inert elements, having apparently no chemical activities whatever. No compounds are known in which any one of them exists as a chemical constituent, and they have resisted all attempts to cause them to enter into chemical combination with any other element. Hence there is at present no chemistry of these strange substances. This being the case, the only light which can be thrown upon the complexity of the molecules of these elements is by the determination of the ratio of their specific heats at constant pressure and at constant volume, deduced from determinations of the wave-length of sound. This ratio is found to be 1.66, which is the same as that obtaining in the case of mercury vapour, the only other monatomic gas in which this ratio has been determined; whereas with diatomic gases, such as hydrogen, nitrogen, and oxygen, the ratio is 1.4. ARGON. Symbol, A. Density, 19.96. Atomic weight, 39.92. Occurrence.-Argon is present in the atmosphere, where it exists to the extent of 0.937 per cent., or rather more than 1 per cent. of the "atmospheric nitrogen" is argon. It is also found in the occluded gases of certain specimens of meteoric iron, and in minute quantities in almost all natural waters, derived doubtless by solution from the atmosphere. Argon has not been met with in chemical combination with other elements, and no compounds containing this element are known. Modes of Preparation.—(1.) Argon may be obtained from the atmosphere by sparking a mixture of air and oxygen. The nitrogen combines with the oxygen, and the oxidised product is absorbed by potash. When no further contraction of volume is obtained, the excess of oxygen is removed by alkaline pyrogallate, and the residual gas is the argon. Unless a high-tension alternating electric discharge is employed the process is extremely slow. (2.) Argon may also be separated from the other atmospheric gases by first withdrawing the oxygen by means of red-hot copper, and after removing the carbon dioxide and aqueous vapour, passing the remaining gas over strongly heated magnesium turnings. The magnesium combines with the nitrogen (p. 232) and leaves the argon. In order to effect the complete absorption of every trace of nitrogen, the gas is passed backwards and forwards over the heated magnesium for many hours. Recently it has been found that the metal calcium is a more efficient agent for the absorption of nitrogen. If, therefore, the "atmospheric nitrogen" be passed over a heated mixture of magnesium filings and pure dry lime the magnesium and lime interact, forming magnesia and calcium, which latter absorbs the nitrogen very rapidly and at a lower temperature than that required by metallic magnesium. The purification of the argon thus obtained, and its complete separation from the other gases with which it is associated, was a problem which was only solved as a result of the later achievements by Dewar of obtaining liquid hydrogen in quantity. By means of the intense cold obtainable by liquid hydrogen, comparatively large quantities of argon were liquefied, and the liquid so obtained was then submitted to a process of fractional distillation. The liquid gases having the lowest boiling-points, namely helium and neon, are the first to evaporate, and by careful adjustment of the temperature of the refrigerating bath the denser gases, krypton and xenon, may be maintained even in the solidified state, while the whole of the argon in a state of practical purity can be distilled away. Properties. -Argon is remarkable for its extraordinary inertness, a property which is indicated by its name, argon signifying "inactive." As already mentioned, it has hitherto resisted all attempts to cause it to unite chemically with any other element. The density of the gas is 19.96, and therefore its molecular weight is 39.92; and since argon is a monatomic element, its atomic weight and its molecular weight are the same. Argon is about two and a half times as soluble in water as nitrogen, 100 volumes of water at 15° dissolving 4.1 volumes of argon. Owing to this superior solubility, the gases which are expelled from rain-water by boiling are slightly richer in argon than the original air before solution. The critical temperature of argon is - 117.4°, at which temperature the gas is liquefied by a pressure of about fifty-three atmospheres (or 40.20 metres of mercury). Liquid argon has a specific gravity of 1.212, and boils at 186.1. The boiling-point of argon, therefore, lies between those of the two chief constituents of the atmosphere, namely, oxygen and nitrogen, while its critical temperature is slightly above that of oxygen, as may be seen by the following comparison : Oxygen Nitrogen A very slight reduction of temperature below its boiling-point is sufficient to freeze argon to a white solid, the melting-point of which is - 187.9°;`that is, less than two degrees below the boilingpoint. The spectrum of argon is very complex. The most characteristic lines are two in the fed (less refrangible than the red lines of either hydrogen or lithium), a bright yellow line (more refrangible than the sodium line), a group of bright green lines, and another group of strong lines in the violet. The general character of the spectrum depends upon the nature of the electric discharge employed. With an intermittent discharge the lines in the red and pale green are the most prominent, while with a Leyden-jar discharge the red and light green lines almost entirely disappear, giving place to lines in the dark green, blue, and violet. HELIUM. Symbol, He. Density, 1.98. Atomic weight, 3.96. Occurrence. The existence of this element in the universe may be said to have been first discovered by Janssen, who during a solar eclipse in 1868 observed a certain line in the yellow of the spectrum of the sun's chromosphere which was not coincident with that of any known terrestrial element. This unknown element was afterwards named helium by Frankland and Lockyer. Terrestrial helium was discovered by Ramsay in 1895, in the gas which is contained in certain rare minerals, and which is evolved from them either when they are heated or when they are treated with dilute sulphuric acid. Chief among these minerals are clèveite, bröggerite, and uraninite, all of them minerals containing the metal uranium.* Helium is present in minute quantities in the atmosphere, namely, to the extent of about 1 or 2 volumes in 1,000,000 volumes of air, as estimated by its discoverer. Helium also occurs in certain natural waters, notably in the water from the Bath springs, which has been found to contain argon mixed with about 8 per cent. of its volume of helium. Method of Preparation.-Helium is isolated from the atmosphere by a method consisting firstly of what may be described as fractional liquefaction, followed by fractional evaporation or distillation. When air is liquefied by the so-called self-cooling or recupe *The amount of helium contained in 1 gramme of clèveite is about 3.2 c.c. (Ramsay), only about one half of which is given off by heat alone. rative method in any of the modern air-liquefiers based upon Linde's original apparatus (see page 77), those portions of the air which escape liquefaction and pass out of the apparatus will obviously contain most of the constituents having the lowest boiling-points. Therefore, by collecting the gas which escapes from the air-liquefier under these circumstances, and compressing it into a vessel cooled by liquid air, a liquid is obtained which contains most of the more volatile constituents (namely, the helium and neon), with, of course, argon and some nitrogen. Thus, by this process of fractional liquefaction liquid air is divided into two fractions, one containing practically all the denser and least volatile constituents, namely, the krypton and xenon, the other containing the helium and neon. The separation of the gases contained in the more volatile fraction is accomplished by fractional distillation or evaporation. At the low temperature obtainable by means of liquid hydrogen both argon and neon exert no vapour-pressure, being reduced to the state of non-volatile solids, and the helium in a state of purity can be pumped away from the mixture. Properties. Next to hydrogen, helium is the lightest known gas, its density being 1.98. Like all the other gases of this group its molecules are monatomic, its atomic and molecular weight therefore is 3.96. Helium is much less soluble in water than argon. The solubility of this gas in water forms an exception to the usual behaviour of gases, for it has been found that while its solubility diminishes with rise of temperature up to about 25°, between 25° and 50° the solubility slightly increases, as is shown by the following table.* 100 volumes of water at 760 mm. dissolve When induction sparks are passed through rarefied helium, the gas emits a brilliant yellow light with a tinge of apricot colour. When viewed through the spectroscope the most prominent and characteristic line is the intense yellow line D3, which is accompanied by one bright red line, two in the green, and two in the blue. On reducing the pressure in the tube, the yellow light due to line D gradually changes to a green, owing to the light from one of the green lines becoming greatly intensified. * Estreicher," Z. Phys. Ch.," 31, 176. Helium was first liquefied by Dewar (May 1898) by the use of boiling liquid hydrogen as a refrigerant; but the exact boilingpoint of the liquefied helium has not yet been determined (1902), although it is believed to be not very far removed from that of hydrogen. Like all the other gases of this group, helium is chemically inactive. NEON. Symbol, Ne. Density=9.96. Atomic weight=19.92. History. From analogy with other natural families of elements and the numerical relations of the atomic weights of the different members, the discoverer of argon and helium was led to believe that another element should exist having an atomic weight between those of these two elements, and about sixteen units higher than that of helium. The long and careful search for this unknown element was at last rewarded by the discovery of neon, whose atomic weight was found to be 19.92, or exactly sixteen units above that of helium. Although only present in minute quantities in the atmosphere, the discoverer estimates the amount as about ten times that of helium; that is to say, 1 or 2 parts of neon in 100,000 parts of air. Neon is more readily liquefied than helium, but no exact determination of its boiling-point has yet been made. The colour emitted by this gas when induction sparks are passed through it is a brilliant orange-pink. Its spectrum is characterised by a bright yellow line, D, and a great cluster of lines in the more orange part of the red end. It also exhibits other fainter lines throughout the spectrum. These two denser gases are obtained by the fractional distillation of the heavier portion of liquid air obtained in the air-liquefier (see Helium). Large quantities of this liquid, amounting to 30 litres, were carefully evaporated, and the residual portion, after being entirely freed from nitrogen and oxygen, was again liquefied by means of liquid air. The constituents of this liquid were then separated by fractionation. As soon as most of the argon was |