer of ozone, a special compounded form of oxygen produced by the action of electrons shot out by the sun, and this ozone is as opaque to ultra-violet as so much cast iron. That left about one third of the ultra-violet range, the visible, and the infra-red -- much good might it do them. They couldn't get anything at all in the infra-red because photographic plates were not sensitive to it.
The result was that such elements as boron could not be detected, even if they were in the Sun. The spectrum of boron is far in the ultra-violet. In fact, nearly all the nonmetallic elements give spectra in the ultra-violet. The metals were easier -- except for some like cæsium, which ionized so easily that it promptly became an ion, with a spectrum in the ultra-violet also, where it couldn't be reached.
To this day, we have not proven the presence of such common elements as chlorine, bromine, neon, argon, gold or bismuth. We have only within the last few years found phosphorous, and then only because plates were developed which were sensitive to infra-red and could find the line far, far out at 10,000 angströms. (7200 is the limit of human vision).
Boron has been found, despite the fact that the element does not give an attainable spectrum, because it forms a compound with oxygen stable at even that tremendous temperature, and the compound does give a visible spectrum. Silicon and fluorine have been found similarly, by the presence of silicon fluoride.
NEXT comes the problem of how much? From the strength of the spectrum lines, we can form an estimate, taking into account the fact that some elements, such as calcium, have enormously strong lines, while the same amount of lead would give very faint lines. Further, some elements can be identified only by the lines of the ionized atom, and the ionized state is the wreck of an atom, a special condition, occurring only when an atom has been violently treated, and accordingly only a small percentage will be in condition to give that line.
Thus, working back from a line on a bit of glass plate, we must estimate the number of atoms needed to produce it, then estimate what percentage of atoms would be ionized under those conditions obtaining on the Sun, and so derive the whole number of atoms of that element.
Through laborious work, fairly accurate estimates have been made possible. We can compare these estimates of the commonest metals in the Sun with similar estimates of the most plentiful metals in the whole Earth -- including the core, as derived from the study of meteorites. As might be expected, the analysis shows that the Sun's metals agree very closely in relative plentitude with those of Earth. The table below gives the most abundant metals in order; the first group is approximately ten times as plentiful as the second; the second, in turn, ten times as plentiful as the third.
Now come the nonmetals, such things as silicon and oxygen, hydrogen and sulphur. Silicon and oxygen seem to be about equally abundant in the Sun and on Earth, in proportion. But carbon appears to be about ten to one hundred times as abundant in the Sun.
But right there the similarity stops. Hydrogen in the Sun is three hundred times as abundant as all the metals put together! Nitrogen is even more widely off; it is ten thousand times more abundant on the Sun! And helium, argon, neon, the rare gases of the atmosphere? From various indications given by hotter stars than the Sun, it would appear that the Sun has five hundred million times more neon, in proportion, than the Earth!
In other words, compared to the Sun, the Earth has practically no atmosphere at all. Put this way, the answer is not hard to see. At some remote past time, the Earth was torn out flaming hot, the gases blazing at immense temperatures. No mere six or eight thousand degrees, but the temperature of the invisible, incredibly hot, deep layers of the Sun. So hot that the light gases, impelled by heat motion of the atoms giving velocities of dozens of miles a second, shot out into space, beyond the Earth's gravitational grip. But -- not from such an Earth as we know to-day, a monster planet, gorged with light gases, for, before they escaped, Earth must have been ten times as massive as it is to-day!
BUT THEY ESCAPED. Fastest of all, because lightest of all, hydrogen escaped, and, in escaping, carried with it much of the heat energy of the planet, rapidly cooling the remainder by its swift expansion into space. Behind it flew helium, nitrogen, vast quantities of oxygen, carbon -- yes, for carbon was a gas at that still-tremendous temperature -- the inert gases neon and argon.
But much oxygen was bound to titanium and calcium, iron and aluminium and silicon in heavy molecules that could not break free. Much nitrogen was caught with carbon in cyanogen -- a carbon-nitrogen compound stable at high temperatures. The active gases were trapped by the heavy substances they combined with; the inert gases escaped. Hydrogen, though fairly active, escaped because there was such a vast amount, and it was so light.
So a cool Earth resulted, the last of the hydrogen uniting with oxygen to form water, the water in turn breaking down many of the compounds that had formed in the hot, new world. The excess of oxygen united with carbon to carbon dioxide, till that still far-future date when life should appear to drive it out to join the cool, and now utterly inactive nitrogen in the air.
Earth was made. And so must Venus and Mars and Mercury have been made, Mars and Mercury losing even more of their atmosphere, since they were even lighter planets.
And what, one wonders, of Jupiter? Immensely heavier, immensely more massive metallic core gripping far more strongly the volatile hydrogen.
That hydrogen never escaped in anywhere near the degree it escaped Earth! It is there to-day, and Jupiter is, by it, made a world of weird seas, and weird, alien chemistry, a world of strange atmospheres and oceans, whose constituents not even the science-fictionists have imagined!
Yet -- a world of an ideal climate on an alien basis, a world where life can more readily exist than on any other planet, save perhaps Earth alone!
Next Month John W. Campbell, Jr., tells of Jupiter in his ninth article in this series.
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