IN THE ANIMAL ORGANISM. 203 



100 feet. The momentum of motion of the first 

 (30X100) is equal to that of the third (60X50), 

 while that of the second (30X200) is twice as 

 great. 



Momentum of force and momentum of motion 

 in mechanics are therefore expressions or measures 

 for effects of force, having reference to the velocity 

 attained in a given time, or to a given space ; and 

 in this sense they may be applied to the effects of all 

 other causes of motion, or of change in form and 

 structure, however great or however small may be 

 the space or the time in which their effects are dis- 

 played to the senses. 



Every force, therefore, exhibits itself in matter 

 either in the form of resistance to external causes 

 of motion, or of change in form and structure ; or 

 as a moving force when no resistance is opposed to 

 it ; or, finally, in overcoming resistance. 



One and the same force communicates motion 

 and destroys motion ; the former, when its manifes- 

 tations are opposed by no resistance ; the latter., 

 when it puts a stop to the manifestation of some 

 other cause of motion, or of change in form and 

 structure. Equilibrium or rest is that state of 

 activity in which one force or momentum of motion 

 is destroyed by an opposite force or momentum of 

 motion. 



We observe both these manifestations of activity 

 in that force which gives to the living tissues their 

 peculiar properties. 



