ESTABLISHMENT OF THE GENERAL PRINCIPLE. 405 



spring of our mechanical power ; the vis viva of our whole 

 microcosm. Modern physical inquiry ventures even one step 

 further, and seeks the source of Light and Heat of the Sun 

 itself. Are these, as formerly supposed, the result of com- 

 bustion, or are they, as surmised by Mayer and Thomson, 

 the expression of the motive power continually generated in 

 the fall of aerolites towards the Sun, and as continually anni- 

 hilated by their impact on its surface ? Leaving the discus- 

 sion of this question to Physical Philosophers, I proceed now 

 to my own proper subject. 



It is now about twenty years since Dr. Mayer first broadly 

 announced, in all its generality, the great principle now known 

 as that of " Conservation of Force ;" as a necessary deduc- 

 tion from two axioms or essential truths ex nihilo nil fit, and 

 nil fit ad nihilum the validity of which no true philosopher 

 would ever have theoretically questioned, but of which he 

 was (in my judgment) the first to appreciate the full practical 

 bearing. Thanks to the labours of Faraday, Grove, Joule, 

 Thomson, and Tyndall, to say nothing of those of Helmholtz 

 and other distinguished Continental savans, the great doctrine 

 expressed by the term " Conservation of Force " is now 

 amongst the best-established generalizations of Physical Sci- 

 ence ; and every thoughtful Physiologist must desire to see 

 the same course of inquiry thoroughly pursued in regard to 

 the phenomena of living bodies. This ground was first broken 

 by Dr. Mayer in his remarkable treatise, *' Die Organische 

 Bewegung in ihrem Zusammenhange nit dem Sloffwechsel " 

 (" On Organic Movement in its relation to Material Changes," 

 Heilbronn, 1845) ; in which he distinctly set forth the princi- 

 ple that the source of all changes in the living Organism, 

 animal as well as vegetable, lies in the forces acting upon it 

 from without; whilst the changes in its own composition 

 brought about by these agencies, he considers to be the imme- 

 diate source of the forces which are generated by it. 



In treating of these forces, however, he dwells chiefly on 



