ON MATTER AND FORCE. 73 



disintegration, but with formation ; that, when 

 it is in exercise, it increases ; and, when it is 

 quiescent, it lessens in bulk ; that is, that it 

 more rapidly disintegrates during rest than 

 during exercise. This view can only be proved 

 by a vast amount of further experimental 

 research. 



When this question, which regards the value 

 of this one item in the expenditure and income 

 of matter and force, is solved by experiment, 

 there will be left still more difficult problems 

 as, for example, how this conversion of heat 

 into contraction of muscular fibre takes place ; 

 how the nerves are made to be able to increase 

 or lessen the conversion of latent into active 

 energy at will. 



This most extensive and important inquiry, 

 which the doctrine of the conservation of energy 

 has opened to our work, is good evidence that 

 the third stage of ideas on the union of matter 

 and force is beginning to be received in phy- 

 siology. 



What the worth of these ideas is may be seen 

 in the comparative simplification of the problem 

 of the source of animal heat, when the doctrine 

 of the conservation of energy is admitted. 



