THE INSTABILITY OF THE HOMOGENEOUS. 439 



gate, are different in their directions, the effects produced 011 

 these two parts by like forces must be different in their di- 

 rections. Why must they be different? They must be dif- 

 ferent because such unlikeness as exists between the two 

 sets of factors, is made by the presence in the one of some 

 specially-directed force that is not present in the other ; and 

 that this force will produce an effect, rendering the total re- 

 sult in the one case unlike that in the other, is a necessary 

 corollary from the persistence of force. Still more 



manifest does it become that the dissimilarly-placed parts of 

 any aggregate must be dissimilarly modified by an incident 

 force, when we remember that the quantities of the incident 

 force to which they are severally subject, are not equal, as 

 above supposed; but are nearly always very unequal. The 

 outer parts of masses are usually alone exposed to chemical 

 actions ; and not only are their inner parts shielded from the 

 affinities of external elements, but such affinities are brought 

 to bear unequally on their surfaces; since chemical action 

 sets up currents through the medium in which it takes place, 

 and so brings to the various parts of the surface unequal 

 quantities of the active agent. Again, the amounts of any 

 external radiant force which the different parts of an aggre- 

 gate receive, are widely contrasted: we have the contrast 

 between the quantity falling on the side next the radiat- 

 ing centre, and the quantity, or rather no quantity, falling 

 on the opposite side; we have contrasts in the quantities re- 

 ceived by differently-placed areas on the exposed side; and 

 we have endless contrasts between the quantities received 

 by the various parts of the interior. Similarly when me- 

 chanical force is expended on any aggregate, either by col- 

 lision, continuous pressure, or tension, the amounts of strain 

 distributed throughout the mass are manifestly unlike for 

 unlike positions. But to say the different parts of an aggre- 

 gate receive different quantities of any incident force, is to 

 say that their states are modified by it in different degrees 

 is to say that if they were before homogeneous in their 



