SEGREGATION. 473 



not previously been able to reach the bottom. And in 

 a way that is different though equally significant, this segre- 

 gative effect of water in motion, is exemplified in the carry- 

 ing away of soluble from insoluble matters — an application 

 of it hourly made in every laboratory. The effects of 



the uniform forces which aerial and aqueous currents exer- 

 cise, are paralleled by those of uniform forces of other 

 orders. Electric attraction will separate small bodies from 

 large, or light bodies from heavy. By magnetism, grains of 

 iron may be selected from among other grains; as by the 

 Sheffield grinder, whose magnetized gauze mask filters out 

 the steel-dust wdiich his wheel gives off, from the stone-dust 

 that accompanies it. And how the affinity of any agent act- 

 ing differently on the components of a given body, enables 

 us to take away some component and leave the rest behind, is 

 shown in almost every chemical experiment. 



What now is the general truth here variously presented? 

 How are these several facts and countless similar ones, to be 

 expressed in terms that embrace them all? In each case we 

 see in action a force which may be regarded as simple or uni- 

 form — fluid motion in a certain direction at a certain veloc- 

 ity; electric or magnetic attraction of a given amount; 

 chemical affinity of a particular kind : or rather, in strictness, 

 the acting force is compounded of one of these and certain 

 other uniform forces, as gravitation, etc. In each case we 

 have an aggregate made up of unlike units — either atoms of 

 different substances combined or intimately mingled, or 

 fragments of the same substance of different sizes, or other 

 constituent parts that are unlike in their specific gravities, 

 shapes, or other attributes. And in each case these unlike 

 units, or groups of units, of which the aggregate consists, 

 are, under the influence of some resultant force acting indis- 

 criminately on them all, separated from each other — segre- 

 gated into minor aggregates, each consisting of units that are 

 severally like each other and unlike those of the other minor 

 aggregates. Such being the common aspect of these 

 32 



