Helium

HELIUM is a colourless, odourless gas of a family of ele ments, which on account of their extreme chemical inertness are termed the "inert" or "noble" gases. It is the lightest gas known except hydrogen. It was discovered in the sun (1868) many years before it was found on the earth. The spectroscope was used for the first time to examine the chromosphere round the sun during an eclipse, and it was observed that there was a brilliant yellow line, which the French astronomer J. Jannsen showed was not coincident with the already well-known D, and D, lines of sodium. J. N. Lockyer and E. Frankland shortly afterwards con cluded that this yellow line (known as was due to an unknown element which they called helium (Greek, iiXtos, sun), symbol He, atomic number 2, atomic weight 4. Terrestrial helium was dis covered by Sir W. Ramsay during the year 1894 while engaged in experiments on the sources of argon. He found that the inert gas obtained in considerable volume by heating the mineral cleveite, and hitherto thought to be nitrogen, was only partially removed by sparking with oxygen in the presence of caustic soda. The residual gas on spectroscopic examination showed a brilliant yellow line which W. Crookes definitely proved to be identical with the line observed by Jannsen. H. Kayser (1895), whose work was later confirmed by S. Friedlander, first discovered the presence of helium in the atmosphere by spectroscopic means. The actual sep aration of helium from the atmosphere was effected by W. Ramsay and M. W. Travers, who obtained, by a method of fractional dis tillation, not only helium, but three new elements belonging to the same family of inert gases, namely neon, krypton and xenon. In the years that followed the discovery of helium, extensive work was carried out on its occurrences, properties and physical con stants. W. Ramsay (1903) demonstrated that helium was a prod uct of the radioactive disintegration of radium. This experiment stands out as being the first example of a transmutation of ele ments. The history of the production of helium on a commercial scale began when a German Zeppelin which had been pierced by incendiary bullets did not take fire (1914) . Sir R. Threlfall sug gested to the British Admiralty that the Germans had possibly dis covered a new source of helium and had utilized this non-inflam mable gas for filling the Zeppelin. The various possible sources of helium in the British empire were then reviewed. The natural gas supplies of Canada appeared to be the only source likely to yield helium in sufficient quantity for war purposes. A survey of the gas fields was instituted by J. C. McLennan of Toronto, and experi mental plants for the extraction of helium were set up at Hamilton in Ontario and Calgary in Alberta (1918). At the conclusion of the work (1920), about 6o,000cu.ft. of helium of 6o to 90% purity had been extracted. The work of H. P. Cady and D. F. Macfarlane (190 7) on the helium-bearing gases of Kansas had already shown that the United States were in possession of supplies of helium which might be developed on a commercial scale, but nothing further was done until America entered the war (1917). Opera tions were then prosecuted with such vigour that at the time of the Armistice 147,000cu.ft. of helium of 93% purity were on the dock ready for despatch to Europe for aeronautical purposes. A large scale plant was built at Fort Worth (1925), capable of turn ing out 1,000,000cu.ft. per month, and this gas was proved by the aeronautical experts to be most eminently suitable for the filling of airships and balloons, provided it could be obtained in sufficient quantities at a reasonable price.

Occurrence.

Helium occurs, not only in the atmosphere of the sun, but also in certain of the fixed stars and many nebulae. It is a constituent of the earth's atmosphere and is found in most of the older rocks and minerals, in gases evolved from mineral springs, in natural gas, and in sea and river water in minute quanti ties. It has also been detected in certain fumarole and volcanic gases. W. Ramsay, using Dewar's method, estimated the amount in the atmosphere as one part in 2 50,00o by volume. W. Watson, later, by analysis of the gas separated from air by G. Claude's method, gave the value as one part in 185,000, which is probably a more exact figure. J. H. Jeans, on theoretical grounds, calculates, that at considerable altitudes, the proportion of helium in the atmosphere is materially increased, and that at a height of 5oom. the atmosphere is made up of helium and hydrogen. J. Stoney states that a planet of the earth's dimensions cannot permanently retain the helium in its atmosphere and that the helium together with the hydrogen is continually lost into space. The minerals which contain helium are widely distributed throughout the earth's crust, but with few exceptions it is only in those containing the radioactive elements, thorium and uranium, that it is found in measurable quantity. The most important helium minerals are cleveite, monazite, fergusonite, broggerite, samarskite, thorianite and euxenite. Helium is found in the gases evolved from mineral springs. The French springs which have been systematically exam ined by C. Moureu and A. Lepape evolve considerable amounts of helium. The Mazieres spring gives a gas containing 6% of helium, whilst that at Santenay gives over 9%. This spring alone is calcu latUd to give more than 600cu.ft. of helium per yeah. The only mineral spring in England containing helium is the one at Bath, estimated by R. T. Elworthy (1918) to contain 0.21 %. The natural gases of America are now recognized as the chief source of helium, as already mentioned. R. D. Moore estimates that more than i,000,000cu.ft. of helium run to waste in natural gas every 24 hours. The gas fields of Ontario and Alberta contain 0.33 to o.34% helium, hut the total Canadian supply is small compared with that of the United States.

Isolation and Purification.—Helium may be prepared from any of the sources mentioned above. The preparation from the minerals is effected by heating them either alone or with sulphuric acid. Thorianite, which consists mainly of thorium oxide, is one of the most convenient minerals for the preparation of helium, since I gram yields as much as 9.5c.c. of the gas. Monazite, which is largely used in the incandescent mantle industry, contains less helium than thorianite but is more abundant. One gram of the mineral yields 1 c.c. of helium. The monazite is heated with strong sulphuric acid, and the evolution of gas is complete within two hours. The gas, so obtained, after washing with caustic soda solu tion, contains about 9o% helium. The isolation of helium from natural gas is most easily effected on a small scale by Dewar's method. The gases are passed over charcoal, cooled to the tem perature of liquid air, and under these conditions only the helium escapes absorption. All gases, other than neon and hydrogen, may be completely separated from helium by Dewar's charcoal method. In order to separate neon from helium, the gas must be cooled to the temperature of liquid hydrogen, when the neon condenses. Hydrogen is usually removed by chemical means. Oxygen is added, and the mixture is exploded by an electric spark, with the result that the hydrogen is converted into water. Alternatively, the gas is passed over copper oxide, when also conversion of the hydrogen to water takes place.

Properties.—Helium possesses several remarkable properties of special interest to the scientist. Its atom has a relatively simple structure, and it has been possible to draw many theoretical con clusions from its behaviour. The density (o. 1368, air = i) is less than that of any other known gas except hydrogen (0.0696). The molecular weight of helium calculated from its density is approxi mately 4. Helium is a monatomic gas. The calculated ratio of the two specific heats (those at constant pressure and at constant vol ume) of a monatomic gas is 1.667. W. Ramsay found the ratio for helium to be 1.652, and K. Scheel and W. Heuse found it to be i.66 at i8° and 1.673 at 180°. Additional evidence that helium is monatomic is derived from a consideration of other physical prop erties, such as dielectric constant, thermal conductivity and Zee man effect ; also from its position in the periodic classification of the elements. The rate of diffusion of helium through a porous tube is less than the value calculated from Graham's law of diffu sion of gases. F. G. Donnan has suggested that this is due to the rise in temperature which is known to occur on free expansion of the gas through a small orifice. The permeability of various fabrics to helium has been investigated on account of its use in airships and balloons. Rubbered fabrics are found to be only about 0.7 times as permeable to helium as to hydrogen. In the case of skin lined fabrics, however, the ratio is nearly unity. Quartz glass is easily permeable to helium at red heat, and it has recently been discovered that it is also permeable at ordinary temperatures to helium at a pressure of ioo atm. Helium is less soluble in water than any other known gas : the solubility at io° is a minimum, 0.0100. The coefficient of increase of pressure with rise of tem perature at constant volume is perfectly normal. The value found by W. Ramsay for this coefficient between o° and ioo° is 0.0036616, and H. K. Onnes observed that this value is independ ent of the original pressure. The constancy of the pressure co efficient of helium has led to its application to thermometry where it is of especial value in the measurement of low temperatures. The thermal conductivity of helium has a comparatively high value, approaching that of hydrogen. The Shakespeare cathar ometer, whose operation depends on this physical property, has been applied to the analysis of helium produced on a com mercial scale where nitrogen usually is the only impurity. The spectrum of helium was studied by C. Runge and F. Paschen, who found that it contained six series of lines which fall into two groups, each group consisting of a special series and two sec ondary series. The first group is a series of doublets of which the D3 doublet is the principal. The second group is a single line series, the most important of which is the green line 5015. The colour of the light emitted by a Plucker tube filled with helium under the influence of an electric discharge depends on the pressure of the gas : at 7-8mm. the colour of the glow is a brilliant yellow, since the (5876) line then reaches its maximum inten sity. If the pressure is gradually reduced the intensity of the green line (5o15.6) increases, till at i to 2mm. the tube emits a brilliant green light. The presence of impurities in helium causes a marked change in the appearance of the spectrum and may completely mask the helium lines; this masking effect is considerably reduced at low pressures. Helium was liquefied by H. K. Onnes (1908) who used a specially constructed liquefier of the Hampson type. It was necessary to pre-cool the helium to a temperature of —258° C with liquid hydrogen boiling under reduced pressure, since at ordinary temperatures heating is produced by the expan sion of helium through a nozzle. Onnes obtained 6oc.c. of liquid from 300 litres of gas, and determined its physical constants. It is a colourless mobile liquid boiling at 4.25° A. The critical temperature is 5.25° A., the critical pressure 2.26 atm., and the critical density 0.066. Liquid helium has proved an invaluable agent for the attainment of extreme cold. By its evaporation under reduced pressure it is possible to reach a temperature slightly lower than 0•9° A. Helium was solidified by W. H. Keesom (1926). When a tube system containing liquid helium became blocked he assumed that the helium had solidified. The following solidification points were determined: i•i° A., 2.6 atm. ; A., so atm. ; 3.2° A., 86 atm. ; 4-2° A., 150 atm. Helium, as is also common with the other rare gases, is characterized by its extreme chemical inactivity. All the attempts of W. Ram say and J. N. Collie to make helium combine with a second ele ment ended in failure. Helium was circulated over various ele ments and substances at a bright red heat, but in no case was there any evidence of a reaction. Berthelot (1895) subjected helium to the action of a silent electric discharge in the presence of benzene and mercury. A large proportion of the gas was absorbed and a resinous solid produced. A green glow was also seen which gave the spectrum of mercury. J. J. Manley (1926) obtained evidence of the formation of a gaseous mercury helide formed from helium and mercury under the influence of an elec tric glow discharge. Boomer subjected helium, in conjunction with mercury, iodine, sulphur and phosphorus, to electronic bombardment in the presence of a surface cooled with liquid air, and obtained evidence of the formation of compounds of helium with the elements mentioned.

Radioactivity.—Whenever helium occurs in nature it appears, with few exceptions, to be associated with the phenomenon of radioactivity, and it is generally believed that helium in nature has arisen from the various radioactive elements. The best known of these elements, radium, thorium and uranium, are characterized by the emission of radiations which can be detected by the elec trical or photographic effects produced by them. These radiations have been divided into three classes, namely, alpha-, beta-, and gamma-rays (briefly, a-, (3-, and 7-). E. Rutherford (1907) was the first to suggest that the a-rays consisted of a stream of helium atoms carrying two unit positive charges. W. Ramsay and F. Soddy demonstrated the truth of this theory by a spectroscopic study of the emanation from a solution of radium bromide. The emanation gradually developed a helium spectrum, as shown by the characteristic line. Later E. Rutherford and T. Roydes conclusively showed the identity of the a-particle with helium. The emanation from radium was stored in exceedingly thin-walled glass tubes surrounded by an outer exhausted jacket. The tubes were impervious to ordinary gaseous helium but allowed nearly all the a-particles to escape into the outer jacket. The gas which collected there was passed into a tiny spectrum tube and shown to be helium. The radioactive elements are distributed with great uniformity in the rocks which constitute the earth's surface. Though the amount in most rocks is small, the sum total is suffi cient to account for the helium that exists in nature.

Commercial Production.—Helium is separated from natural gas on a commercial scale by liquefaction methods, similar to those employed in the separation of oxygen from the atmosphere. A typical analysis of natural gas from the Petrolia field in the United States is as follows: helium 0.93%; carbon dioxide o.25%; oxygen 0•54%; methane 56.85%; ethane and heavier hydrocar bons 10.30%; nitrogen 31.13%. In the large-scale extraction plant at Fort Worth, carbon dioxide is removed from the gas by washing with lime water; the gas is then compressed and cooled, first by a carbon dioxide cycle in a fore-cooler, and next in heat inter changers, when it meets the cold gases returning from the still. The gas is then expanded through a nozzle, and enters the bottom of the still, partly as a liquid and partly as a gas. The still consists of three units, each of which has a rectifying column with a con denser at the top and a receiver at the bottom. The liquids which collect in the upper receivers are used to cool the condensers underneath. The top condenser is cooled by a nitrogen cycle. The heavier hydrocarbons condense in the Lowest unit, while methane and most of the nitrogen are liquefied in the top unit. Helium mixed with a little nitrogen, escapes from the top of the still and is passed to storage. The cold gases, obtained by the evaporation of the liquefied hydrocarbons and nitrogen, pass out through the heat interchangers. The natural gas thus stripped of its helium is returned to gas mains for subsequent industrial or domestic use. The plant at Fort Worth consists of six complete units each designed to handle approximately 44,00ocu.ft. of natural gas per hour, and extracts on the average 6o% of the helium of the gas.

Helium up to 95% purity may be obtained by this process, at a cost of about $24 per i,000cu.ft. The patents for this process are owned by the Linde Air Products Co., which also designed and still operates the plant for the United States Government.

Technical Applications.

Various tests made in the United States have shown that helium is the ideal gas for filling airships and balloons, and its use has necessitated new methods in the operation of airships. The German practice has been to waste large amounts by valving, so that as much gas as 20 times the volume of the airship was used each year. In order to prevent wastage of helium by valving, the loss in weight due to consump tion of fuel has been compensated by condensation of water from the exhaust gases of the engines. Also, when possible, the airship is driven down by means of her motors instead of by valving. Helium which has become too impure for flying purposes, through diffusion of air into the gas bag, is repurified either by cooling the compressed gas with liquid air, or by Dewar's charcoal method. Owing to the low solubility of helium in the fluids of the body, experiments have been made on the use of helium-oxygen mixtures in deep diving (q.v.) and caisson operations, as it is evident that the use of such mixtures in place of air will allow divers to work at greater depths or for longer periods. Many other technical appli cations of helium have been suggested, but the relatively small amount available for industrial use and the restriction on its expor, from the United States have hindered developments.

BIBLIOGRAPHY.—E. H. Weaver, Bibliography of Literature ofBibliography.—E. H. Weaver, Bibliography of Literature of Helium, United States Bureau of Standards, Circular No. 81, 1919; R. T. Elworthy, Helium in Canada, Mines Branch, Department of Mines, Canada. No. ; R. B. Moore, Properties, Occurrence, and Production of Helium, Journal Franklin Institute, vol. cxci., pp. '99, Feb. 1921 ; I. N. Friend and H. V. C. Briscoe, Textbook of Inorganic Chemistry (vol. i., 1917) . (R. TA.)