242 THE SCIENTIFIC MONTHLY
ered that there muflt be the element and had formed a pretty fair idea of what its appearance and properties wonld be, still, this element, that Davy in 1813 stated to exist in fluorspar, was not isolated till, in 1886, Moissan by the use of the electric current at a low temperature, with specially resistant apparatus, obtained the element in the form of a gas whose properties were almost identical with those predicted.
Another class of elements are elusiye because their presence is not suspected, and their properties are somewhat similar to those of known elements. The metal cssium belongs to this class. In 1846, Flattnei analyzed the mineral pollux, but could not get the constituents as found by his analysis to add up to one hundred per cent. He published his figures, however. In 1860 Bunsen and Kirchhoff discovered the element caesium, and it turned out that Flattner's analysis needed only the correction that the mineral pollux contained caesium, instead of what he thought was potassium. The properties of potassium and caesium are very similar; potassium ia a common metal, while caesium is not, and it was not till the delicate methods of the spectroscope were devised that any ready means of distingnishing between the two ele- ments was available. The five elements discovered in the air are suffi- ciently similar to nitrogen as not to be distinguished from it until chemists had their attention turned to the matter by experiments under- taken with an entirely different object in view.
In 1785 the Honorable Heniy Cavendish, of whom the Frenchman Biot in his obituary notice remarked that he was '^the richest of all scientists, and the most scientific of all the rich, made some experi- ments with air by passing electric sparks through it, in this way pro- ducing nitric acid and potassium nitrate (saltpeter). It is notable that this process that Cavendish first applied on the small scale and with excessive toil is now carried out on the commercial scale in Nor- way and Sweden, where electric power is cheap. Up till three or four years ago, however, not more than one per cent of the world's supply of nitric acid was made in this way, since, for the most part, it was cheaper to get it from sodium nitrate or Chili saltpeter, so named from its place of origin in South America. But as 50-80 per cent, of all the more important explosives consist of nitric acid and as Germany must be pretty well shut off from South America, she must either have laid in an enormous stock of nitric acid before the war or the electric process must have since been greatly developed, unless^ indeed, she has gone back to the primitive method by which saltpeter is made in the villages of India, which is very unlikely.
When Cavendish passed electric sparks through air and oxygen added as required, he found that though the volume of air diminished until it became very small, it was impossible for him to reduce it to zero; a little gas remained. He had recognized the air as containing
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