Page:The Scientific Monthly vol. 3.djvu/247

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UBSTANCES withoit chemistry must necessarily be elements, for, if substances form compounds with other substances, they thereby come into the realm of chemistry. The group of elements to be considered are found in the atmosphere, but their presence was not suspected until less than twenty years ago, when within a comparatively short space of time, helium, neon, argon, krypton and xenon were discovered. Of these, xenon is present only to the extent of one part in 170,000,000 of air, but argon constitutes nearly one per cent. of the atmosphere. Yet, with the exception of the chemist Cavendish, no one seems to have suspected its presence and Cavendish merely suggests the existence of some unknown gas.

In this paper there will be several digressions that may seem foreign to the subject in hand, but that are intended to help in elucidating the main topic or to explain words or phrases necessarily employed.

Two classes of elements have eluded for a longer or shorter time the researches of chemists. One class is represented by fluorine. The story is told of a man who claimed to have discovered a universal solvent and, when asked to exhibit it, replied "How can I? It is impossible to get any dish to contain it." Fluorine is somewhat of this nature. The existence of the element was suspected. The mineral fluorspar was known to contain calcium, which is the metallic part of limestone, and was suspected of containing another element similar to chlorine, which is the non-metallic part of common salt. But this element could not be got from fluorspar in a manner analogous to that by which chlorine was obtained from common salt. The element could be detached from the calcium, but only to combine with something else. When common salt is acted on with concentrated sulphuric acid (the oil of vitriol of the newspaper reporter) hydrochloric add, a very irritating gas, is produced, and this gas with suitable chemicals gives chlorine. When fluorspar is acted on with concentrated sulphuric acid a still more irritating gas, hydrofluoric acid, is produced. Its solution in water produces, on the flesh, very distressing sores exceedingly difficult to heal. Both the gas and its solution act on glass and the etching of glass is frequently done by the use of hydrofluoric acid. But hydrofluoric acid could not be made to act on any chemical in such a way as to set free fluorine. The element entered into innumerable combinations, but did not appear alone. Thus, though chemists had consid-