PREFACE BY THE AUTHOR. 
Tus 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 au 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- 
