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 [Pg vi] 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.