Sir William Ramsay.
Helium is a monatomic gas having the atomic weight 4. It is less soluble in water than argon. Like argon, it shows no tendency to enter into chemical union with any other substance. It has been found in many minerals, particularly in those containing uranium and the so-called rare earth metals. It also occurs among the gases issuing from certain mineral springs, such as those of Bath and at Cauterets in the Pyrenees, and also at Adano near Padua. The spectrum of helium contains, in addition to the characteristic yellow line—by which its presence had been recognised not only in the solar chromosphere, but also in certain of the fixed stars—two lines in the red, and lines in the green, blue, and violet. The character of the light emitted by the spark-discharge is modified by the intensity of the discharge in a manner similar to that of argon. It has been shown by Collie that its spectrum is altered by the presence of mercury vapour. It is the least refractive of all the gases. Helium was liquefied by Kammerlingh Onnes in 1908. It forms a colourless liquid of sp. gr. 0.154, boiling at -268.5; that is, 4°.5 above the absolute zero of temperature. Its critical temperature is about 5° absolute, and its critical pressure above 2¼ atmospheres.
The methods now in use for obtaining liquid air, referred to in a subsequent chapter, enable large quantities of that material to be obtained readily; and it was in investigating spectroscopically the residues left after volatilising a quantity of liquid air that Ramsay and Travers, in 1898, detected the existence of two new monatomic gaseous constituents of the air which they named respectively krypton (χρυπτός, hidden) and neon (νέος, new), the former heavier and the latter lighter than argon. By fractional distillation of the argon, simultaneously procured, a gas was obtained which in the spectroscope showed the characteristic lines of helium—previously recognised in atmospheric argon by Kayser and Friedländer—together with a complicated spectrum consisting of a number of lines in the red, orange, and yellow due to the new element neon. On cooling this mixture to -252° by means of liquid hydrogen, the neon solidified, while the helium remained gaseous and could thus be separated.
Krypton was obtained from the residues left on the evaporation of a large quantity of liquid air. Mixed with the krypton was a third gaseous constituent of air, to which the name xenon (ξενος, the stranger) was given. The boiling-point of krypton at atmospheric pressure was found to be -152°, and its melting-point -169°; the boiling-point of xenon was -109° and its melting-point -140°. Their critical temperatures were respectively -62°.5 and +14°.7. Hence xenon could be liquefied by pressure a very little below the mean temperature of the air. Neon boils at -243° and freezes at -253°. They form colourless liquids freezing to ice-like solids. All of them, with the exception of argon, which is present to the extent of about 1 part in 107 parts of air, are contained in extremely small amounts in the atmosphere, approximately in the following proportions:
| Helium | 1 | part in | 245,300 | parts by volume. |
| Neon | 1 | ” ” | 80,800 | ” ” |
| Krypton | 1 | ” ” | 20 millions | ” ” |
| Xenon | 1 | ” ” | 170 ” | ” ” |
Many tons of liquefied air have since been systematically fractionated, but no other gas than those above named has been obtained.
Julius Thomsen, of Copenhagen, in a paper published in 1895, entitled On the Probability of the Existence of a Group of Inactive Elements, pointed out, in relation to Mendeléeff’s Law of Periodicity (see ante), that in periodic functions the change from negative to positive value, or the reverse, can take place only by a passage through zero or through infinity; in the first case the change is gradual, and in the second case it is sudden. The first case corresponds with the gradual change in electrical character with rising atomic weight in the separate series of the periodic system, and the second case corresponds with a passage from one series to the next. It therefore appears that the passage from one series to the next in the periodic system should take place through an element which is electrically indifferent. The valency of such an element would be zero, and therefore in this respect also it would represent a transitional stage in the passage from the univalent electronegative elements of the seventh to the univalent electropositive elements of the first group. This indicates the possible existence of a group of inactive elements with the atomic weights 4, 20, 36, 84, 132—numbers corresponding fairly closely with the atomic weights respectively of helium, neon, argon, krypton, and xenon.
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No discovery of recent years has created more widespread interest than that of the radio-active elements.