PREFACE. 



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 writ- 

 ten 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 chrono- 

 logical order an impossibility. It was his habit to 

 w^ork more or less simultaneously at several subjects. 

 Experimental work was often carried on as a refresh- 

 ment 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 Avhat way I have attempted to avoid this 

 result. 



In printing the letters I have followed (except in a 



