To Lord Rayleigh the writer desires to express his best thanks, for kindly consenting to read the ms. Of that part of Chapter V. Which relates to the discovery of argon, and for notes thereon, which have been embodied in the text. Mr. W. G. Ramsay has taken the trouble to compile the chronological list of honours conferred on his father, which will be found at the end of the book. The long series of letters addressed by William Ram say to his cousin, Miss Ramsay, the eldest daughter of his uncle, the late Sir Andrew C. Ramsay, has afforded much insight into his character and the intimate rela tions subsisting in the family.