478 SEGREGATION. 



force of mutual polarity impresses unlike motions on the 

 mixed units in proportion as they are unlike ; and therefore, 

 in proportion as they are unlike, tends to deposit them in 

 separate places. 



There is a converse cause of segregation, which it is 

 needless here to treat of with equal fulness. If different 

 units acted on by the same force, must be differently moved ; 

 so, too, must units of the same kind be differently moved by 

 different forces. Supposing some group of units forming- 

 part of a homogeneous aggregate, are unitedly exposed to a 

 force that is unlike in amount or direction to the force acting 

 on the rest of the aggregate; then this group of units will 

 separate from the rest, provided that, of the force so acting 

 on it, there remains any portion not dissipated in molecular 

 vibrations, nor absorbed in producing molecular re-arrange- 

 ments. After ail that has been said above, this proposition 

 needs no defence. 



Before ending our preliminary exposition, a comple- 

 mentary truth must be specified ; namely, that mixed forces 

 are segregated by the reaction of uniform matters, just as 

 mixed matters are segregated by the action of uniform 

 forces. Of this truth a complete and sufficient illustration 

 is furnished by the dispersion of refracted light. A beam 

 of light, made up of ethereal undulations of different orders, 

 is not uniformly deflected by a homogeneous refracting 

 body; but the different orders of undulations it contains, are 

 deflected at different angles: the result being that these 

 different orders of undulations are separated and integrated, 

 and so produce what we know as the colours of the spectrum. 

 A segregation of another kind occurs when rays of light 

 traverse an obstructing medium. Those rays which consist 

 of comparatively short undulations, are absorbed before 

 those which consist of comparatively long ones; and the red 

 rays, which consist of the longest undulations, alone pene- 

 trate when the obstruction is very great. How, conversely, 

 there is produced a separation of like forces by the reaction 



