History of Helium discovery

During the solar eclipse visible in India on the 18th August 1868, a spectroscope was for the first time turned upon the solar chromosphere - the luminous atmosphere of gas which surrounds the sun. Many observers noticed in the chromospheric spectrum a yellow line, supposed by them to be the D lines of sodium. Janssen pointed out that this line did not exactly coincide with the sodium lines D₁ and D₂, and he proposed to call it D₃. Shortly afterwards, Frankland and Lockyer came to the conclusion that this line could not be attributed to any known terrestrial substance, but must be due to a new element existing in the sun. To this hypothetical element they gave the name helium (the sun); a name which was generally accepted by astronomers to denote the substance giving rise to the line D3. As observations accumulated, certain other lines were seen always to accompany this line and to vary with it in intensity, and they were consequently attributed to the same source. The chief of these were λ7056 λ4472, and λ3970; D₃ itself has λ5876.

Until the year 1895 the only reference to the possible existence of terrestrial helium is found in a note by the astronomer Palmieri, who observed that a lava-like product from Vesuvius gave a yellow spectral line of wave-length λ = 5875, and concluded that it contained helium. Unfortunately no details of his method of experiment are given, and it is possible that his observation was mistaken. Helium is known to occur in Vesuvian minerals, but it is not possible to obtain the helium spectrum from helium minerals either by heating in the flame or by the spark.

The actual discovery of terrestrial helium was made by Sir William (then Professor) Ramsay in the latter part of 1894 when searching for new sources of argon, then recently discovered. While engaged in this investigation he received a letter from Miers, the eminent mineralogist at that time connected with the British Museum, in which it was suggested that it might be worth while to examine certain uraninites (varieties of pitch-blende) from which Hillebrand had obtained a gas which he had supposed to be nitrogen. Ramsay considered it improbable that nitrogen could have been obtained from its compounds by the methods Hillebrand had used, and therefore proceeded to re-examine cleveite, one of the minerals from which the supposed " nitrogen " had been obtained.

It was really a most unfortunate chapter of accidents that prevented Hillebrand from making the discovery of helium. He had confirmed the presence of nitrogen in the cleveite gas in various ways: (a) the gas when sparked with oxygen gave nitrous fumes; (b) sparked with hydrogen in presence of hydrochloric acid it gave ammonium chloride, the identity of which was proved by conversion into ammonium platinichloride and estimation of platinum in that salt; (c) when subjected to an electrical discharge in a vacuum tube the gas gave a strong nitrogen spectrum. Ramsay was able to confirm the accuracy of these results, as he found about 12 per cent, of nitrogen in the helium from oleveite.

Hillebrand, writing to Ramsay after the discovery of helium had been announced, explained that he had noticed that in his experiments the formation of nitrous fumes and ammonia proceeded very slowly, and that the spectrum contained many lines not attributable to nitrogen. To the first phenomenon he attached but little significance as he was using only a small current. He was aware that the spectra of gases are profoundly influenced by changes of pressure, and therefore, though he and his assistant jocularly suggested that they might be dealing with a new element, the matter was allowed to drop, and helium remained undiscovered for another five years. Truly a great discovery narrowly missed!

Ramsay heated powdered cleveite with dilute sulphuric acid, sparked the resulting gas with oxygen over soda, removed excess of oxygen with alkaline pyrogallate, washed with water, dried, and transferred to a vacuum tube. The light given by the passage of electricity through this tube was examined visually in a spectroscope alongside that from a Pliicker tube containing argon, as a comparison. It so happened that this seconcl tube, owing to impurities contained in the magnesium electrodes, gave the spectra of hydrogen and nitrogen as well as the argon spectrum. It was at once evident that the cleveite gas contained some argon and hydrogen, but it gave also a brilliant line in the yellow, nearly, but not quite, coincident with the yellow sodium lines.

The wave-length of this line was measured by Crookes and proved to be exactly that of the solar D₃ line. It thus became known that helium could thenceforward be reckoned among the number of terrestrial elements.

This discovery was quickly confirmed by Cleve and by Lockyer, who prepared a sample of the new gas from broggerite, and identified in its spectrum many lines which had previously been attributed to helium.

Before long doubt was cast both on the elementary nature of the gas and on its identity with solar helium. Runge and Paschen showed that the spectrum lines of helium fell naturally into six series which were related to one another in sets of three. In each set there was a Principal Series composed of strong lines, and two Subordinate Series, consisting of weaker lines, which converged to a common limit. The series showed general resemblances to the series of hydrogen, on the one hand, arid to that of lithium on the other. Moreover, when the gas was allowed to stream through a porous plug into a Pliicker tube, the light at first was green, the line λ5016 of the single line group being equal in intensity to D₃, and then gradually became yellow as D₃ became relatively stronger. This observation was confirmed by Brauner. These investigators therefore concluded that they had separated cleveite gas into two components: helium of density 2.2, and a lighter gas for which a name, parhelium, was actually proposed. This view received some support from the fact that helium from different minerals showed considerable variations in density (from 2.181 to 2.114), and by diffusion through porous earthenware could be separated into two fractions differing still more in density. It was even suggested that there might be two sizes of molecules in the gas.

The homogeneity of helium was subsequently proved in two ways. Travers showed that on passing an electrical discharge through helium contained in a vacuum tube with platinum electrodes the pressure fell steadily, owing to the absorption of the gas by the finely divided platinum deposited on the walls of the tube, and that with this fall in pressure the colour of the glow changed from orange-yellow, through bright yellow and yellowish-green, to green. At this point the tube was allowed to cool, the residual gas was pumped out, and the tube was heated with a flame in order to drive out the gas occluded in the platinum. According to the hypothesis advanced by Runge and Paschen, this gas should have contained an excess of that constituent to which the yellow line of helium was due, but when the discharge was again passed through the tube it showed exactly the same behaviour as the original gas.

Further, Ramsay and Travers conducted an exhaustive fractional diffusion of cleveite gas and found that, though it could certainly be separated into two portions of densities 1.979 and 2.245 respectively, the lighter fraction, which possessed all the properties attributed to helium, was unchanged by further diffusion, while the heavier portion under this treatment gave still heavier fractions which were ultimately shown by spectroscopic observation to contain argon. The uncertainty caused by the differing densities of natural helium was thus satisfactorily removed and the elementary nature of the new gas demonstrated.

At one stage of its history the identity of cleveite gas with solar helium was also open to doubt as the former gave a yellow line which was undoubtedly double, while the solar line D₃ had not, at that time, been resolved. Later, however, both Huggins and Hale. showed that the solar line was also double.

During the first year following the discovery of helium - argon being the only member of the group then known - its position in the periodic classification was matter for much discussion, and even as late as 1899 Brauner showed considerable ingenuity in devising reasons for considering helium and argon to be inert compounds or allotropic modifications of known elements. With wider knowledge of the group to which helium belongs it becomes, however, increasingly probable that the commonly accepted views as to its elementary nature and position in the periodic classification are correct.

Up to 1903 the work done on helium consisted mainly in the detailed examination of its properties, but in that year Ramsay and Soddy made the sensational discovery that this gas was a product of the atomic disintegration of radium. This discovery will be dealt with more fully later; it is merely necessary to state here that it has been shown since that helium is also produced in the disintegration of other radioactive elements, and that the atom of helium is identical with the α-particle.

It has been supposed that helium can be produced by the passage of an electrical discharge through hydrogen. Sir J. J. Thomson has obtained evidence by his positive-ray method of the continuous evolution of helium from salts by the action of cathode rays.

After many fruitless attempts to liquefy helium had been made by Olzewski and Dewar, that difficult task was accomplished in 1908 by Onnes.