FINDING HELIUM

Evening Star, Issue 22448, 19 September 1936, Page 19

FINDING HELIUM

USE OF SPECTROSCOPE THE CLUE OF THE ECLIPSES At the time of a total eclipse of the sun there is no very striking phenomenon until shortly before totality occurs, says a writer in the Melbourne ‘ Ago.’ But just at the instant of totality, as the last rays of the eclipsed sun disappear, there is revealed to the eye a picture of unequalled beauty. The disc of the moon, of deepest black, appears to hang in the sky, surrounded by the soft silvery light of the corona. In this corona bright, rose-coloured , tongues and clouds of fantastic shapes appear to project from different points of the moon's limb. Of these two phenomena the corona was known to the ancients, but apparently there is no record of these strangely beautiful and weird appendages of the sun, which have been variously referred to as red flames, protulierances or prominences, until early in the eighteenth century. In 1706 Captain Stannyan .was at Berne observing the total solar eclipse, the duration of which was four minutes and a-half. In a letter to Flamsteed, the first “ Astronomer Royal,” he says: “His getting out of his eclipse was preceded by a blood-red streak of light from his left limb, which continued not longer than six or seven seconds of time; then part of the sun’s disc appeared all of a sudden as bright as -Venus was ever seen in the night—nay, brighter; and in that very instant gave a light and shadow to things as strong as the moon used to dti.” THE PROMINENCES. ,A. similar phenomenon was recorded by Halley and Louville in 1715, and probably has been observed at every total eclipse of the moon since that time seen in Europe. Flamsteed considered that the “ blood-red streak ” belonged to the moon, and at any rate till 1842 the idea that the red flames were due to a lunar atmosphere held sway, and was not banished from men’s minds till 1860. At the eclipse of 1842, the first at which full and accurate scientific observations wore made, the rose-coloured prominences were measured, and their various colours and general appearance noted. At the Swedish eclipse of 1851, where many distinguished British astronomers were present, the prominences were very well observed. .Airy, the Astronomer Royal, was convinced that the prominences belonged to the sun and riot to the moon, but Dunkin, a well-known British observer, formed t|ie contrary At the Spanish eclipse of 1860 the prominences were photographed by de la Rue and by Seechi, and were proved beyond all doubt to belong to the sun—the prominences were eclipsed and uncovered exactly as the sun itself was. Immediately the question of what the prominences were arose. At first there seemed no answer, but a new method of attack was developed by Lockyer in 1866. When the light from the sun passes a spectroscope the light is drawn out into a rainbowcoloured band, crossed by numerous black lines. THE LAWS STATED. Kirchhoff and Bunsen (1859-62) explained these lines and enunciated the following laws;— (1) An incandescent solid, liquid, or dense gas gives a continuous spectrum. (2) A glowing rarefied gas composed of atoms of elementary chemical substances gives a spectrum consisting of bright lines. The line spectrum of each chemical element is distinctive, and is characteristic of the element. (3) When light from a source giving a continuous spectrum is passed through a relatively cool gas, which by itself would emit a bright line spectrum, the continuous spectrum is crossed by narrow dark lines, corresponding exactly in wave length to the bright lines that the gas js capable of emitting. The lines only appear dark when the temperature of the intermediate gas is lower than that of the source giving the continuous spectrum. If the temperature of the intermediate gas is higher, the spectrum of the transmitted beam consists of bright lines due to the gas on a less bright background due to this source. Applying these laws, Lockyer argued that if the prominences were gaseous, it might be possible to see them apart from an eclipse. Suppose the narrow slit of the spectroscope is set so that part of the slit is filled by the image of the sun and part by the image of a prominence. On passing through the spectroscope the light falling in the slit from the sun’s image will _be drawn out into a long band. The light therefore will be spread out over a large area, and its intensity correspondingly diminished. If the prominence consists of a rarefied gas or gasses, its spectrum will be formed of narrow bright lines. On passing through the spectroscope therefore, these lines will not undergo appreciable widening, and in consequence their intensity will not be diminished. THE SPECTRUM OF THE SKY. But superposed on the spectrum of the prominence there will be the spectrum of the sky in the neighbourhood of the sun’s limb; which will be similar to the spectrum of sunlight. If the spectroscope is sufficiently powerful, this sky spectrum will be spread out to such an extent that the lines due to the prominence will appear brighter than the neighbouring continuous spectrum due to the sky. On the other hand, if the spectrum of the prominence were ■continuous, it would be spread out in the same way as the solar spectrum, and the method would fail. In 1866, Lockyer made his first attempts, but without result, as his spectroscope was not sufficiently powerful to give. a dark enough' background against which to see the bright lines, if such existed. He communicated his idea to the Royal Society, and obtained a grant from the Government Grant Committee for the construction of a more powerful spectroscope. In the beginning of 1867 an instrument was being constructed, but, owing to a chapter of accidents, it was still incomplete when it was delivered at Lockyer on October 16, 1868. Four days later he sent a preliminary communication to the Royal Society, in which he announced that he had established the existence of three bright lines in the spectrum of a prominence. Two of these were coincident with well-' known lines of hydrogen in the solar spectrum, and showed the presence of hydrogen in the prominence. The third was near, but not coincident with, the yellow sodium lines. Meanwhile the bright lines had been independently observed by several astronomers at the Indian eclipse of August 18, 1868. These were equipped with spectroscopic apparatus, so that the nature of, the spectrum of the prominences .could be observed. Several bright lines were seen, but the observations left the exact positions of these lines in douhfc. t

A NEW ELEMENT. The element which was responsible for the production of this bright line in the neighbourhood of the yellow lines of sodium was for long a mystery. No substance could be found which gave rise to a line in this position in the spectrum, and Lockyer was of opinion that the lino was produced by some element not yet known in terrestrial laboratories. To this new element he gave the name helium, but in many quarters the existence of this hypothetical element was ridiculed. In the spectrum of many stars this same line of unknown origin appeared, but all attempts to produce it in the laboratory failed. In 1895 a fresh step forward was made. In that year Lord Rayleigh and Professor Ramsay discovered the gas argon. In the course of the researches they examined various sources of nitrogen, amongst others the mineral uraninite. The gas obtained was examined spectroscopically, and it so happened that the pressure of the gas and the electrical conditions employed were such that the spectrum obtained 'was not that usually associated with nitrogen, but consisted of a bright line in the yellow. Careful measurement showed that this line was identical with the yellow line observed in the spectrum of the prominences 26 years before. Ramsay sent a tube of the gas to Lockyer on March 28, but Lockyer obtained some gas for himself, and from March 30 onward he and his assistants identified not only the yellow line, but a number of other lines of which the source was till then unknown. These lines were compared - , not only with lines of unknown origin in the sun, but also with those in some nebulae and stars. More recently helium has been detected in much larger quantities in the natural gases from wells in the United States and Canada, the United States producing 600,000 cubic feet of helium per month, so that it has been possible to use helium instead of inflammable hydrogen in filling airships. Liquid helium has proved to he exceptionally suited for the production of very low temperatures, while in the laboratory the helium nucleus plays a very important part in one of the products of radio-active disintegration.