14 THE DATA OF BIOLOGY. 



liarities we have been tracing: bringing to their solution, 

 the general mechanical principles which are now found to 

 hold true of molecules as of masses. But it must suffice 

 briefly to indicate the conclusions which such an inquiry 

 promises to bring out. 



Proceeding on these principles, it may be argued that the 

 molecular mobility of a substance must depend partly on the 

 inertia of its molecules; partly on the intensity of their 

 mutual polarities ; partly on their mutual pressures, as deter- 

 mined by the density of their aggregation; and (where the 

 molecules are compound) partly on the molecular mobilities 

 of their component molecules. Whence it is to be inferred 

 that any three of these remaining constant, the molecular 

 mobility will vary as the fourth. Other things equal, there- 

 fore, the molecular mobility of molecules must decrease as 

 their masses increase; and so there must result that pro- 

 gression we have traced, from the high molecular mobility 

 of the uncombined organic elements, to the low molecular 

 mobility of those large-moleculed substances into which they 

 are ultimately compounded. 



Applying to molecules the mechanical law which holds of 

 masses, that since inertia and gravity increase as the cubes 

 of the dimensions while cohesion increases as their squares, 

 the self-sustaining power of a body becomes relatively 

 smaller as its bulk becomes greater; it might be argued that 

 these large, aggregate molecules which constitute organic 

 substances, are mechanically weak — are less able than simpler 

 molecules to bear, without alteration, the forces falling on 

 them. That very massiveness which renders them less mo- 

 bile, enables the physical forces acting on them more readily 

 to change the relative positions of their component atoms; 

 and so to produce what we know as re-arrangements and de- 

 compositions. 



Further, it seems a not improbable conclusion, that this 

 formation of large aggregates of elementary atoms and re- 

 sulting diminution of self-sustaining power, must be accom- 



