                    London

                        MACMILLAN AND CO., LTD.
                      NEW YORK: THE MACMILLAN CO.
                                 1896


                         _All rights reserved_

        “Modern discoveries have not been made by large
      collections of facts, with subsequent discussion,
      separation, and resulting deduction of a truth thus
      rendered perceptible. A few facts have suggested an
      _hypothesis_, which means a _supposition_ proper
      to explain them. The necessary results of this
      supposition are worked out, and then, and not till
      then, other facts are examined to see if their
      ulterior results are found in Nature.”
      --DE MORGAN, _A Budget of Paradoxes_, ed. 1872, p. 55.




PREFACE


The discovery of new elementary gas in the atmosphere in 1894 aroused
much interest, and public attention has again been directed to the
air, which was, for many centuries, a fruitful field for speculation
and conjecture. The account of this discovery, communicated to the
Royal Society in January 1895, was, however, necessarily couched in
scientific language; and many matters of interest to the chemist and
physicist were written in an abbreviated style, in the knowledge that
the passages describing them would be easily understood by the experts
to whom the communication was primarily addressed. But persons without
any special scientific training have frequently expressed to me the
hope that an account of the discovery would be published, in which the
conclusions drawn from the physical behaviour of argon should be
accompanied by a full account of the reasoning on which they are based.
An endeavour to fulfil this request is to be found in the following
pages. And as the history of the discovery of the better known
constituents of the atmosphere is of itself of great interest, and
leads up to an acquaintance with the new stranger, who has so long been
with us incognito, an effort has here been made to tell the tale of the
air in popular language.