In choosing letters for publication I have been largely guided by the wish to illustrate my father's personal character. But his life was so essentially one of work, that a history of the man could not be written without following closely the career of the author. Thus it comes about that the chief part of the book falls into chapters whose titles correspond to the names of his books. In arranging the letters I have adhered as far as possible to chronological sequence, but the character and variety of his researches make a strictly chronological order an impossibility. It was his habit to work more or less simultaneously at several subjects. Experimental work was often carried on as a refreshment or variety, while books entailing reasoning and the marshalling of large bodies of facts were being written. Moreover, many of his researches were allowed to drop, and only resumed after an interval of years. Thus a rigidly chronological series of letters would present a patchwork of subjects, each of which would be difficult to follow. The Table of Contents will show in what way I have attempted to avoid this result. In printing the letters I have followed (except in a few cases) the usual plan of indicating the existence of omissions or insertions. My father's letters give frequent evidence of having been written when he was tired or hurried, and they bear the marks of this circumstance. In writing to a friend, or to one of his family, he frequently omitted the articles: these have been inserted without the usual indications, except in a few instances, where it is of special interest to preserve intact the hurried character of the letter. Other small words, such as "of", "to", etc., have been inserted usually within brackets. I have not followed the originals as regards the spelling of names, the use of capitals, or in the matter of punctuation.