PREFACE TO THE SECOND EDITION

SINCE the publication of the first edition, atmo- spheric air has been found to contain other four inactive gases, belonging to the same class of elements as argon. These are helium, discovered by myself in 1895 in certain rare minerals, but first separated from the atmosphere in 1900 ; and neon, krypton, and xenon, discovered in conjunc- tion with Dr. M. W. Travers in 1898, and separated from argon and from each other during the years 1899 and 1900. An additional chapter has been added, giving an account of these elements. I have also to thank Mr. S. Lupton and Dr. Harfcog for some interesting details of the lives of Mayow and of Priestley, which, along with some errata, will be found at the end of the book.

December 1900. vii