1861.] Conservation of Farce applied to Organic Nature, 357 



bodies were determined by one principal agent which they chose to 

 call the " vital principle." The physical forces in the living body they 

 supposed could be suspended or again set free at any moment, by the 

 influence of the vital principle ; and that by this means this agent 

 could produce changes in the interior of the body, so that the health of 

 the body would be thereby preserved or restored. 



Now the conservation of force can exist only in those systems in 

 which the forces in action (like all forces of inorganic nature) have 

 always the same intensity and direction if the circumstances under 

 which they act are the same. If it were possible to deprive any body 

 of its gravity, and afterwards to restore its gravity, then indeed we 

 should have the perpetual motion. Let the weight come down as 

 long as it is heavy ; let it rise if its gravity is lost ; then you have 

 produced mechanical work from nothing. Therefore this opinion 

 that the chemical or mechanical power of the elements can be sus- 

 pended, or changed, or removed in the interior of the living body, 

 must be given up if there is complete conservation of force. 



There may be other agents acting in the living body, than those 

 agents which act in the inorganic world ; but those forces, as far as they 

 cause chemical and mechanical influences in the body, must be quite of 

 the same character as inorganic forces, in this at least, that their effects 

 must be ruled by necessity, and must be always the same, when acting 

 in the same conditions, and that there cannot exist any arbitrary choice 

 in the direction of their actions. 



This is that fundamental principle of physiology which I men- 

 tioned in the beginning of this discourse. 



Still at the beginning of this century physiologists believed that it 

 was the vital principle which caused the processes of life, and that 

 it detracted from the dignity and nature of life, if anybody expressed 

 his belief that the blood was driven through the vessels by the 

 mechanical action of the heart, or that respiration took place accord- 

 ing to the common laws of the diff*usion of gases. 



The present generation, on the contrary, is hard at work to find 

 out the real causes of the processes which go on in the living body. 

 They do not suppose that there is any other difference between the 

 chemical and the mechanical actions in the living body, and out of it, 

 than can be explained by the more complicated circumstances and 

 conditions under which these actions take place ; and we have seen 

 that the law of the conservation of force legitimizes this supposition. 

 This law, moreover, shows the way in which this fundamental question, 

 which has excited so many theoretical speculations, can be really and 

 completely solved by experiment. 



