PREFACE BY THE AUTHOR. 



THIS volume contains a series of essays written and 

 published at different times, some of a general charac- 

 ter, and others more special, and all relating to the 

 activity of natural forces, especially those of life. The 

 mere bringing together of these fragments has present- 

 ed an opportunity of completing a methodical and uni- 

 form whole, combining exactness in details with gen- 

 erality of doctrines, and distinctly tracing the precise 

 aspect of each group of phenomena in the picture of 

 the close and universal relations that bind the whole 

 together. An exposition is thus offered under an ele- 

 mentary form, in language freed from technical dress, 

 of the most essential truths established of late by phys- 

 ics, chemistry, and biology, regarding the mechanism 

 of natural forces, and the arrangement and combina- 

 tion of the fundamental springs of being in the world, 

 especially in the living world. I indulge the hope that 

 such a work might meet a kindly welcome from minds, 

 ever increasingly numerous, that regard science as the 

 subject neither of idle curiosity nor of passing entertain- 

 ment, but as the object of earnest sympathy and of se- 



