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 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. It will be seen, for instance, that the second 



