476 SEGREGATION. 



Through a mixed aggregate of soluble aud insoluble sub- 

 stances, let water slowly percolate. There will in the first 

 place be a distinct parting of the substances that are the 

 most widely contrasted in their relations to the acting 

 forces: the soluble will be carried away; the insoluble will 

 remain behind. Further, some separation, though a less 

 definite one, will be effected among the soluble substances; 

 since the first part of the current will remove the most solu- 

 ble substances in the largest amounts, and after these have 

 been all dissolved, the current will still continue to bring 

 out the remaining less soluble substances. Even the undis- 

 solved matters will have simultaneously undergone a certain 

 segregation; for the percolating fluid will carry down the 

 minute fragments from among the large ones, and will de- 

 posit those of small specific gravity in one place, and those 

 of great specific gravity in another. To complete 



the elucidation we must glance at the obverse fact ; namely, 

 that mixed units which differ but slightly, are moved in but 

 slightly-different ways by incident forces, and can therefore 

 be separated only by such adjustments of the incident forces 

 * as allow slight differences to become appreciable factors in 

 the result. This truth is made manifest by antithesis in the 

 instances just given; but it may be made much more mani- 

 fest by a few such instances as those which chemical analy- 

 sis supplies in abundance. The parting of alcohol from 

 water by distillation is a good one. Here we have atoms con- 

 sisting of oxygen and hydrogen, mingled with atoms consist- 

 ing of oxygen, hydrogen, and carbon. The two orders of 

 atoms have a considerable similarity of nature: they similar- 

 ly maintain a fluid form at ordinary temperatures; they 

 similarly become gaseous more and more rapidly as the tem- 

 perature is raised ; and they boil at points not very far apart. 

 Xow this comparative likeness of the atoms is accompanied 

 by difficulty in segregating them. If the mixed fluid is 

 unduly heated, much water distils over with the alcohol: 

 it is only within a narrow range of temperature, that the one 



