ON THE CORRELATION OP THE PHYS- 

 ICAL AND VITAL FORCES. 



I. RELATIONS OF LIGHT AND HEAT TO THE VITAL FORCES 

 OF PLANTS. 



IN every period of the history of Physiology, attempts 

 have been made to identify all the forces acting in the 

 Living Body with those operating in the Inorganic universe. 

 Because muscular force, when brought to bear on the bones, 

 moves them according to the mechanical laws of lever action, 

 and because the propulsive power of the heart drives the 

 blood through the vessels according to the rules of hydraulics, 

 it has been imagined that the movements of living bodies may 

 be explained on physical principles ; the most important con- 

 sideration of all, namely, the source of that contractile power 

 which the living muscle possesses, but which the dead muscle 

 (though having the same chemical composition) is utterly in- 

 capable of exerting, being altogether left out of view. So, 

 again, because the digestive process, whereby food is reduced 

 to a fit state for absorption, as well as the formation of va- 

 rious products of the decomposition that is continually taking 

 place in the living body, may be initiated in the laboratory 

 of the chemist ; it has been supposed that the appropriation 



