26 



THE HUMAN MOTOR 



The influence of molecular structure (stereochemistry) on 

 physiological operations is still an obscure problem. As for the 

 reality of the molecule, recent works show that it does not admit 

 of a doubt. By the calculations of M. Jean Perrin ( x ) it is known 

 that 1 gramme of glucose contains 100 milliards of milliards 

 (10 20 ) of real molecules. 



20. Centre of Mass or Gravity. By centre of mass is under- 

 stood that point in a body through which the resultant of all the 

 weights of its parts passes, in every position the body can 

 assume. More precisely, if all the 

 forces, which represent the effects of 

 gravity on the molecules of the body, 

 are composed, their resultant G R, is at 

 the centre of its bulk, which is the centre 

 of gravity, G (fig. 45). The position of 

 the centre of gravity only depends on 

 the form of the body and the distribution 

 of its matter at a given moment ; it 

 depends on the form alone if the body 

 is homogeneous, that is to say, of similar 

 structure in all its parts. But that is 

 a comparatively rare case, and in hetero- 

 centre of gravity must be determined 



Fio. 45. 



the 



geneous bodies, 

 experimentally. 



The vertical line which traverses a body at its centre of 

 gravity is its axis of gravity ; such an axis has special properties 

 ( 41). 



M 



Ptane, 



M 



Fio. 46 



FIG. 47 



H Jean Perrin (Journal de Physique, 1910, p. 10 ; Revue Scientifique, 1911, 

 p. 774) ; Les Atomes (Paris, 1913). 



