ADVERTISEMENT

The Inert Constituents of the Atmosphere

THE discovery of an element always awakens interest; for the total number of the known elements does not exceed seventy-five, and all the various forms of matter which exist on this globe are necessarily composed of these elements. Elements must not be regarded as isolated entities, each self-dependent, having no relations with its compeers; on the contrary, all the elements exhibit certain connections with their neighbors; and there is to be traced an orderly progression from one class of elements, strongly electro-positive in character, metallic iu appearance, very inflammable when heated in the air, and at once attacked by water, to another class, highly electro-negative, transparent, unattackable by oxygen, and without perceptible action on water, through a number of connecting Jinks, each of which serves to soften the transition. These elements have been arranged in series, and it is by considering the method of arrangement that cur interest is awakened. The revival of the hypothesis of the atomic constitution of matter by Dalton and of his attempt to determine the atomic weights of the elements was not long in provoking the guess that perhaps there could be found some connection between the numbers representing the relative atomic weights of kindred elements. But, as is well known, the state of knowledge in Dalton's day was not sufficiently advanced to enable him to attribute to elements their correct relative atomic weights; and it was not until the eminent professor of chemistry in Rome, Cannizzaro, whose jubilee has recently been celebrated, pointed out the bearing on Dalton's numbers of all the facts accumulated up to the year 1856 that the dose relationship between the atomic weights and the properties of the elements was suggested by John Newlands. Some years later Lothar Meyer and Dmitri Mendeleef amplified and elaborated the ideas which had first been propounded by Newlands; and the periodicity of the atomic weights and the gradual variation of the properties of the elements and their compounds were established on a firm basis. The division of the elements into metals and non- metals corresponds broadly with another well-marked division—that into basic and acidic. Generally speaking, it is the oxides of the metallic elements which react with water to form bases, and those of the non- metals whch form acids with water. According to modern ideas, bases, by the mere act of solution in water, are supposed to be split up into portions, for which the term ion, invented by Faraday, has been retained; one ion is charged by the process of solution with a positive charge, and that portion is usually a metal; the other portion, which consists of one or more groups of hydrogen and oxygenin combination, termed “hydroxyl”—OH—has a negative charge. A base, indeed, is a compound which splits in this manner. On the other hand, an acid, when dissolved in water, undergoes an analogous split; but in this case the electro-positive ion is always hydrogen, while the electro-negative ion may either be an element such as chlorine, or a group of elements such as exist in nitric acid (NOJ. The order of the various elements in the electric series has been determined; and not merely determined, but to each has been attached a numerical value. This value is identical with what is termed “chemical affinity” and it represents the electric potential of the element with reference to an arbitrary starting point, which does not differ much from that of nickel, an element closely related to iron. Only a few such values have as yet been determined numerically; instances may be chosen from the magnesium group, where the numbers run: Magnesium = + 1.2; Zinc = + 0.5; Cadmium = + 0.19; or from the fluorine column, where the numbers are: Fluorine = — 2.0; Chlorine = — 1.6; Iodine = — 0.4. In each case the potential, positive or negative, is the highest for the element with smallest atomic weight, and decreases with increase of atomic weight, for elements in the same column. The order of some of the elements is: Cs Rb K Na Li Ba Sr Ca Mg Al Mn Zn Cd Fe” Co Ni Pb H Cu Ag Hg'Pt“Au” and for electro-negative ions, S O' I Br CI F; the first element, c<Bsium, being the most electro-positive, and the last, fluorine, the electronegative. The order given above corresponds fairly well with the order in the periodic table, passing from left to right. But, as in the tabie, the atomic weights follow each other continuously round the cylinder or round the spiral, the abrupt change from elements of an extreme electro-negative character like fluorine to sodium, an element of highly electro-positive character, or from chlorine to potassium, has always appeared remarkable. The old dictum, Natura nihil fit per salturn, if not always true (else we should have no elements at all, but a gradual and continuous transition from one kind of matter to another—a condition of affairs hardly possible to realize), has generally some spice of truth in it; and it might have been predicted (and *Abstract of an evening lecture delivered at the meeting of the British Association at Glasgow, September 13, by Prof. W. Ramsay, F.R.S. the forecast seems to have been made obscurely by several speculators) that a series of elements should exist which should exhibit no electric polarity whatever. Such elements, too, should form no compounds, and, of course, should display no valency; they should be indifferent, inactive bodies, with no chemical properties. The discovery of argon in 1894, followed by that of terrestrial helium in 1895, and of neon, krypton and xenon II 1898, has shown the justice of the foregoing remarks. Inasmuch as the methods employed for the isolation of these elements illustrate their properties and confirm the views as to their inertness and lack of electric polarity, I propose to sketch shortly the history of their discovery. An accurate investigation of the density of atmospheric nitrogen and of nitrogen prepared from its compounds led Lord Rayleigh to inquire into the cause of the density of the nitrogen of the atmosphere exceeding that of “chemical nitrogen” by about one part III two hundred, whereas the accuracy of his experi- Fig. 1. ments was such that it would have excluded an error of one part in five thousand. I need not here allude to the reasons which were at first put forward to account for this anomaly; suffice it to say that they offered no explanation; and that we ultimately traced the discrepancy to the presence in “atmospheric nitrogen” of a gas nearly half as dense again as nitrogen. A convenient form of apparatus for isolating this gas shown II Fig. 1. The gas, air mixed with oxy- ?en is confined over mercury in an inverted test-tube, II contact with a few drops of a solution of caustic potash; and by connecting the rings with wires from the secondary coil of an induction apparatus, sparks pass between the platinum terminals in the interior o f the test-tube. The volume of the gas rapidly diminishes; and II a few hours the gas is removed to a clean tube, and the excess of oxygen absorbed by burning phosphorus; the inert gases remain behind. On a larger scale, the apparatus used by Lord Rayleigh , consisting of a balloon of six liters' capacity, in the interior of which an electric flame is kept alight by means of a transformer. while a jet of caustic alkali forms a fountain in the interior, gives good results. By its help seven or eight liters of mixed gases can he made to combine per hour. Such experiments show the inactive nature of the argon group of gases toward an electro-negative element. oxygen. The gases are absolutely incombustible. No other elements can withstand such treatment, save platinum and its congeners and gold. But even these FlG.2. metals combine with fluorine or chlorine when heated in a current of one or other gas. Argon, however, is wholly unaffected when electrie sparks are passed through its mixture with chlorine or fluorine, the two other most electro-negative elements. To them, too, it shows itself completely indifferent. A more convenient method of separating the nitrogen from its admixture with argon in atmospheric aii' is by means of red-hot magnesium. The metal magnesium, which is now made on a considerable scale for photographic and signaling purposes, is a white, silvery metal, which can be planed or turned into shavings. In the early experiments a measured quantity of atmospheric nitrogen dried by passing over suitable drying agents was brought into contact with magnesium turnings, heated to redness, in a tube of hard glass. It has been found. however, by M. Ma- quenne that the metal calcium, which, for this purpose is most easily produced by heating together a mixture of magnesium filings and pure dry lime, is a more efficient absorbing agent for nitrogen, for it does not require such a high temperature and can be effected without danger of melting the glass tube. Indeed, the operation is a very easy one and can he carried out with the very simple apparatus shown in Fig. 2. M. Guntz has also found that lithium, an element belonging to the same column in the periodic table as sodium and potassium, is an exceedingly good absorbent for nitrogen, for it tarnishes in nitrogen even at atmospheric temperature owing to the formation of a nitride. On a large scale the magnesium turnings are contained in iron tubes and the gas-holders are made of copper or of galvanized iron. By this means fifteen liters of argon were separated from about two cubic yards of air. The inactivity of argon in contact with such highly electro-positive elements as lithium, magnesium and calcium again demonstrates its want of electric polarity. No other element would have resisted such treatment except those of the argon group. But these are not the only data from which such a conclusion can be drawn, for it was found that no action takes place between argon and hydrogen, phosphorus, sulphur, tellurium, caustic soda, potassium nitrate, sodium peroxide, sodium persulphide, nitro-hydrochloric acid, bromine water and many other reagents which it would be tedious to mention, all of which are remarkable for their chemical activity. We may therefore take it that the name “argon,” which means “inactive,” has been happily chosen. In attempting to form compounds of argon, however. another consideration was not lost sight of; if compounds of argon were capable of existence they ought to exist in nature, and, as in all probability they would be easily deeomposed by heat, it ought to be possible to decompose them with evolution of argon, which could be collected and tested. Prof. Miers, in a letter which he wrote me the day after an account of the fruitless attempts to cause argon to combine had been given to the Royal Society, drew my attention to experiments by Dr. Hillebrand, of the United States Geological Survey, in course of which he obtained a gas, which he believed to be nitrogen, by treating the rare mineral clevite, a substance found in feldspathic rocks in the south of Norway, with sulphuric acid. The chief constituents of clevite are oxides of the rare elements uranium and thorium, and of lead. The gas obtained thus, after purification from nitrogen, was examined in a Plucker tube with the spectroscope and exhibited a number of brilliant lines, of which the most remarkable was one in the yellow part of the spectrum, similar in color to the light given out by the glowing tube. The position of this line, and of others which accompany it, established the identity of this gas, not with argon, as was hoped, but with a supposed constituent of the sun's chromosphere, first observed by M. Janssen, of Paris, during an eclipse which was visible in India in 1868. The late Sir Edward Frankland and Sir Norman Lockyer, who studied the spectrum of the chromosphere, gave to the supposititious element, which they regarded as the cause of these lines, the name “helium,” a word derived from the Greek for “the sun.” Having been placed on the track, I examined, with the assistance of Dr. Collie and Dr. Travers, no fewer than 51 minerals. while Sir Norman Lockyer examined 46 additional ones, which we had not examined, and in 19 minerals. almost all of them containing uranium, helium was found. Only one gave an argon spectrum, namely, malacone. We also sought for argon and helium in meteorites, which all give off gas on heating; but in only one specimen, a meteorite from Augusta County. Virginia, was helium found, in this case accompanied by argon. All natural waters contain argon, for that gas is somewhat soluble in water (4.1 volumes per 100 of water at 15 deg. C.); but some also contain helium, as, for instance, the gas from the Bath springs, which Lord Rayleigh found to contain argon mixed with about 8 per cent of its volume of helium; and helium has also been found in mineral springs at Wild- bad, and at Cauterets, in the Pyrenees. It would appear, then, that helium is not such a very rare constituent of our globe; and indeed, it is probable that it is continually escaping from the earth in small quantities in certain regions. In 1897, as president of the Chemical Section of the British ' Assoeiation, I chose the title “An Undiscovered Gas” for the address to the Section. The arguments in favor of the existence of such a gas were briefly these: The differences between the atomic weights of consecutive elements in the columns of the periodic table are approximately 16 to 20; thus 1(;.5 Is the difference between the atomic weights of fluorine and ehlorine; 16 between those of oxygen and sulphur. and so on. Again, stepping one pace down the scale, we have 19.5 as the difference between chlorine and manganese; 20.3 between sulphur and chromium; 19.8 between silicon and titanium. etc. The total difference between manganese and fluorine is 36; between chromium and oxygen, 36.3; between vanadium and nitrogen, 37.4; and between titanium and carbon, 36.1. This is approximately the difference between the atomic weights of helium and argon, 36. I quote now from that address: “There should, therefore, be an undiscovered element between helium and argon, with an atomic weight 16 units higher than that of helium, and 20 units lower than that of argon, namely, 20. And if this unknown element, like helium and argon, should prove to consist of monatomic molecules, then its density should be half its atomic weight, 10. And pushing the analogy still further, it is to be expected that this element should be as indifferent to union with other elements as the two allied elements.” Those who care to read the story of the search for this undiscovered element may find it in the address. Minerals from all parts of the globe, mineral waters from Britain, France and Iceland, meteorites from interstellar space: all these were investigated without results. Helium from various minerals was separated by long and tedious processes of diffusion into a possibly lighter portion, diffusing more rapidly, and a possibly heavier portion diffusing more slowly, but with no positive result. The systematic diffusion of argon. however, gave a faint indication of where to seek for the missing element, for the density of the more rapidly diffusing portion was 19.93. while that of the portion which diffused more slowly was 20.01. The invention by Dr. Hampson of an apparatus by means of which it is possible to obtain liquid air at small expense and with little trouble placed a new instrument in our hands; and Dr. Travers and I prepared 15 liters of argon from the atmosphere, with the purpose of distilling it fractionally, after liquefaction; for we know, from the researches of Prof. Olszewski of Cracow, who has done so much to determine the properties of liquefied gases, that argon could be liquefied easily by compressing it into a vessel cooled by help of liquid air. And, moreover, we were in hope that by fractionating the air itself gases of even higher atomic weight than argon might possibly be obtained. Both expectations were realized; on distilling liquid argon the first portions of gas to boil off were found to be lighter than argon, and on allowing liquid air to boil slowly away heavier gases came off at the last. It was easy to recognize these gases by help of the spectroscope, for the light gas, to which we gave the name neon. or “the new one,” when electrically excited emits a brilliant flame-colored light; and one of the heavy gases, which we called krypton, or “the hidden one,” is characterized by two brilliant lines, one in the yellow and one in the green part of the spectrum. The third gas, named .renon. or “the stranger,” gives out a greenish-blue light and is remarkable for a very complex spectrum, in which blue lines are conspicuous. Although neon was first obtained by the fractional distillation of argon, it was afterward found convenient to prepare it direct from air. The torpedo-compressor, which is used for compressing the air before it enters Dr. Hampson's liquefier, was made to take in the air which had escaped liquefaction in the liquefier; the denser portions were thus liquefied, and the lighter portions were liquefied by compressing them into a vessel cooled by the denser fractions, boiling under reduced pressure, and consequently at a specially low temperature. This liquefied portion was again fractionated, am! yielded neon; and it was not long before we discovered that helium was also present in the mixture. The presence of helium in atmospheric air had previously been noted by Prof. Kayser of Bonn, and by Prof. Friedlander of Berlin, on submitting the spectrum of argon to a searching examination. The purification of this mixture of neon and helium from argon, although a lengthy process, was not attended by any special difficulty. It was accomplished by repeated distillation, the lighter portions being always collected separately from the heavier portions, and again distilled by themselves. But after this separation had been accomplished, we found that we were unable by means of liquid air to liquefy the mixture, or indeed any portion of it. We effected a partial separation by diffusion; but it is not possible to separate by this method two gases of which the quantity is limited. Another attempt was made by dissolving the gases in liquid oxygen, on the supposition that neon might prove more soluble than helium; but without satisfactory results. It was evident that a lower temperature than that possible by help of liquid air was necessary. Prof. Dewar had by that time succeeded in producing liquid hydrogen in quantity, and had indicated the principle, which is identical with that of Dr. Hampson's air-liquefier, although he has not published any detailed account of his apparatus. Dr. Travers undertook to investigate the subject; and after four unsuccessful trials he made a liquefier, with the help of Mr. Holding, the laboratory mechanician, by means of which a hundred cubic centimeters of liquid hydrogen could be easily and cheaply produced. There was then no difficulty in effecting the separation of neon from helium; for, while neon is practically nonvolatile, when cooled by liquid hydrogen, remaining in the state of solid or liquid. even that enormously low temperature is not sufficient to convert helium into a liquid. Hence the gaseous helium could be pumped away from the non-gaseous neon, and the latter was obtained in a pure state. The residues obtained from the evaporation of about thirty liters of liquid air, after being freed from oxygen and nitrogen, were liquefied by help of liquid air, and fractionated from each other. The separation offered no special difficulty, but was long and tedious. It soon appeared that when most of the argon had been removed the residue solidified when cooled; but while it was possible to remove the krypton by pumping, for it goes into gas slowly even at a low temperature of liquid air, very little xenon accompanied it; for at that temperature xenon is hardly at all volatile. Having finally separated the gases, their densities and other properties were carefully determined; and it was also proved that they were like argon and helium, inasmuch as their molecules consist of single atoms. Neon, as was expected, turned out to be the missing link between helium and argon; the atomic weight of krypton was found to be 81.6, and that of xenon 128. The volumes occupied by equal numbers of molecules of the liquefied gases were determined; and also the boiling-points and melting-points of argon, krypton and xenon. These figures are shown in the following table: Helium. Neon. Argon. Krypton. Xenon. Density of gaB.... 198 996 19.96 40.78 64.0 Atomic weight 3.96 19.92 39.92 81.1,6 128.0 Density ofliquid ... 0.3(?) 1.0(1) 1.212 2.155 3 52 Boiling-points — -186.1°C. -lS1.7°0. -109.l°C. Melting-point8 — — 187.9°C -189°C^. -140 °C. Critical temperatures — — -117.4°C. - f)2.5°C. + 1"".75°C Critical pressures ... — —(Meters.) 40.20 -41.24 43.50 Refractivity of p:as.. 0.124 0.235 0.968 1.450 2.368 In every case there is seen what is termed periodicity; that is, a gradual alteration with rise of atomic weight, of the densities of the liquids, of the melting- points, of the boiling-points, and of the retardation of light when passed through the gas. Let us consider, in conclusion, the position of these elements in the periodic table; and it will be sufficient to confine our attention to the groups of elements which form the neighboring columns. The atomic weights are given in round numbers. Hydrogen. Helium. Lithium. Beryllium. 1 4 7 9 Fluorine. Nf'on. Sodium. Magnesium. 19 20 23 24 Chlorine. Argon. Potassium. Calcium. 35.5 40 39 4 Bromine. Krvpton. Rubidium. Strontium. 80 82 85 87 Iodine. Xf'noii. Cresiiim. Barium. 127 128 137 It is evident that these new elements fall into their natural places between the strongly electro-negative elements of the fluorine group, and the very electropositive elements of the lithium group, and that, in consequence of their lack of electric polarity and their inactivity they form, in a certain sense, a connecting- link between the two. It is curious, too, to notice that iodine, xenon, cmsium and barium form the ends of their respective columns. It is, of course, not impossible that other elements may be discovered possessing similar properties and yet higher atomic weights than these; but as yet there is no clew to guide us where to search for them. It is difficult, owing to the impossibility of. effecting a complete separation of the inactive elements from each other, to do more than hazard a guess as to their relative amount in air. As they are easily separated from the other constituents of air, there is no doubt as to their total amount; air contains 0.937 part of argon and its companions by volume in 100 parts. Perhaps the table below may be taken as affording some indication of their relative amounts. Air contains by volume: 0.937 part of argon per- hundred. One or two parts of neon per hundred thousand. One or two parts of helium per million. About one part krypton per million. About one part of xenon per twenty million. It is of course not impossible that xenon may contain an even smaller proportion of a still heavier gas: hut it is unlikely. Sea-water sometimes contains a grain of gold per ton; that is one part in 15,180,000; a grain of xenon is container! in about four hundredweights of air. The problems suggested by the periodic table are by no means solved by the discovery of these aerial gases; but something has been done to throw light upon one obscure corner of the field. The gap between the electro-positive and the electro-negative elements has been bridged.—Nature.

Scientific American Back To School

Back to School Sale!

12 Digital Issues + 4 Years of Archive Access just $19.99

Order Now >

X

Email this Article

X