OF VITAL MOTION. 25 



.relative with physical motion, heat, chemical affinity, 

 and electricity. Instead of having any distinct and 

 independent mode of action, therefore, light is rather 

 to be regarded as a condition of one and the same 

 action, ^higher in character, perhaps, than heat, but 

 still related to this and the other physical agents, just 

 as the several prismatic colours in light are related to 

 each other, all being varying aspects of one central 

 principle. 



It is difficult indeed to imagine any real difference 

 in the mode of operation of the several physical agents 

 foreign to the body ; and it is more consistent with 

 the spirit of philosophy to seek in secondary causes 

 for the explanation of any apparent differences. 

 Instead of supposing light to have any special action 

 upon the organism, it is better, therefore, to regard 

 this influence as expressive of that condition of the 

 atmosphere most favourable to evaporation and radia- 

 tion, and most likely to bring about that degree of 

 inspissation in the organic fluids which naturally 

 results in the deposition of solids that were previously 

 in a state of fluidity. 



The phenomena of "rotation" refer also to the 

 agencies concerned in the free discharge of the other 

 functions of nutrition ; and here, likewise, it is possible 

 to meet with a partial explanation without having 

 recourse to the unintelligible mystery of vital power. 

 Let it be supposed indeed that the cell (itself acted 



