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. CONTENTS CHAPTER I PAGE THE EXPERIMENTS AND SPECULATIONS OF BOYLE, MAYOW, AND HALES 1 CHAPTER II “FIXED AIR” AND “MEPHITIC AIR”--THEIR DISCOVERY BY BLACK AND BY RUTHERFORD 38 CHAPTER III THE DISCOVERY OF “DEPHLOGISTICATED AIR” BY PRIESTLEY AND BY SCHEELE--THE OVERTHROW OF THE PHLOGISTIC THEORY BY LAVOISIER 68 CHAPTER IV “PHLOGISTICATED AIR” INVESTIGATED BY CAVENDISH--HIS DISCOVERY OF THE COMPOSITION OF WATER 119 CHAPTER V THE DISCOVERY OF ARGON 146 CHAPTER VI THE PROPERTIES OF ARGON 181 CHAPTER VII THE POSITION OF ARGON AMONG THE ELEMENTS 216 LIST OF PORTRAITS STEPHEN HALES _Frontispiece_ ROBERT BOYLE _To face page_ 8 JOHN MAYOW " 17 JOSEPH BLACK " 48 DANIEL RUTHERFORD " 62 JOSEPH PRIESTLEY " 72 ANTOINE AUGUSTE LAVOISIER " 102 HON. HENRY CAVENDISH " 121 CHAPTER I THE EXPERIMENTS AND SPECULATIONS OF BOYLE, MAYOW, AND HALES