THE FORCE BEHIND NATURE. 351 



experience as the basis of our knowledge, we recognize the down- 

 ward tendency of every body heavier than air, by our sense of 

 muscular tension in hfting it from the ground, or in resisting its 

 descent towards the earth ; and that our cognition of force 

 through this form of sensation, being thus quite as immediate and 

 direct as our cognition of motion through the visual sense, ought 

 to be equally taken account of. 



The promulgation, about the same time, of the doctrine of 

 the " Correlation of the Physical Forces " by Professor (now Sir 

 William) Grove, and the researches of Mr. Joule on the "Me- 

 chanical Equivalent of Heat," seemed to me to bring this view of 

 dynamical causation into yet greater importance; by showing 

 that what is true of that form of force which produces or resists 

 mechanical (or what is now distinguished as jftolar) motion, may 

 be legitimately extended to those other forms which are mani- 

 fested in the molecular changes that express themselves in 

 chemical action, or impress us with the sensations of heat, light, 

 etc. Partaking of the general ignorance at that time prevalent in 

 this country of the doctrine of " Conservation of Energy," already 

 promulgated in Germany by Mayer and Helmholtz, I myself 

 endeavoured to carry Professor Grove's principle into the domain 

 of biology ; by showing that what physiologists had been accus- 

 tomed to call vital force, may be regarded as having the same 

 "correlation" with the various forms of physical force as they 

 have with each other.* And in the introduction to the fourth 

 edition of my "Human Physiology" (published in 1853), I thus 

 explicitly defined my position : — 



When this assemblage of antecedents is analyzed, it is uni- 

 formly found that they may be resolved into two categories, 

 which may be distinguished as the dynamical and the material, 

 the former supplying the force or power to which the change 

 must be attributed, whilst the latter afford the conditions under 

 which that power is exerted. Thus in a steam-engine we see 

 the dynamical agency of heat made to produce mechanical 

 power by the mode in which it is applied : first, to impart a 

 mutual repulsion to the particles of water ; and then, by means 



* " On the Mutual Relations of the Vital and Physical Forces," in Philos. 

 Transact., 1850. 



